Apollo 15

The main focus of the mission, as far as the public and media are concerned, is with Falcon and its occupants, Dave Scott and Jim Irwin, on the surface of the Moon. Readers can follow their progress in the Apollo Lunar Surface Journal.

Many details of Falcon's descent and touchdown are shown in this video. which includes the 16mm film that was shot from high gate to the landing itself.

Meanwhile, Al Worden continues orbiting the Moon in Endeavour at an altitude of about 110 km (60 nautical miles), carrying out a busy program of science data gathering. Most of the instrumentation for this is contained within the SIM bay, mounted in sector 1 of the Service Module.

During Falcon's descent and landing, Al had kept largely quiet. Now after the landing, his own workload begins to build up. At 105:12 GET, he completes a routine P52 realignment of Endeavour's inertial platform using the landing site REFSMMAT as the reference orientation. The Flight Plan also includes an update from Mission Control of the start and stop times of the Mapping and Panoramic Cameras, and an uplink of the most up-to-date state vector to the spacecraft's computer. The transcript does not include any hints that these tasks took place though a post-mission summary does include a note of the P52 realignment.

Rev 15 begins at about 105:41.

During the far-side pass, as Endeavour commences its 15th rev, Al prepares the SIM bay for his solo mission. After disabling those RCS jets which might affect the SIM bay's instruments, he opens the covers for the Mapping Camera and Laser Altimeter and begins extending its lens out to its operational position. He powers up the Panoramic Camera and sets it to take stereo images, then switches on the Laser Altimeter. He then takes 20 photographs out of a planned 36, using the Hasselblad camera and magazine O, of details within the 240-kilometre crater, Gagarin; a large but heavily eroded circular formation named after the first man to orbit Earth, Yuri Gagarin, 1934-1968.

The first two images, AS15-97-13137 and 13138, composited here, along with 13139, show a small region of the crater's eastern rim.

Al continues his photography as Endeavour crosses the floor of Gagarin. This composite of AS15-97-13140 to 13152 shows the rough floor of Gagarin where the debris blankets flung in from distant craters are pockmarked by smaller craters, old and new.

Three photos cover Gagarin's western rim.

This spectacular photo of a rayed crater manages to display virtually the entire ray system. The result of a relatively recent impact, this feature illustrates the transition from the continuous to the discontinuous ejecta blanket. This transition is roughly two crater diameters out but is irregular and lobate in shape. The light colour of the ray material is due to the highly shocked and fractured nature of the ejected rock. Time, and the irradiated environment of the Moon will eventually darken this material to match its surroundings. Consider that every other crater in this photograph, from the truly ancient and barely visible depressions to the more recent, neat bowls peppering the landscape, once was graced with a similar stunning ray system.

Al's next target for the Hasselblad is Tsiolkovsky; first pointed out by the crew soon after they settled into lunar orbit at 083:16:41 GET. First, he gets the Mapping and Panoramic Cameras going, then takes AS15-97-13157 to 13168, 12 frames taken, out of the planned 28, showing the crater and its northwest hinterland. The first three in this series AS15-97-13157 to 13159, combined here into a composite image, show the steep slopes of the crater's eastern rim with the slumped terrain below.

Al then concentrates on the crater's distinctive dark floor.

The final eight frames of the series start where the dark interior of the crater meets its western rim. Each succeeding shot moves around to the northwest rim and out to the landscape beyond.

Of particular note in the last four images is the radial "sculpture"; evidence of the outrushing sheet of ejecta from the impact which carved long furrows in the ground. These are smaller scale examples of similar landforms seen around the Imbrium basin on the front side, particularly around Mare Vaporum, which are traces of the tremendous effect the formation of the Imbrium Basin had on the Moon's surface. The correct interpretation of the origin of sculpture would prove to be crucial to understanding the history of the Moon's features. For vertical context, this is a more recent image from the Lunar Reconnaissance Orbiter.

Frames AS15-97-13169 to 13172 continue the coverage of Al's photography beyond Tsiolkovsky's sculpture to the 50-km crater, Lütke.

This crater exhibits an unusual ridge- or rille-like feature on its floor.

Al takes another shot back towards Tsiolkovsky's northwestern sculpture.

As Al returns to the near side of the Moon, he takes a photograph looking obliquely towards the central peak of Neper.

This 137-km dark-floored crater was apparently named after John Napier, 1550-1617, a Scottish mathematician who invented logarithms. The Sun is almost overhead in this picture and the changes in albedo (reflectance) within crater rims and around the base of the Neper's central peak are particularly distinct.

This journal continues as Endeavour comes around from the far side of the Moon in the early minutes of its fifteenth lunar orbit. During the far-side pass, Mission Control has had a change of shift and in the process, has effectively split into two. With separate CapComs and flight control teams, Houston is now running two missions, each with heavy and demanding workloads. Whereas previously, Ed Mitchell was the CapCom for both spacecraft, now Joe Allen is working to the surface crew while Gordon Fullerton deals with Al's mission in orbit. Milt Windler replaces Glynn Lunney at the Flight Director console.

The Panoramic Camera can be switched to a mode for taking stereo photographs suitable for making topographic maps of the surface.

The Optical Bar Panoramic Camera, a modified version of the U.S. Air Force's KA-80A 'spysat' camera, uses a 610-mm f/3.5 lens which rotates about an axis parallel to the spacecraft's longitudinal or X-axis to obtain panoramic coverage. Exposure is by a slit and motion of the film to achieve long strips of coverage, 330 by 21 km, the long measure of which is perpendicular to the orbital track, on a frame 11.4 by 114.8 cm. Each exposure begins 54° to one side, sweeping across the ground track for a total scan of 108° with exposure duration being set by the width of the slit and the rotation rate of the roller cage carrying the film. During the 2 seconds it takes to expose a complete frame, 1.2 metres of film will be pulled over the roller cage and past the exposing slit. Consecutive images are made 6 seconds apart and if stereo coverage is required, the lens assembly can be swung fore and aft by 12.5° so that a stereo pair are taken with every fifth exposure.

Compensation for the motion of the spacecraft is provided by the V over H sensor which detects the rate at which surface detail moves across the camera's field of view, generating a signal that controls the compensating movement of the film. This allows the camera to achieve a resolution of better than 2 metres from the nominal orbital height of 110 km. On this, its first flight to the Moon, this sensor will not operate properly, degrading the quality of the imaging by introducing a small degree of smearing. The camera is used during 11 orbits and 1,529 usable photographs are obtained on the 2-km roll of Kodak EK-3414 film contained in a cassette of 25-kilograms mass which Al will retrieve later in the mission. Journal Contributor Brian Lawrence has more to add on the camera's pedigree.

From Brian Lawrence: "The United States' first photo-reconnaissance program was authorised by President Eisenhower in February 1958. Known by the secret code name CORONA, some 140 satellites were launched between 1959 and 1972. The USAF was responsible for development of the launch vehicle [Lockheed Thor-Agena], spacecraft, and control and recovery, while the CIA took responsibility for development of the camera and satellite recovery vehicle. The camera was designed and built by the Itek company in Boston, although the first ten flight models were fabricated under a subcontract by the Fairchild Camera and Instrument Corp. The first camera was known as the CORONA KH-1, or simply as C. The camera then developed into the C' [C Prime], and eventually to the C''' [C Triple Prime] - the C'' was never built. The C' was installed in an updated spacecraft [KH-2 - KH is said to be short for 'Key-Hole']. The C''' was used in the KH-3 and then mounted in pairs in the KH-4. The same camera, again mounted in pairs [known as MURAL] was also used in the final two CORONA spacecraft versions [the KH-4A and KH-4B].

Brian Lawrence (continued): "The C Triple Prime was manufactured by Itek, and used a lens design called Petzval. It used three 7 inch elements at the front and two 5.5 inch elements at the rear, all encased in a 22 inch long cylindrical cell. A sixth element was mounted just 0.25 inches in front of the film. The completed lens unit weighed about 20 pounds. All of the glass was imported from the Schott company in Germany, since the US did not have a source of glass of the requisite quality - the Corning Glass Company was developed as a 'home grown' source of quality glass, but did not supply glass for the CORONA program.

Brian Lawrence (continued): "The first 39 launches [1959-62] were publicly known as the Discoverer program, but after Discoverer 39 in April 1962, the program was conducted in secret.

Brian Lawrence (continued): "In the mid-sixties Itek produced a version of the C Triple Prime for use in the U-2, A-12 and later in the SR-71/Blackbird aircraft recon flights. The CIA called this version the Optical Bar Camera [OBC], while the USAF named it the KA-80A. The SIM-bay Pan Camera used on Apollos 15, 16 and 17 was basically the same camera, although in fact only the lens system was unchanged."

Journal contributor Robin Wheeler has written a comprehensive illustrated essay on the cameras in the SIM bay.

On Al's first pass over Falcon, now sitting on the plain between Hadley Delta and Mount Hadley, he will attempt to pinpoint its position. Mission Control are going to read up timings to aid him in tracking the LM as he manoeuvres Endeavour in P24.

This PAD is to allow Al to get marks on Falcon with the spacecraft optics which will allow the guidance systems to determine the LM's position by empirical means. Al will also be able to describe verbally where the LM has come to rest. Explanation of the P24 PAD follows:

• T-1 (106:33:59 GET) is the time that the landmark is on the horizon and is 6 minutes, 30 seconds before closest approach.
• T-2 (106:38:06) is 4 minute, 7 seconds after T-1 and is when Al begins rotating the spacecraft at orbital rate to keep it pitched down 10° from the local horizon.
• TCA (106:40:29) is the time of closest approach.
• T-3 (106:40:57) is 6 minutes, 58 seconds after T-1 and 28 seconds after TCA. It marks the end of the landmark tracking mode.

Al will have about 2 minutes, 51 seconds to view the landing site. Omni antenna C will be best placed to provide communications to Earth at the attitude used for the tracking.

The Gain Step Shield on the Gamma-ray Spectrometer discriminates between true gamma-ray events within the instrument and events caused by the detection of cosmic-rays.

Long comm break.

Comm break.

Just before Al stops the Pan and Mapping Cameras, he loads the Digital Autopilot (DAP) with new settings in preparation for the landmark tracking exercise.

Nearly every page of the Flight Plan includes two sets of five digits each which define the current status of the DAP, a program in the computer that keeps the spacecraft attitude within set limits, firing the appropriate jets to achieve this. By executing Verb 48 and entering new values into register 1 and 2, the crewman can control how the DAP operates. For the last three hours the DAP status has been 11111 X1111. Al will change it to 11102 X1111. Here's an explanation of what each digit means.

Now we can interpret the new settings for the DAP (11102 X1111):

• 1: Vehicle configuration is the CSM only.
• 11: Quads A/C and B/D are used as coupled pairs.
• 0: The deadband will be ±0.5° for tight control during the P24 pass.
• 2: The relatively fast rotation rate of 0.5° per second will be used for any corrective manoeuvres. Again, tight, fast control of the spacecraft's attitude is required in this tracking situation.
• X: The Flight Plan does not define which pair of quads are used to control roll. Therefore the letter "X" is used and either a one or a zero can be entered.
• 1111: These final four 'one' digits simply tell the DAP that all the quads are available for its use.

These DAP settings won't change until about 109:30.

Worden, from 1971 Technical debrief: "My hat is off to the people who designed the DAP because that just worked so smoothly it was almost unreal. It was so smooth all the way around you never noticed the thruster firings. We stayed right in the orb-rate attitude all the time."

Scott, from 1971 Technical debrief: "The confidence factor in the RCS goes up by orders of magnitude every day to the point where I think some of the training we do on RCS failures might be superfluous, because everybody powers down and goes to sleep. I guess my confidence factor on those jets is 100 per cent."

Dave's comment here is greater praise for the fact that he and Neil Armstrong were almost casualties aboard Gemini VIII in March 1966 when a thruster failed "on", putting their spacecraft into a violent roll. He has every reason not to trust RCS thrusters.

Scott, from 1971 Technical debrief: "I don't think we ever worry about a jet failing on or failing off because you could hardly do the mission and have to worry about that. We were running through the orb-rate during the sleep, and they [the thrusters] weren't bothering anybody."

Worden, from 1971 Technical debrief: "That's right. As a matter of fact, during the sleep periods, I don't recall hearing a thruster fire or any maneuvers at all."

Scott, from 1971 Technical debrief: "I don't either."

Worden, from 1971 Technical debrief: "It was just as quiet as it could be the whole time."

Comm break.

The Mapping and Pan Cameras have been placed in Standby, then with Mission Control confirming that the Pan Camera's lens has returned to its parked position, both cameras are switched off.

Long comm break.

With the Mapping Camera powered down, Al retracts it.

The Mapping Camera actually consists of two cameras, the Metric Camera and the Stellar Camera, with the Laser Altimeter also occupying the same housing. The Metric Camera's 76-mm lens has a field of view of 74° so that from the nominal orbital height of 110 km with the camera looking straight down, each square frame of film will cover 165 km on a side. The camera's resolution from this height is of the order of 20 metres.

Three devices aid the use of the resultant photographs when it comes to mapping the Moon. A Reseau plate imprints a grid of crosses similar to the crosses seen in the photos taken on the surface. Calibration of the lens before the flight then allows analysts to compensate for changes in the film's dimensions after exposure and through the processing lab and storage. Altitude information, accurate to within one metre, is supplied by the Laser Altimeter and coded onto the film while the precise pointing direction of the Metric Camera during exposure can be determined by a simultaneous exposure onto 35-mm film by the Stellar Camera of whatever stars are incident to its 85-mm lens. The Stellar Camera takes an image off to the side at 96° to the Metric Camera's axis and to accommodate this, the entire system is moved out from the body of the spacecraft along a track.

The Stellar Camera is also used during times when the Laser Altimeter is taking altitude measurements over the night-time side of the Moon, if there is sufficient film available. It helps determine the exact direction the laser is pointing so that the altimeter's results can be correlated with maps of the lunar surface. The camera's pedigree is related by journal contributor Brian Lawrence.

From Brian Lawrence: "In the early sixties a new program codenamed ARGON was developed to take geodetic measurements to provide precise locations for targets identified by the CORONA system. Fairchild produced the cameras for this KH-5 satellite. In 1965 Fairchild were asked to produce a new version of the ARGON camera which would be mounted on the CORONA spacecraft. The system was called the Dual Image Stellar Index Camera [DISIC], and included a terrain camera with a focal length of 3 inches using an f4.5 Ikogon lens. Stellar photography was provided by two 3-inch focal length f2.8 cameras mounted to port and starboard of the terrain camera. This system flew on the 17 CORONA KH-4B satellites. The SIM-bay Metric Camera System was an adaptation of the DISIC design which included a single stellar camera and added a ruby laser altimeter. The CORONA systems were not declassified until 1995 at which time their use in Apollo became public knowledge."

Apollo 15's Mapping Camera will yield 2,240 usable photographs out of 3,376 taken during 18 orbits of operation and the early hours of the return home. Every image is available at the Apollo Image Archive at Arizona State University. Each frame, 114 mm on a side, has been scanned at about 140 pixels per millimetre yielding images in excess of 16,000 pixels to a side.

As Al manoeuvres to the starting attitude for the landmark tracking exercise, Mission Control are waiting for him to select omni-directional antenna C. This is the best placed of the four to give clear communications.

Long comm break.

Comm break.

Al is executing the P24 landmark tracking manoeuvre with the hope of seeing Falcon on the surface. Knowing the exact position of the LM helps mission planners plan the traverses and interpret the photography returned by the crew. Al is looking through the 28-power sextant, which is also used for sighting stars when aligning the platform.

Two switches on panel 3 of the Main Display Console (the right most panel) allow Al to cycle through all four of the CM's omni-directional antennas. These flush mounted antennas are distributed around the periphery of the CM. Despite their name, the four omnidirectional antennas mounted around the periphery of the Command Module do not radiate equally in all directions, as the spacecraft structure interferes with the line of sight in most directions. The attitude necessary for the landmark tracking is blocking a clear path of the Omni Charlie antenna to Earth.

As the attitude information and other data is important for this manoeuvre, Gordon Fullerton has asked Al to reduce the data rate transmitted by the antenna, which in turns reduces the need for a strong signal, and to start the Data Storage Equipment recorders to ensure that data is not lost.

Al begins pitching Endeavour to keep Hadley Base visible through the sextant.

Al is making his closest approach to the landing site and is taking marks on Falcon so the LM's position can be computed.

These two images, taken about 4 decades after the event, are included to show the LM's final position. They were taken by the Lunar Reconnaissance Orbiter after it reached lunar orbit in 2009 and they show the Apollo 15 landing site with the remaining descent stage and the soil lightening caused by engine exhaust disturbance. Nearby craters Index and Luke are indicated on the closer version.

Comm break.

Long comm break.

Worden, from 1971 Technical debrief: "As I came over the landing site, I saw the LM shadow very clearly, and once I had identified the shadow, then I could also see the LM in the sextant. I watched the LM until I was near nadir, until I was almost to TCA, and then I took out the visual map, the 1 to 25,000 scale, in the CSM Lunar Landmark Map Book, and marked the spot where I saw the LM. That was BR.5 and 75.5, in the Lunar Landmark Book. One more comment on the LM acquisition, and, again, it's a comment that's been made before on landmark tracking. Once the LM was spotted, there was no problem at all tracking with the optics. Of course, at this time, I was in a 60-mile circular in orb-rate, and the [angular] rates were very low. But even at the low altitudes, there was no trouble tracking any landmark that you selected with the optics, in either orb-rate or inertial hold [keeping the spacecraft steady with respect to the stars]. The optics were very smooth in tracking and very positive. As long as the trunnion angle is great enough so that you don't go or close to zero trunnion angle, any landmark you pick is fairly easy to track."

The 28-power sextant has two degrees of freedom by virtue of an articulated optical path. An exterior view of the Apollo 11 Command Module Columbia shows the orifices in the spacecraft's wall for the sextant and scanning telescope. The sextant looks through the slit in the disc that is nearly flush with the surface of the CM. Rotation of this disc represents the shaft angle. The slit across this disc accommodates the objective lens which can scan across the slit up to 57° from the shaft axis. The angle of the lens within this slit is called the trunnion angle with the position that is coincident with the shaft axis being the zero angle.

Tracking an object; a star, planet or landmark; is achieved with a combination of motion in the shaft and trunnion axes, providing the attitude of the spacecraft has brought the object into the range of the optics. However, if the tracking of an object brings the trunnion near the middle of the slit, the zero calibration of its travel, then there has to be a relatively large rotation of the shaft to keep alignment and if it gets too close to zero, the shaft cannot be rotated fast enough to maintain a fix on the object.

As seen in the picture of Columbia, the sextant, through which Al was looking at the LM, is mounted on the side of the Command Module opposite the main hatch. Therefore, the windows and, more importantly, the SIM bay are all pointing essentially out into deep space. With the upcoming operation of the SIM bay instruments in mind, Al reorients Endeavour with the bay facing the Moon and the spacecraft pointing forward in the direction of motion. To keep the SIM bay properly pointed, the spacecraft must be rotated in orb-rate. P20 is the program in the computer which takes care of this.

With the A, B or C omni-directional antenna selected, Mission Control have a degree of control over which is in use as they can remotely select Omni D or reset to the crew defined condition. As Omni B is opposite D, the ground controllers have maximum control when it is selected. Since this facility is normally only used during the translunar coast, Mission Control may have requested it simply because it is the most suitably placed antenna.

Comm break.

With the spacecraft correctly oriented, Al opens covers for the Mapping Camera and Laser Altimeter, and for the Alpha Particle and X-ray Spectrometers. The Mapping Camera is extended out on its track to allow the Stellar Camera a clear view to the side. The boom carrying the Gamma-ray Spectrometer is also deployed. Al will time these deployments using the talkback indicators on the SIM bay's control panel to tell him when they have reached the end of their movement.

Comm break.

Al now switches on the X-ray Spectrometer and the Laser Altimeter experiments.

When solar X-rays strike certain elements, particularly those up to atomic number 14 (silicon) they cause the atoms to fluoresce X-rays with a well defined energy. By monitoring the X-ray flux from the ground, measuring its spectral characteristics and comparing it to the direct solar flux, it is possible to determine the composition of the upper layers of the lunar surface, particularly with respect to those elements which form the bulk of planetary mass (Oxygen, Silicon, Aluminium, Magnesium and Iron).

Two X-ray detectors form the basis of the X-ray Spectrometer. The main unit, mounted in Endeavour's SIM bay, consists of three counters, two of which have filters to help determine the energy of the X-rays coming from the Moon. A square-celled baffle restricts the field-of-view of the instrument to approximately 60° and calibration sources can be commanded to face into the counters when desired.

A second detector is mounted on the opposite side of the Service Module from the SIM bay to measure the intensity and spectral shape of the incident solar radiation.

Comm break.

The Mapping Camera is scheduled to be operated for one complete orbit of the Moon, including the pass over the night-time side which Al has just commenced. This will allow the Laser Altimeter and Stellar Camera combination within the assembly to gather data around the entire globe. Because of later problems with the Laser Altimeter, this dataset will prove to be the only one which gives experimenters coverage around the Moon's girth.

Comm break.

SEVA is the Stand-up Extra Vehicular Activity which, at Dave Scott's suggestion, was added to the surface Flight Plan to allow him, as a newly-trained geologist, to survey the pristine site around the vehicle and describe his first impressions of what he sees in geological terms. While he and Jim remain connected to the LM's life support systems, Dave stands on the ascent engine cover with the top half of his suited body out of the overhead, or docking hatch for half an hour. In the Apollo 15 Lunar Surface Journal, Dave comments that the SEVA was an excellent compromise between relieving the strong desire to get out and the recognition that they needed a good night's sleep before the first EVA.

Long comm break.

Al is beginning a 30 minute period of exercise.

In the time leading up to Apollo 15, the duration of space flights had barely exceeded two weeks, yet people were becoming aware that the weightless environment affected the human body in different ways. Initial exposure to weightlessness causes about half of astronauts to experience some nausea and in some instances, vomiting. This is only a short term effect. Longer periods without the pull of gravity cause the muscles and bones to atrophy, or waste away, especially in the legs which are no longer needed to support the weight of the body. This, and weakening of the heart due to the lack of work in the leg muscles led to the adoption of regular exercising as a way to ameliorate these effects.

Comm break.

Long comm break.

Long comm break.

Very long comm break.

Rev 16 begins at about 107:40.

On this far-side pass, Endeavour will commence its sixteenth orbit. After his exercise period finishes at 107:35:00, just before Endeavour re-enters lunar day, Al will operate the Mapping Camera image motion compensation system and retract the Gamma-ray boom. He will report the time taken for the retract mechanism to operate after AOS. As he passes over the daylit portion of the far side, Al is scheduled to photograph a pair of craters using the telephoto lens (250-mm) on the Hasselblad and shooting on mag S. He does not restrict his photography to the planned features.

AS15-94-12737, 12738 and 12739 are marked in the index as being of the north west rim of Gagarin, also photographed by Al during his last pass. These three images are rather dark and show three small craters.

Later in the pass, Al coasts over Tsiolkovsky. AS15-94-12740, 12741 and 12742 are of an area on the north east ejecta blanket of the great crater. The images concentrate on a raised, lobate formation which appears to be a landslip or a lava flow inside an old, eroded crater, emanating from a line on one side of the crater.

One of the photographs taken during rev 23 by the Mapping Camera is AS15-M-0757. It is a spectacular oblique shot of the entire Tsiolkovsky crater and its ejecta blanket with north to the top.

This image gives a much better impression of the flow's vertical relief. The structure can be seen just outside Tsiolkovsky's northeast rim. Al discussed this feature during the post-flight debriefings with Farouk El-Baz, a geologist from Bellcomm Inc. who was intimately involved with Al's training.

Worden, from 1971 Visual Observations Debrief: "I could see down into some of the craters. The lava was piled up on one side, and a cross-section of the area where the lava had been piled up along the rim of the crater would [show] this. When the Sun angle was low, I could see a shadow all the way around. The flow appeared to have flowed down into the crater, up the far side, and then curled back on itself. There was a great big lobe of lava at the end."

El-Baz, from 1971 Visual Observations Debrief: "You could actually trace one of these flows through the fault."

Worden, from 1971 Visual Observations Debrief: "Yes, I could."

AS15-M-0757 also gives the context of the next two Hasselblad images, 12743 and 12744, which show the double terrace of Tsiolkovsky's northeastern rim and the westernmost end of the central peak respectively.

Directly north of Tsiolkovsky, and just south of the crater Perepelkin, are two adjoining craters of about 60 and 40 km diameter. This pair are Al's intended target from the Flight Plan and he devotes seven frames, AS15-94-12745 to 12751, to them. The larger of these two, Shirakatsi, has a subdued central peak while the smaller, Dobrovolsky is of a simple, though eroded, bowl type. Al will discuss this pair in two days time, at 143:42:51. What is particularly noticeable is the how the saddle between the two craters has slumped into the smaller, and apparently older of the two craters.

Anania Shirakatsi was a 7th century polymath and natural philosopher from Armenia. Georgy Dobrovolsky (1928-1971) commanded the Soyuz 11 mission that ended in tragedy a month before the Apollo 15 mission.

Before continuing his photography over the near side, Al replaces the 250-mm telephoto with the 80-mm lens.

Fullerton is referring to problems with the Panoramic Camera's motion compensation system which are causing will last throughout the mission. However, the camera continues to operate, albeit at a degraded level. The quantity of good pictures returned is apparently sufficient so that special procedures are unnecessary. Al Worden will not get an update on the details of the problem until 122:11:07.

This comment is not a flip as it may first appear. Apollo 15's orbit takes it further north and south than any other flight in the Apollo program, and there is some uncertainty over the cumulative effects of the Moon's irregular gravity field. A comprehensive gravity map is not available to the Flight Dynamics Officers (FIDOs). Indeed, it was only created in the late 1990's by the Lunar Prospector spacecraft.

As well as the S-band communication system, operating at about 2,200 MHz, the Apollo spacecraft can also use VHF, intended for short distance communication, at frequencies of around 275 MHz. The SM has two semi-circular antennae (also known as scimitar antennae) mounted on either side of the module's body. One of these will be used for communicating with the crew on the lunar surface via one of Falcon's two VHF in-flight antennae.

In the Command Module, there are panels adjacent to either side of the Main Display Console. The upper panels on each side, panels 6 and 9, and a central panel, 10, just in front of the console, have most of the controls for audio communications with a few others on the console itself.

The A channel for VHF communication will only allow one person to speak at a time and it will use the right hand antenna.

Panel 9 is to the left of the Main Display Console and would normally be used by the Commander.

Just as he reaches closest approach to Falcon, Al is scheduled to take 5 photos of the Montes Caucasus, a range of mountains running north from the point where the lavas of Mare Serenitatis and Mare Imbrium meet. This range is a continuation of the Apennines, beneath which Dave and Jim are having their evening meal. Like the Apennines, they were created near-instantaneously by the upthrusting of crustal blocks when the Imbrium basin was formed and even now, nearly 4 billion years later, they rise up to 6,000 metres above the mare surface below. In the event, the photo sequence will not be taken as Al will be occupied with the comm check.

Worden, from 1971 Technical debrief: "The orbital science photography, for the most part, went as per the Flight Plan. There were a few instances where some other activities were scheduled real time which interfered with orbital photography, and in those places, the photography was not accomplished. In the Flight Plan, the orbital photography was almost invariably strip photography, with the camera being held in the window and pictures taken at some prescribed interval, such as 15 seconds or 20 seconds. Those at 20 seconds were done with the intervalometer, and those at the other times were done just by counting on the clock.

Worden (continued): "In almost every case of orbital photography, the ground site had been analyzed preflight, so that I knew what the targets were. In flight, rather than just take pictures looking straight out the window, I concentrated on taking pictures of the sites that we are interested in. That worked in almost every case. There were several strips of photography taken from Crisium to Serenitatis. I think there were five strips scheduled to cover some of the Lunar Orbiter photos that were of very poor quality. We got all of those, except one strip, which was replaced real time by some other activity. I don't see it in the flight right now, but, as I recall, there was one strip that we didn't get. The rest of the orbital photography all went pretty much as planned."

Comm break.

Comm break.

This is the retraction Al carried out while over the far side. The Flight Plan quoted an expected time of about 3 minutes, 15 seconds for retraction of the Gamma-ray boom.

Long comm break.

CapCom Gordon Fullerton is acting as a prompt for Al.

Long comm break.

Al is scheduled to take a sequence of 19 photographs in the northwest region of Mare Crisium. In all, he takes 25 images, including a couple taken after his next comment. First is a strip of images, AS15-94-12752 to 12762, of the landscape from the northwest rim of Crisium across the crater, Macrobius. The Sun's elevation is very high and shadows are non existent. In the first three of the sequence, the light, mountainous rim of the Crisium basin stands out against the much darker basalt of Mare Crisium itself. Some of Peirce is visible at the bottom-left of frame 12752, along with the smaller, 11-km crater Swift, named after Lewis Swift, 1820-1913, an American astronomer.

The rays of Proclus, just to the west of Mare Crisium, can be seen gently streaking the mare surface.

By 12757, the 64-km crater, Macrobius, dominates the left of the frame with Lacus Bonitatis (Lake of Good), a dark area northwest of Macrobius showing up beyond. The high lighting renders the 37-km crater, Tisserand, almost invisible. Though it is over half the diameter of Macrobius and is well placed in the foreground of 12757, almost the only clue to its presence is a darkening of its floor.

Lacus Bonitatis stretches across the width of frame 12760, which also includes the dark irregular crater Macrobius W to the right.

Frame 12762 looks vertically down upon Lacus Bonitatis and at the top of frame is a curious keyhole-shaped which was formerly called Macrobius L but is now known as Esclangon after a French astronomer, Ernest B. Esclangon, 1876-1954.

Al continues his photography with a sequence of nine images, AS15-94-12763 to 12771. The first part of the series is rather washed out because Al is aiming the camera into the zero-phase illumination.

When the 40-km Römer comes into view, Al holds the crater centred in the camera's view while the spacecraft's motion moves the viewpoint, giving him a series of stereo photographs. The best from this sequence are 12769 and 12770. Römer A can be seen as a dark-floored crater to the right of frame.

The final image of the sequence allows Römer and its little internal crater to move out of frame.

During this time the Panoramic and Mapping Cameras have been busy. A frame from each at around this point is presented here.

Note the long, narrow aspect of these frames. On closer inspection, the foreshortening at either end caused by the camera looking sideways at the extremes of its sweep is apparent.

This Mapping Camera image shows the region that Al is about to discuss with Mission Control.

The 'King' Worden is referring to is Farouk El-Baz. His involvement with the crew's training is depicted in Tom Hanks' remarkable series 'From Earth to the Moon'. The episode dealing with the training and flight of Apollo 15, 'Mr Galileo Was Right', shows El-Baz taking Al for aircraft rides over various geological structures, particularly volcanics, while teaching him how to describe what he sees in terms which would convey information efficiently to a geologist.

Worden, from 1971 Technical debrief: "I thought that the training that I received on orbital geology was better than I had anticipated. I was very well prepared when we got there. The only comment I'd have is that most of that detailed training we had came very late in the game. It had to be sandwiched in with other things at the Cape and some meetings through the isolation booths on the final stages of the training. It would be helpful if we got into the detailed part of that a little bit earlier in the training cycle."

As he approaches the landing site, Al continues his photographic coverage as he comes across the eastern margin of Mare Serenitatis.

In the foreground of AS15-94-12772 is le Monnier, 61 km in diameter and clearly flooded by the lavas from Mare Serenitatis to form a bay. Beyond, the light coloured rim of Posidonius and its internal crater is also visible.

As the camera clears le Monnier, 12774 shows the boundary between the dark material which borders the mare, and the lighter material which dominates the central bulk of Serenitatis and which can be seen towards the upper left of the image.

During the run-up to Apollo, lunar mapping exercises and their associated geologic interpretations - the practice of photo-geology - led to the belief that the dark material seen around the shore of Mare Serenitatis was younger than the centre of the mare. Further dark deposits, so-called "dark mantling", in the mountains of the basin's rim were interpreted as being from fire fountains, gas-rich magma vents under tremendous pressure which sprayed molten rock many kilometres above the Moon where the countless tiny droplets were shock-cooled in the vacuum to form little spheres called "beads".

This thinking informed the theories about the mare's dark border which was also thought to be due to these deposits. Also, it was believed that the older surface would be brighter from the countless little craters on its surface sustained from a longer exposure to the micrometeoroid rain. As would be expected, Apollo changed these views.

By counting the numbers and distributions of craters across a mare surface, some understanding of the relative ages of lava flows that comprise the mare could be gleaned. Crater counts across Mare Serenitatis suggested that actually the lighter surface covering the centre of the mare was younger than the outer, dark regions. Additionally, "ground truth" from Apollo 17 proved that the dark mantling outside the mare, while indeed coming from fire fountains, was actually just as old as the mare lavas, and was essentially a phenomena that accompanied major eruptions. As the gassy lava was vented in a fire fountain, the rest oozed out to form the flows. Dates obtained from Apollo 17 rocks date a basalt sample to 3.72 billion years ago while another dates the pyroclastic deposits from the fire fountain to 3.64 billion years.

Our current understanding of the two tones of Mare Serenitatis is centered around the different chemistry of the lavas emanating from the various vents which fed the mare at different times to form the mare basalts. The dark basalts around the margins of the mare have a higher concentration of titanium. Increased proportions of aluminium give a lighter tone to the younger basalts near the center. Therefore, in Serenitatis, we see two, maybe three distinct episodes of filling of an impact basin. The early covering of the basin floor by dark, high-titanium lava was followed by the centre of the mare sinking under the weight of the resulting basalt. Meanwhile, around the margins of the basin, fire fountains had sprayed the surrounding terra with titanium-rich deposits. Many millions, probably hundreds of millions years later, aluminium-rich lavas filled the central depression, leaving the regions at the shore exposed.

As Endeavour crosses the expanse of Mare Serenitatis and approaches the terminator, the lengthening of shadows becomes apparent. 12775 shows the scattering of craters of relatively small proportions which is typical of mare surfaces and records the cratering which has occurred since the basalt was laid down over three billion years ago. The largest crater in this image is Banting, 5 km in diameter and formerly known as Linné E. It is now named after Sir Frederick G. Banting, 1891-1941, a physician from Canada. It is typical of the small, raised-rim craters seen on the Moon and is notable as having been once named as a subsidiary of a crater less than half its diameter, the tiny Linné. This latter feature, a very bright, young, ray crater is only 2.4 km across and lies about 120 km west of Banting. The reason it is dominant in the nomenclature is the brightness and visibility of its ray system. The crater itself is a stringent test of visual acuity for Earthbound observers.

The Mapping Camera is continuing its photography during this orbit, including AS15-M-0408 which shows Linné and the other nearby craters in the region.

The Panoramic Camera also images Linné, taking a stereo pair, AS15-P-9348 and 9353. Linné is the striking bright crater near the top, wrinkle ridge Dorsum von Cotta meanders into the right centre, the irregular depression Krishna encroaches on the left two-thirds way down, and Dorsum Buckland passes across the bottom.

Some of the following transcript is taken from the Apollo Lunar Surface Journal which follows the surface activities. Joe Allen is the CapCom for the LM crew.

Index was the target for the Lunar Module in that determinations of its position relative to the spacecraft's orbit were used as part of the guidance for Falcon prior to the descent. It sits between the North Complex and South Cluster at Hadley. Leading up to it in a row, and providing a visual cue for Dave in piloting the LM, were three other craters, Matthew, Mark and Luke. In the Apollo Lunar Surface Journal, Eric Jones spoke to Dave Scott and Jim Irwin about where these names came from.

Jones, from the Apollo 15 Lunar Surface Journal: "[To Irwin.] Did you and Dave do much of the crater naming?"

Irwin, from the Apollo 15 Lunar Surface Journal: "Joe Allen did a great deal of it. We did a few of the craters right around our site. There was a Scott. There was an Irwin. There was a Matthew, Mark, and Luke. There was Index. But we thought that was a good thing for the Mission Scientist to do. And Joe's a very resourceful Ph.D. and he did a good job on that."

Scott, from the Apollo 15 Lunar Surface Journal: "One of the problems we had on the descent ... once we pitched over and looked out, we had four craters that we lined up on the model and the maps for our landing chute: Matthew, Mark, Luke, and Index. Now, we couldn't use 'John' because of Madalyn O'Hair. That's the truth. And, boy, those were great craters and they were lined up and we spent hours training on those craters. We called the last one Index, because Index was where we were supposed to land. And I'm sure that - because the available Lunar Orbiter photos had such poor resolution - they enhanced out maps and photos with features that probably either were different or were not there. And the problem was, when we pitched over and looked out the window, there was nothing there! I mean, Matthew, Mark, Luke and Index were there, but very subtle."

Eric adds in the ALSJ; "Madalyn Murray O'Hair was a well-publicized atheist who, among other things, sued NASA after the Apollo 8 crew read from Genesis during a Christmas Eve broadcast from lunar orbit in 1968. That suit was eventually dismissed."

As this is VHF communication, Al will switch from the Right to the Left antenna on the Service Module.

Dave was surprised at how difficult it was to see the landmarks he expected during the descent. He had expected the crater shadows to be better defined and wants to know if Al's view from orbit in anyway corroborates what he saw.

As Endeavour passes across Hadley, the SIM bay cameras continue to take images. A stereo pair from the Panoramic Camera are shown here from this sequence. On the left of each of these images are the peaks of the Apennine Front, the landing site is in the centre and the grooves of Rimae Fresnel are on the right.

A detail from panoramic frame 9370 shows the landing site at this time.

The Mapping Camera shows the current wide-angle view:

To the west is Palus Putredinus, the little bit of Mare Imbrium they have landed in. The LM is sited near the centre of the image between Hadley Rille and Mount Hadley. Note that the face of Mount Hadley towards the Rille is still in shadow. When Dave and Jim see it tomorrow, it will be lit by a very low-angled sunlight causing the crew to believe they see lineations, initially interpreted as layering, across its face.

Although almost no substantive information passed between the two spacecraft during the above exchange, the comm check does allow both crews to know that everything is operating as it should.

Scott, from the Apollo 15 Lunar Surface Journal: "This [comm sequence] is interesting from two standpoints. One is the complexity of trying to run two spacecraft in parallel on separate loops, but using the same earthbound antenna. So the same DSN [Deep Space Network] antenna is being used for the S-band - I'm assuming that - and they're separating it out. And now they're bringing it together. And there's one times two times three - six - separate combinations of things you can do, which is even multiplied further by two CapComs. You've got two spacecraft, two CapComs... it works out pretty well but, in the beginning, there is a little bit of confusion here. It finally got worked out. The other point is, being on the surface, we never really know where Worden is. Because we don't try to keep track of his coming over the hill - AOS/LOS - so, as far we're concerned, he's sort of up there all the time. I think it's an interesting perspective. It doesn't affect what we do; but, on the other hand, it's nice to know he's there and, when he comes over and we get to talk to him - as I think you can probably tell from the voices - it's nice that we do get to talk to each other. Because we haven't since we separated. And, you've got to remember it's home that he's taking care of. So it's nice to be able to confirm that linkage. Not only comm but, Al's up there, we're down here, everything's going good and that's a nice way to end the day."

Jones, from the Apollo 15 Lunar Surface Journal: "No substantive exchange of information. Just contact."

Scott, from the Apollo 15 Lunar Surface Journal: "Just contact. 'Hey, you're doing good, we're doing good', and it's comforting to know that everything's working because Al's got a tough job. He's got this whole big SIM bay and all these new procedures and all that sort of stuff. Another big step for the guy in the Command Module, because he's going three days alone. Which is longer than a day and a half alone (as was the case for the Command Module Pilots on the previous mission), and he's doing all sorts of new things that, themselves, take a lot of training. It's another step in the expansion of the system - which makes it a more interesting and productive mission, but is also more complex and takes more people to run it."

The Flight Plan requires that the Panoramic Camera be switched out of its mode for taking stereo pictures as Endeavour approaches the terminator.

Linking up what are essentially two separate missions could be a complex affair with many permutations of communication. Two frequencies are used for VHF communication between the spacecraft and each spacecraft has two S-band frequencies for communication with Earth. Some coordination is required to ensure the spacecraft and the correct consoles in Mission Control are listening to the correct loops.

Al does catch one more photo before Endeavour reaches the terminator. Looking forward towards the line between night and day, AS15-94-12776 shows the rims of four craters rising out of the basalt of Mare Imbrium.

At the right of the foreground is Bancroft. This 13.1-km crater was formerly known as Archimedes A but is now named after a chemist, W. D. Bancroft, 1867-1953. In the center of the image is Beer and Feuilée, a pair of craters, 10.2 and 9.5 km in diameter respectively. Crater Beer gets its name from one of the 19th century's best mappers of the Moon, Wilhelm Beer, 1797-1850. Beer and his collaborator, Johann Mädler produced a monogram, Der Mond, in 1837 which included a highly detailed lunar map at the limits of telescopic observation. The other crater of this pair is named after a Frenchman, Louis Feuilée, 1660-1732, who was a naturalist and Director of Marseilles Observatory. The raised rim of Timocharis, one of the major craters of the mare, is just poking up into the sunlight.

The same view is captured soon after by the Mapping Camera across two images, AS15-M-0422 and AS15-M-0424, as the spacecraft crossed the terminator.

These images show how low sunlight profoundly accentuates the topography of the landscape. Compressional wrinkle ridges dominate the centre and a series of grabens are to the right. Note the string of craters next to Beer. These are probably from a comet or asteroid that was loosely assembled and which was separated by the gravity of Earth or Moon before crashing sequentially into the lunar surface.

Al is preparing Endeavour for data gathering over the coming evening and night. For the next three hours, once he gets both experiment booms deployed, heaters will be energised within the Mass Spectrometer to promote the process of outgassing. While the Mass Spectrometer loses contaminant molecules and Al has his evening meal, Endeavour's radio signals will be used in a radar experiment.

Long comm break.

During this comm break, four items from the presleep checklist are scheduled, the rest being left until Al is ready to go to sleep. Al spreads these tasks out over the period until LOS on this pass, not necessarily doing them exactly when the Flight Plan dictates. They are: a report on crew status regarding medication, a read-out of the onboard gauges for O₂, H₂ and RCS propellants, cycling of the H₂ cryo fans, and a dump to Earth of the contents of the erasable electronic memory for analysis. These items all require the availability of the radio link to Earth. However, throughout the next near-side pass, the Bistatic Radar Test will occupy the VHF and S-band transmitters and they will not be able to carry information while the experiment is progressing.

Long comm break.

As happens throughout the period that Apollo 15 is operating near the Moon, Al has been given the information necessary to get the CSM home in case of an emergency, not that he'd head home without Dave and Jim. The surface crew would have to lift-off and rendezvous first. Al always has an up-to-date "get home" PAD available to him and a new one is sent before the old one runs out in case the voice link with Earth should fail.

An interpretation of the PAD follows:

• Purpose: The purpose of the PAD is a return to Earth at the end of Rev-26 in case of an emergency.
• System: The burn, if required, would utilise the SPS engine under control of the Guidance & Navigation system.
• CSM Weight (Noun 47): 37,354 pounds (16,943 kg).
• Pitch and yaw trim (Noun 48): +0.6° and +0.97°.
• Time of ignition, T_IG (Noun 33): 129 hours, 22 minutes, 25.54 seconds.
• Change in velocity (Noun 81), fps (m/s): X, +3,031.2 (+923.9); Y, -1,327.1 (-404.5); Z, -308.7 (-94.1). These velocity figures are expressed with respect to the local vertical frame of reference. The burn is mainly prograde and the large Y component shows a significant change of orbital inclination.
• Spacecraft attitude: Roll, 180°; Pitch, 092°; Yaw, 339°.

There are two additional notes to this PAD. The SPS propellants would be settled in their tanks by firing the plus-X thrusters on all four quads around the Service Module for 12 seconds. The spacecraft's longitude over the Moon at T_IG would be 172.93°E.

The crew had a minor problem with the System Test Meter at 081:41:29. Mission Control are happy that it will operate fine, should Al need it.

Long comm break.

The health of the SIM bay's deployment systems is monitored by timing the duration of the deploy and retract operations. With Apollo 15 being the first flight with a SIM bay, there is much interest from the engineers in how it performs in space.

Al is performing a P52 platform realignment, the second since the landing. He uses option 3, the REFSMMAT option in which the platform is aligned to a defined orientation and in this case, the orientation of the landing site at the time of touchdown is used as the reference. Once he has made the two star sightings, Al will bring up three angles on the DSKY (Display and Keyboard) that the three gimbals are to be moved to re-establish alignment. While they are being displayed, Mission Control can also view them via telemetry.

Long comm break.

As listed in the Mission Report, the details of this realignment are as follows: Stars 04 (Achernar, Alpha Eridani) and 42 (Peacock, Alpha Pavonis) were used and Al achieved a perfect, 'all-balls' star angle difference. In other words, the known angle between the two stars and Al's measured angle were the same. The angles to which the guidance platform had to be rotated or 'torqued' were; X, +0.015°; Y, -0.004°; Z, -0.014°.

Al is closing down the X-ray Spectrometer and the Laser Altimeter. He also retracts the Mapping Camera, expecting to take about 4 minutes to do so, whereupon he will close its cover.

E-mod refers to the erasable memory dump to Earth, a task which is part of the presleep checklist.

Al is right about the PRD (Passive Radiation Dosimeter). At 094:39:53, yesterday morning, CapCom Bob Parker asked Dave to take Al's PRD to the surface as Mission Control suspected Dave's was faulty.

Comm break.

Al Worden is the first crewman to operate the SIM bay and has no one's previous experience to call on when deciding the most efficient way to operate it. During a busy schedule of photography on Apollo 14, when the CM cabin had to be darkened for the sake of a high resolution camera, Stu Roosa used tapes of instructions he had previously made up to cue him for various operations. Al's task is different because he has three days instead of two to fly the spacecraft solo, his high resolution cameras are outside, he has much more equipment to operate and he has lost the use of the mission timer on the Lower Equipment Bay ever since the circuit breaker supplying it popped on the way to the Moon.

Mission Control are awaiting the final item from the abbreviated presleep checklist: the read-out of the onboard gauges. If they don't get them within the next few minutes, the opportunity will be lost for this evening.

Very long comm break.

Each time Endeavour goes behind the Moon and Mission Control lose contact with it, the telemetry from its many sensors and the data from the SIM bay are recorded onto the Data Storage Equipment (DSE). During the subsequent near-side pass, the tape is replayed to Earth on a separate channel and rewound before LOS by remote control. Crew controls are available at the bottom of panel 3 on the Main Display Console.

Now nearing the end of its sixteenth revolution, Endeavour's Mass Spectrometer experiment is continuing its outgassing, allowing possible contaminant molecules to escape from the instrument. The boom, on which the experiment is mounted is presently extended to its full 7.3-metre length to take it away from gases expressed by the spacecraft.

The Gamma-ray and Alpha Particle Spectrometers are still operating, though the latter's covers are closed. As the spacecraft's attitude is about to be changed from the SIM bay attitude, data from both experiments is unreliable. During this far-side pass, at the start of orbit 17, Endeavour will be manoeuvred to the initial attitude for the bistatic radar test.

Rev 17 begins at about 109:38.

Since the landmark tracking exercise, some 2½ hours ago, the SIM bay has been kept pointed at the Moon using P20 orb rate tracking mode. In preparation for the Bistatic Radar Test, Al manoeuvres Endeavour to an initial attitude so that at 110:00 hours GET, a slow, automatically controlled pitch can be started. This is similar to the orb-rate manoeuvre in that it also uses P20. The difference is that the orb-rate manoeuvre uses option 5 of this program whereby the computer endeavours to keep one face of the spacecraft pointed at a celestial object, while the radar test will use option 2 and a specific rate of rotation will be entered in, in this case 0.083° per second.

From the 1971 Apollo 15 Press Kit: "The downlink Bistatic Radar Experiment [BRE] seeks to measure the electromagnetic properties of the lunar surface by monitoring that portion of the spacecraft telemetry and communications beacons which are reflected from the Moon. The CSM S-band telemetry beacon (f = 2.2875 gigahertz), the VHF voice communications link (f = 259.7 megahertz), and the spacecraft omni-directional and high gain antennas are used in the experiment. The spacecraft is oriented so that the radio beacon is incident on the lunar surface and is successively reoriented so that the angle at which the signal intersects the lunar surface is varied. The radio signal is reflected from the surface and is monitored on Earth. The strength of the reflected signal will vary as the angle at which it intersects the surface is varied. By measuring the reflected signal strength as a function of angle of incidence on the lunar surface, the electromagnetic properties of the surface can be determined. The angle at which the reflected signal strength is a minimum is known as the Brewster Angle and determines the dielectric constant. The reflected signals can also be analyzed for data on lunar surface roughness and surface electrical conductivity. The S-band signal will primarily provide data on the surface. However, the VHF signal is expected to penetrate the gardened debris layer (regolith) of the Moon and be reflected from the underlying rock strata. The reflected VHF signal will then provide information on the depth of the regolith over the Moon. The S-band BRE signal will be monitored by the 210-foot antenna at the Goldstone, California, site and the VHF portion of the BRE signal will be monitored by the 150-foot antenna at the Stanford Research Institute in California. The experiment was flown on Apollo 14. Lunar Bistatic Radar Experiments were also performed using the telemetry beacons from the unmanned Lunar Orbiter I in 1966 and from Explorer 35 in 1967."

At 110 hours, shortly before coming around to the near side, Al starts the Bistatic Radar Test and commences his evening meal period. As with SIM bay operations, the automatic pitch rate is carried out with the spacecraft's DAP set to ±0.5° deadband and the test will last the whole of the near-side pass of orbit 17. Therefore, there will be no voice communication between Endeavour and Mission Control during this time.

The Apollo 15 Preliminary Science Report includes the above diagram showing some of the preliminary results from the Bistatic Radar Experiment. Each line across the graph represents the spectrum of the reflected signal, as received on Earth, at 2½ second intervals. The features on the surface which tend to reflect the spacecraft's radio wave also modify its frequency due to their motion relative to the spacecraft. This is the Doppler effect and it varies between objects as they approach then pass beneath Endeavour. From one time-slice to the next, you can see the shifting frequency of each dominant reflection moving as the spacecraft moves.

As Endeavour goes around the far side and the Bistatic Radar Test ends, Al manoeuvres to the SIM bay attitude which allows the Mass Spectrometer's inlet to face the direction of motion (engine bell forward, SIM bay towards Moon) as it did last night. The DAP is set to a wide (5°) deadband as those instruments still powered (the Gamma-ray Spectrometer, X-ray Spectrometer, Alpha Particle Spectrometer and Mass Spectrometer) do not require such accurate pointing and a wide deadband saves RCS fuel.

Rev 18 begins at 111:36 and the next communication takes place during this orbit's front side pass. By the Flight Plan, it is Al's rest period and Robert Parker is the CapCom on the fresh shift.

Parker is referring to what settings the High Gain Antenna (HGA) should be in during Al's rest period.

Endeavour is keeping a constant orientation with respect to the Moon. With this in mind, Mission Control can calculate what attitude, with respect to Earth, the spacecraft will be in each time it reappears around the Moon's eastern limb. By translating the attitude to angles for the HGA, they can ensure that at AOS, it will be pointing at Earth. The "Reacquire" mode then makes the HGA track Earth until the signal is lost, upon which, it automatically slews the antenna to the preset angles, waiting for Earth to reappear within its sights. The Narrow setting for the antenna beamwidth maximises the received signal strength and improves the signal to noise ratio.

PAO then replays the above conversation.

PAO's further commentary relates to the surface mission.