This month there has been an important development in the Mars program, so we will defer other matters to describe it in some detail.
The Revised Mars Global Surveyor Mission Plan:
Mars Global Surveyor’s problems with a damaged solar panel had just become apparent as I wrote last time. After a month’s study, the Project has decided to reduce the maximum drag force during aerobraking by about a factor of three, to avoid the risk of further weakening the solar panel hinge joint. The panel is attached to the spacecraft by a honeycomb sandwich structure that is believed to have been damaged in the original anomaly shortly after launch, and the top and bottom layers of the sandwich seem to have begun to come apart under the stress of aerobraking. The new target force level is 0.2 N (a little less than an ounce), which will bring the apoapsis down to the desired 400 km considerably more slowly than originally planned. For many kinds of mission this would not matter at all, but there is a slightly complicated (but interesting) additional issue related to the orbit and the lighting desired for the MOC, or Mars Orbiter Camera. As all amateur astronomers well know from viewing the Moon, the visibility of features on a planetary surface is highly dependent on the angle between the viewing direction and the solar illumination. If this angle is too small, very little detail can be seen at all, as is the case when viewing the full Moon.
As originally planned, MGS was to be in a polar orbit, crossing the martian equator at 2 PM local time, moving from north to south. Of course, since Mars orbits the Sun in about 2 Earth years, in order to maintain the 2 PM orientation, the MGS orbit must precess at just the same rate. This is possible if the orbit does not quite pass over the poles, but is slightly inclined to the equator. Then the oblateness of Mars (caused by its rotation, as for the Earth) exerts a torque of the right amount to make the orbit "sun-synchronous", ie, to cause the precession to exactly keep up with the apparent motion of the Sun across the sky as the martian year passes.
However, and this is the important point, for this synchronicity to be maintained, the orbit must be also be low and circular; the current high elliptical orbit precesses much too slowly to keep up, because it is too high to feel much of the gravitational tug of the planet’s equatorial bulge. The current orbit would remain almost fixed in inertial space, while the terminator of the planet rotates below it; counter clockwise as seen from above (ie, north). At the time of its arrival in September, MGS would have approached slowly from the dawn side, passed over the north pole, and been captured into an orbit nearly above the terminator. With the passage of time, the terminator moves; but the orbit essentially has not. If you think about it (not too hard!) and draw a few simple sketches, you can see that MGS would reach the desired 2 PM south-going orbit about 4 months after arrival, ie, January 1998. At that point the orbit would precess at the correct rate to maintain it fixed at 2 PM. But, if there is any interruption in aerobraking, the equator-crossing slips towards the west, and the dreaded condition, with the Sun directly overhead, begins to occur. To avoid this awful fate, the new plan is to suspend aerobraking altogether starting early next year, for a whole year (half a martian year) until the geometry is again correct, and then going down into an orbit which crosses the equator from south to north, still at 2 PM. This new mission plan entails some additional risks, as a failure may occur before we get there. However, the scientific advantages are large, so that in the end the Science Team thought it worthwhile to accept the danger. The new plan could also entail additional cost, since the science and operations teams would obviously have to be kept together for a year longer. The current plan is to neatly avoid the latter problem by shortening the 2-year mapping phase to one year, although if the mission is doing well additional funding (or a delay of a future Mars mission) might allow it to continue. The spacecraft is actually designed for a 6-year lifetime. On the other hand, during the 1-year hiatus, the periapsis will be only about half as high as in the final mapping orbit, so it will be possible to obtain a large amount of data at twice the normal mapping resolution.
Meanwhile, beautiful pictures are coming in. The most recent (as of this writing in late Nov.) show striking layering in 1000-m high cliffs overlooking the great Valley of Mars, reminiscent of the Canadian Rockies. The layering (sedimentary?) suggests yet again that Mars has had a more complex history than the earlier consensus view.
For those of you with WWW access, the MGS team has provided a nice touch by putting the spacecraft telemetry pages, presumably as shown on the operators’ consoles, on the Web in real time. They have also recently added a section explaining the rather cryptic telemetry page format, without which the information on would be a little hard to interpret. Since it is mostly text, even those denied access to high-speed connections should be able to enjoy it. However, I am pleased to see that over twenty mirror sites have proliferated around the world, including one, http://www.southafrica.co.za/mars, obviously intended for readers of Canopus!
Other News in Brief:
As you have probably heard, the Pathfinder Mission to Mars has had to be brought to an official close, as all attempts to contact the Lander were in vain. It is believed that the extreme cold, well below design limits, encountered as the northern hemisphere martian winter builds, has caused the electronics to shift frequency, making further communication impossible.
President Clinton, using a newly-acquired authority to veto individual items in the US defense budget, has killed Clementine 2, a small low-cost combination technology-development and science mission planned to go to the near-earth approaching asteroid Toutatis. It would have followed on the success of Clementine 1, which orbited and photographed the Moon in 1994. The lunar investigations of the latter, however, are to be continued by Lunar Prospector, which has just been rescheduled for launch in January 1998, due to the need for further work on its new, all-solid-fueled Athena launch vehicle. Clementine 1 observed by radar what may be ice in a permanently-shadowed crater near south pole of the Moon. If present in even fairly modest quantities, such ice could have a tremendous impact on the economics of lunar development, as it could provide both water needed by explorers and hydrogen and oxygen for rocket fuel. Lunar Prospector will carry a gamma-ray spectrometer which has the potential to settle the matter by observation of the characteristic 2.2 MeV line emitted when hydrogen captures the many neutrons expected to be produced on the Moon by cosmic rays.