Apollo 15

This chapter covers lunar orbit science activities during rev 71. Date: 4 August 1971. Apollo 15 has just gone behind the Moon and is approaching the end of rev 70. Rev 71 begins in about 20 minutes time. In the meantime, the crew's conversation has been caught on the onboard tape recorder.

Rev 71 begins at about 215:56.

Flight Plan page 3-328.

While Endeavour is in the darkness of lunar night, two cameras are prepared with very high speed black and white film for photography of the solar corona as it appears just prior to sunlight. The DAC with magazine H and a Hasselblad with magazine R are mounted in brackets looking out of window 4, the right hand rendezvous window.

At 216:03:55, the mission timer is started which will provide a reference for taking shots. When it reaches 5 minutes, the DAC is started at 1 frame per second and a shutter speed of 1/125th of a second. At 6:40 on the timer, its shutter speed is changed to 1/500th. At 6:50 and every 10 seconds thereafter, a frame is taken on the Hasselblad starting at 1 second exposure, then 1/4 then reducing the exposure by one stop with every frame until they get to 1/500th. At 8 minutes on the timer, the DAC is stopped, both cameras are cycled to fresh film and the Hasselblad prepared for colour photography during the coming daylit pass. Thirteen images, AS15-98-13374 to 13386, are catalogued in the Apollo 15 Hasselblad Index but the first three do not appear to carry any useful imagery. Nine images, AS15-98-13377 to 13385, do show the solar corona while 13386 seems to be a deliberately blank frame fired off to provide protection to the very sensitive film. These photos will be presented at approximately the correct place in the transcript.

Orbit 71 begins about 23 minutes before AOS at about 215:56 and the spacecraft meets the sunlight 14 minutes later. At 216:18:30, as the spacecraft crosses the terminator, the Mapping Camera begins an hour-long period of photography looking to the south, 40° away from vertical.

The DAC should be started at this time.

The DAC's exposure should be set to 1/500th at this time.

Three frames on magazine R prior to the solar corona photos appear to carry no useful imagery.

At this time, the first of a series of Hasselblad photos of the solar corona is taken. This is the first of nine such images on magazine R and should be for an exposure time of 1 second.

The next solar corona exposure is for 0.25 seconds, with each subsequent shot reducing the exposure by a stop.

The last in the solar corona series, AS15-98-13385, appears to have been taken after the first sliver of the Sun has appeared over the Moon's horizon.

This extra frame would appear to be AS15-98-13386 which, as is to be expected, is completely blank.

As read to the crew at 214:59:52, the start time for the upcoming Mapping Camera pass is 216:18:30 which coincides with Endeavour passing over the sunset terminator. During this pass, 130 images will be taken comprising AS15-M-2494 to 2623. The spacecraft's attitude gives the camera an oblique view towards the southern horizon, an aspect which often produces spectacular images.

A selection of these images will be presented here, beginning with the second photograph, AS15-M-2495. Then starting with AS15-M-2500, every subsequent tenth image will be shown. Readers who wish to browse the entire collection can find it at the Apollo Image Archive website at Arizona State University.

This coming near-side pass is the crew's last major photo opportunity before they begin preparing to come home. The picture taking begins on magazine P and the 250mm lens with AS15-93-12639, a stark portrait of the crescent Earth.

Handheld photography continues with an 18-frame sequence of the 207-km crater Humboldt, photo target 13 in the Flight Plan, looking to the south with the Sun illuminating from the west. The Apollo 15 Index of 70mm photographs has this sequence as having been taken using an 80mm lens, a focal length considered to be the standard lens for this format. However, given that previous Earth image and subsequent photos from this magazine are catalogued as having been taken using the 250mm lens, that the photographs themselves appear to be through a long focal length lens, and that the Flight Plan calls for the long lens to be fitted, it seems likely that the crew used the 250mm.

A composite image of some of these frames shows the crater well and the features within. To the east is a triplet of craters, one of which is an unusual double crater.

In the centre of Humboldt, part of a mountain range forms the focus of a splendid network of rilles, both radial and arcuate. Humboldt shares some common features with Alphonsus, another large, flat-floored crater. Both have dark patches on their floors and both have been the site of "Transient Lunar Phenomena", unexplained activity seen from Earth. Subsequent images in this sequence, AS15-93-12647 to 12657, essentially cover the same ground seen in the composite. Paul Spudis is a geologist at the Lunar and Planetary Institute in Houston.

Paul Spudis, from 2000 correspondence: "Humboldt is one of a class of features on the Moon known as 'floor-fractured craters' These craters have anomalously shallow floors and extensional features (cracks) on the floor. Often, they are partly filled with later, volcanic lava."

Spudis (continued): "The thought is that these craters were originally normal impact craters, but have been modified by internal processes. Specifically, liquid magma, injected underneath the crater floor, would (1) shallow the crater; (2) uplift and dome the floor; (3) create an extensional stress field, resulting in cracks; (4) possibly permit lava to reach the surface, accounting for the partial lava flooding."

Spudis (continued): "The 'double-crater' on the floor of Humboldt is an unrelated feature. It's not known how these 'doughnut craters' form, but there are several scattered around the Moon. For small craters (a few hundred meters in diameter), they indicate a solid, bedrock substrate beneath a fragmental over-layer, but the one in Humboldt is quite large (almost 10 km in diameter) and I suspect instead that these double craters represent impacts of clots of debris or clouds of fine debris, either a broken up, fragmented asteroid or it is a secondary crater."

Research by David Trang et al of the Hawai'i Institute of Geophysics and Planetology in 2016 doesn't clear up the question of how doughnut craters (also known as concentric craters) form. However they strongly suggest that subsurface processes are involved. "We find that these craters are regular impact craters that experienced intrusions of magma beneath them. Each intrusion pushed on the material around the crater causing the construction of the concentric doughnut-shaped topographic ridge."

Comm break.

The routine procedure each time Apollo 15 reaches AOS is for the crew to position the HGA (High Gain Antenna) to angles given in the Flight Plan. These are based on the expected spacecraft attitude at AOS. The beamwidth of the antenna is set to Wide so that some signal will be received even with pointing errors. Once a crewmember gets the antenna within range of Earth and sees an appropriate indication on a meter, he switches it to Reacq(uire) and Narrow (beamwidth) whereupon it automatically homes in on its target. Houston are getting a weak signal and believe that the crew have not yet selected Reacq.

Allen's request about the spacecraft's RCS propellants highlights some of the interesting ways in which consumables are managed by Houston. Remaining propellant quantities in the four jet clusters are not directly measured (a difficult thing to do in a zero-g environment). Instead, indirect methods are used. The spacecraft's gauge uses the ratio of pressure and temperature of each quad's helium pressurisation system. These readings are also telemetered to Earth and combined with thruster usage, system history and the like. However, by this stage of the mission, errors in these methods may have built up and it would be desirable to have some way to firm up their knowledge of what is left.

The propellants for each RCS quad are stored in two pairs of tanks; primary fuel and oxidiser and secondary fuel and oxidiser; four tanks in all. The quantity of propellant in the secondary tanks comprises 39.5 per cent of the total available and so when they switch to it, they know that this is the quantity that is actually remaining. It gives them an accurate data point to work from as they keep track of the RCS quantities for the rest of the mission. To avoid running the primary tank to empty, this 'crossover' is carried out when the total has been calculated to have reached 43 per cent remaining.

Houston estimate that the C and D quads will require switching over to the secondary tanks during the RCS burn that settles the SPS propellants prior to the upcoming orbit shaping burn, therefore Houston wants all the quads switched over.

Comm break.

This is a reminder that the valves to the secondary tanks in each RCS quad are still to be opened.

Using the 500-mm lens on magazine OO, AS15-92-12510 and 12511 cover the central peak cluster of Langrenus. These have been composited.

Endeavour is coasting across the prominent crater Langrenus to the south east of Mare Fecunditatis. On magazine P and using a 250-mm lens, frame AS15-93-12658 looks WNW across the mare. Langrenus FF (nearest camera) and FE (beyond and to the left) lead to the largest crater in the picture, the 13-km Lindbergh. Various light-toned rays sweep across the mare and the bright feature near the horizon and to the left is Messier.

AS15-93-12659 looks down to the western rim of Langrenus showing the extensive slumping that has taken place.

AS15-93-12660 is another view WNW across Mare Fecunditatis to the Messier twins and their double ray system. To the left of the twins is Messier B.

Very long comm break.

At 216:40, the Panoramic Camera is powered for a few minutes while Mission Control check its health by telemetry. Once happy, they will ask for it to be switched off again.

AS15-93-12661, 12662 and 12663 are taken at the end of the double ray from Messier, where it meets the highlands bordering Fecunditatis. A distinctly pear-shaped crater, Lubbock H, is visible along with an extension to the Rimae Goclenius system of rilles.

Of particular interest is the string of craters that takes a dogleg north of Lubbock H. These are likely collapse pits that reveal a lava tube which runs beneath the mare surface. Frames AS15-93-12664 and 12665 are a view west to the shore of Mare Tranquillitatis and the bright ray crater Censorinus. Crater Maskelyne A is in the foreground (right) of these images.

Censorinus is also the subject of four images, AS15-92-12512 to 12515, taken on magazine OO with the long telephoto lens.

Within Mare Tranquillitatis is a ring of hills which are the remnants of an inundated crater, Maskelyne F. Under the very high Sun, these hills stand out light-coloured against the mare, as seen in AS15-92-12516 to 12518. These images have also been composited here.

Frames 12519 to 12522 pinpoint areas northwest of Maskelyne F which come under the heading of special features showing pits, mounds and a crater. The first, 12519, includes a patch of mounds surrounded by a lighter toned surface thought to be formed from gas-rich, foamy lava. Image AS15-81-11013 shows a larger version of the same phenomenon in Lacus Felicitatis known as Ina which shows these characteristics very well.

Frames AS15-92-12523 to 12525 are of a small, 6-km pit formerly called Maskelyne H but now named after Otto Wallach, 1847-1931, a chemist from Germany. Wallach is sited about 90 km from Maskelyne next to a cluster of low hills.

Photography of 'special features' continues, mostly on magazine OO.

The rille in these two previous images leads towards the distinctively egg-shaped crater Carrel which is also photographed on colour film on magazine P. Four frames, AS15-93-12666 to 12669, look northwest across Mare Tranquillitatis with the Montes Haemus range on the horizon and Mare Serenitatis beyond. The major craters visible in this montage begin with Carrel, formerly Jansen B but now named after Alexis Carrel, 1873-1944, a French biologist who became a Nobel Laureate for Medicine in 1912. Ross D lies in the middle ground and to its left, only partially visible is Ross itself. The bright patch this side of the mountains is Al-Bakri.

Photography continues on magazine OO.

The wrinkle ridge shown in the previous three images is part of a system of radial ridges across Mare Tranquillitatis which emanate from the ghost crater Lamont. This crater itself only exists as a circular series of wrinkle ridges. All these ridges arise from compression stresses in the basalt sheet that form the mare. Those that form Lamont may be purely due to the distribution of these stresses or they may betray the presence of a subdued impact structure.

The next sequence of photographs on magazine P, AS15-93-12670 to 12674, look to the southeastern corner of Mare Tranquillitatis and the Apollo 11 landing site.

These shots have been composited into a 5-frame pan of the southern shore of Mare Tranquillitatis, including Moltke on the left and Sabine partially visible on the extreme right.

In the Apollo 15 photo index, on magazine OO, AS15-92-12539 is marked as showing part of crater Sosigenes. After some searching, the centre of the image was found to be located nearly 37 km northeast of that crater. There is a small (300- to 400-metre) hill at top left in the photo whose long axis is in line with most of the other hills in the area, part of what is now understood to be the 'sculpture' imparted to the landscape by flying ejecta during the formation of the Imbrium basin to the northwest nearly 4 billion years ago.

The next three photos in this sequence show a 30-km ridge stretching south-southwest from Al-Bakri. These have been composited. The ridge, about 300 metres high, appears to be the rims of two craters, now inundated by the mare lavas. This impression is especially strong in the northern example.

Frames AS15-93-12675 and 12677 show Sosigenes A and the expansion-induced rille system at the western margin of the mare, Rimae Sosigenes. An unusual elongated feature crosses the rilles nearly perpendicular to them. This may be a collapsed volcanic lava feature rather than an impact scar.

The rilles of Rimae Sosigenes continue south past Ariadaeus, as seen in AS15-93-12676. This is part of a double crater as it directly abuts Ariadaeus A. Ariadaeus E is the flooded embayment just in front of the pair and at the bottom of the photograph is Sosigenes C.

Comm break.

The 500-mm telephoto lens is still being used with the black and white magazine OO. Frame AS15-92-12544 looks very obliquely across some of the lakes between Mare Vaporum and Mare Serenitatis. To the left is Lacus Doloris (Lake of Suffering) with the 9-km flat-floored crater, Bowen, on its northeastern shore which is named after Ira Bowen, 1898-1973, an American astronomer. Beyond Bowen is Manilius H To the right is Lacus Odii (Lake of Hate).

A few of the next sequence of photos feature a small bright crater WSW of Menelaus D.

On magazine P, a sequence of four southwest-facing images pan across Boscovich, to the east of Mare Vaporum. These are AS15-93-12679 to 12682 and Boscovich is the dark, degraded bowl to the left, one of a scattering of dark-floored depressions or "lacus" (lakes) which mottle the terrain between Mare Vaporum and Mare Serenitatis.

Frame AS15-93-12683 features an unusual sight, a crater with a dark ejecta blanket - the vast majority have light ejecta - which may be caused by the excavation of dark subsurface material.

Frames AS15-93-12684 and 12685 is of the Haemus mountains, seen under a high Sun with the slopes only recognisable by the light colour of their slopes.

At the centre of the above two images is a small bright crater with a distinctive dark section to its ray system. This is shown in close up in the next two images on magazine OO.

As Jim continues with his secretarial duties, Al and Dave are busy with a camera each shooting off their excess film.

Woods, from 2000 correspondence: "Was there a sense of wanting to use up film rather than bring home unexposed stock?"

Scott, from 2000 correspondence: "Probably a sense of capturing the spectacular scenes before we departed - our high inclination ground track presented us with a great variety of superb views. And of course, why return with unexposed film?"

The next set of images on magazine OO are of various mountains and ray craters along the Apennine Front southwest of the landing site.

Frames AS15-92-12564 and 12565 look down upon the Apennine Front about 50 km west of the Crater Conon and about 150 km southwest of the landing site at Hadley.

Two images are taken with the 250-mm lens and colour film at this time. AS15-93-12686 and 12687 look down down onto the southeast corner of the Apennine Bench Formation near Rima Bradley.

Frame AS15-92-12566 on magazine OO is of a curved ridge, very likely the northern rim of a sunken crater rising out of Mare Imbrium near Mons Huygens.

Frame AS15-92-12567 is of another curved ridge, but this one is just about complete, betraying the crater, known as Wallace, that lies beneath the mare surface.

Comm break.

Flight Plan page 3-329.

The camera with magazine P and the 250mm lens is trained southwest on one of the great lunar craters, Eratosthenes, which is becoming more visible to the crew as the Moon's rotation brings their inclined orbit over new territory.

This composition of AS15-93-12688 and 12689 of the 58-kilometre crater Eratosthenes shows the wall terracing, flat floor and central peak; morphology that is classic for its size. However, notice also that while Eratosthenes itself displays no rays, a vast system of rays from Copernicus, about 250 km WSW, are blasted across it and its surroundings, indicating that Eratosthenes is the older formation. At one time, it would also have displayed an impressive ray system of its own but it is believed that rays fade away after about an aeon or so. Both Copernicus and Eratosthenes lie on the margins of Mare Imbrium, and as they and their secondary craters overlie the mare, both craters are known to be younger than the mare. All this and other evidence had led lunar geologists to classify the ages of all lunar features as follows:

• Copernican System: Applies to features up to about 1 aeon (billion years) old; impact craters with ray systems which define them as being relatively fresh. All geological units which can be linked to these in age.
• Eratosthenian System: Applies to features about 1 to 3 aeons old; slightly eroded impact craters without rays, but with other features that obviously postdate the maria.
• Imbrian System: This includes the formation of the Imbrium and Orientale basins about 3.8 aeons ago, the filling of these and all the other basins during the major mare-laying epoch up until about 3 aeons ago and the features that can be associated with this timeframe.
• Nectarian System: Applies to those features formed at the same time as the major basins, between the formation of the Nectaris and Imbrium basins.
• Pre-Nectarian System: Applies to everything formed prior to Nectaris.

Lacking rays but having damaged the nearby mare surface during its formation, Eratosthenes clearly is part of the Eratosthenian System!

Interpretation of the TEI-73 PAD is as follows:

• Purpose: TEI-73 is the last abort TEI PAD that will be sent to the crew, and is intended to get the crew headed home one orbit earlier than planned. The next PAD, the preliminary TEI-74 PAD, will be the first estimate for the scheduled burn to get them home.
• System: The maneuver would use the Service Propulsion System, and the primary Guidance and Navigation system.
• CSM Weight (Noun 47): 36,188 pounds (16,415 kg).
• Pitch and yaw trim (Noun 48): +0.63° and +0.98°.
• Time of ignition, T_IG (Noun 33): 221 hours, 47 minutes, 42.81 seconds.
• Change in velocity (Noun 81), fps (m/s): X, +2,883.3 (+878.8); Y, -661.3 (-201.6); Z, -195.5 (-59.6). These velocities are expressed with respect to the local vertical frame of reference.
• Spacecraft attitude at T_IG: Roll: 179°, Pitch: 128°, Yaw: 350°.

The final notes of the PAD include the following: The SPS propellants are settled in their tanks by firing the plus-X thrusters on all four of the Service Module RCS quads for 12 seconds. The details assume that the crew have not carried out an upcoming burn that will change the shape of their orbit to extend the life of the subsatellite. The attitude angles are based on the IMU still being aligned per the lunar lift-off REFSMMAT.

Comm break.

The last images on magazine OO mostly look towards the southwest horizon and the great crater Copernicus.

The final image on magazine OO is an oblique view of Pytheas, an 18.8 km crater in the southern reaches of Mare Imbrium.

Photography on this pass is completed by exposing the final 47 frames on magazine P using the 250-mm lens. Twenty frames, AS15-93-12690 to 12709, are taken looking north, essentially letting the spacecraft's motion alter the point of view. This is Photo Target 27 and is the area in which the Soviet-built Luna 17 landed, launched from Earth on 10 November 1970, and carrying a Lunokhod unmanned rover. The Lunokhod 1 rover is still operating while Apollo 15 carries out its exploration and will finally expire in October 1971, having traversed 10.5 kilometres of Mare Imbrium.

Frame AS15-93-12691 looks north across Mare Imbrium with Montes Jura, the mountain range that forms the rim of Sinus Iridum, on the horizon. In the foreground are two small craters, Carlini A nearest the bottom of frame, Carlini S in the middle distance. Lighting is from the east.

As the spacecraft continues its progress westwards, Promontorium Heraclides (Hercules Promontory) comes into view. This represents the southwestern cusp of Sinus Iridum.

The Lunokhod 1 rover is within the field of view of many of these images. In AS15-93-12699, it is sited roughly at the image's centre. The final resting position of the rover, stated as being 38.3150°N, 35.0081°W, is shown in the following image from the Lunar Reconnaissance Orbiter probe. The rover's tracks are also clearly visible.

The wrinkle ridge adjacent to Caroline Herschel U which runs north to Sinus Iridum is shown better in AS15-93-12702.

Very long comm break.

By frame AS15-93-12707, the camera has moved across to the light-coloured terrain that forms the western margin of Mare Imbrium, just southwest of Sinus Iridum. The two major craters visible are Mairan E (smaller and near the centre) and Mairan A (larger and to the left).

This light-toned terrain ends further south near the crater Gruithuisen, where the basalts of Mare Imbrium and Oceanus Procellarum meet. This area has some unusual features that will be the subject of much of the remaining photography.

Frames AS15-93-12708 and 12709 show some of the interesting features in this region. At the bottom left of 12709 is Gruithuisen H and centre-right is the 9.4-km Gruithuisen B with a branch of Dorsum Bucher running between. Beyond and to the left of Gruithuisen B are two dome-shaped hills, each rising about 1,800 metres above the surrounding mare basalt. Nearest to crater Gruithuisen B is Mons Gruithuisen Delta and beyond is Mons Gruithuisen Gamma. This latter hill has a 900-metre craterlet at it peak, giving it a look of an unusually domed volcano.

East of the two domes is a low hill that rises 400 to 500 metres above the surrounding mare. It is seen in frame AS15-93-12710, near Caroline Herschel E, and is known as the Epsilon Prominence.

A sequence of three images are taken looking towards the north at the western side of Sinus Iridum.

The Gruithuisen domes are well shown in AS15-93-12717 with Gruithuisen B to the right. The 900-metre craterlet atop Mons Gruithuisen Gamma is easily apparent. Mons Gruithuisen Zeta is at bottom left and is not dome-like in profile. It rises only about half of the height of the other two mountains.

The 16-kilometre crater after which so many of these features are named is nicely framed in AS15-93-12719. Franz von Paula Gruithuisen, 1774-1852, was a German astronomer who pioneered lunar study in two opposing ways. He was the first to postulate an impact theory to explain the lunar craters, an idea now very widely accepted. However, through his active imagination, he announced the discovery of buildings and cities on the Moon, writing a book on the subject. Centuries later, there are still people who claim to detect non-human intelligent artifacts through close study of the imagery from manned and unmanned photography of the lunar surface, a concept that is very widely rejected. To the northwest of Gruithuisen is a large, dense cluster of small craters. These are likely to be secondary craters formed by the impact of a loose clump of debris, themselves ejected from a large, distant impact.

The final 17 images from this pass can be summarised in three montages and a couple of frames from the Gruithuisen area. Looking vertically down at the terrain beneath them, AS15-93-12720, 12721 and 12723 show part of the Montes Harbinger range and Rimae Prinz collection of rilles.

The next set of frames, AS15-93-12724 to 12728, follow a wrinkle ridge northwards across Oceanus Procellarum to where it becomes a string of collapsed pits, presumeably betraying a lava tube.

AS15-93-12729 returns to the string of collapsed pits.

The next five images, AS15-93-12730 to 12734, look north/northwest along the eastern shore of Oceanus Procellarum. All include Rima Mairan and the first four show Mairan T, an isolated hill rising about 700 metres above the mare, but with an irregular depression at its summit.

Two final images of the Gruithuisen hills completes the photography for this pass. These include Mairan A in the distance.

According to the SIM camera index, the last image in the current sequence is AS15-M-2623. However, this is a double exposure with the first image of the next sequence. The last clear image, AS15-M-2622, is shown here instead.

At 217:18:15, just as the spacecraft reaches the terminator, the Mapping Camera finishes its sequence of southward facing oblique photography. Immediately after, Al maneuvers Endeavour into the "Plus-X Forward SIM Attitude" which has the spacecraft flying pointy-end forward.

As described earlier in the journal, the boom which carries the two extendible instruments is made of thin sheet metal with a natural curl to it. Its resistance to twisting forces is limited and, in the case of the Mass Spectrometer which has a directional intake, any twist might be relevant to the results obtained from the instrument.

Mission Control are mentioning this because they have had problems with the Panoramic Camera's V over H sensor throughout the flight. Poor contrast in this device has meant that many of the frames from this camera have been slightly smeared. Al's EVA tomorrow is an opportunity to get a close look at what might be the problem, something that can very rarely be done during any space flight.

This is the maneuver, controlled by P20 in the computer, to change the attitude of the spacecraft.

Very long comm break.

The "50 18" refers to the computer display Verb 50, Noun 18, meaning "Please perform an Auto maneuver" though in this case they are only using it to recall the attitude angles.