What’s happening at JPL  

Greetings to the ASSA from the United States!

I will be writing a little column about astronomy and space science news at NASA and in the United States, specially at the Jet Propulsion Laboratory in Pasadena, California. I am an physicist and astronomer who has seen working at JPL for some years in the field of gamma-ray astronomy. Recently I have become involved n infrared astronomy as well, and have lately moved to IPAC, the Infrared Processing and Analysis Center at altech.  (Like JPL, IPAC is operated by Caltech for NASA.)

In future issues I will try to cover JPL missions. present and future, including activities at the Caltech observatories, at Mt.  Wilson, and other subjects for amateur astronomers and people with an interest in space. There is certainly no shortage things to report. It is clear to all of us that this is the Golden Age of astronomy, as well as the pioneering beginning for space exploration. While no doubt the achievements of future astronomers will awe and amaze, still one must admit no other generation will ever see the electromagnetic spectrum opened from the one bare octave of frequency known to classical optical astronomy, to the nearly 20 decades we can now observe, from below at least 10 MHz in the radio to above 10^26 Hz in very high-energy gamma-rays, just as no other generation will witness the initial exploration of all the major planets in the solar system, save only Pluto. So I will have to try to pick from this mass of potential subjects a few which I hope will be of special interest to readers in the ASSA. I would be especially grateful for feedback (my e-mail address is attached, or write in care of the Editor), either comments or questions about previous columns, or suggestions for the future.

Mars Pathfinder

As I write, the Mars Pathfinder Mission is winding down, both due to harvesting of the most accessible science data (formally signaled by the end of the Extended Mission on August 3), availability of ground support funds and resources of the Deep Space Network, and also the exhaustion of the lander's battery, which is estimated to have only 10-20 charge/discharge cycles left in its life. The combination of lander and rover is now being operated so that only one or two such cycles are performed per week.  After the battery dies, some operation during daylight hours will still be possible.

MGS

At the same time, Mars Global Surveyor (MGS) is slowly approaching on a longer orbit, designed especially to minimize the rocket propulsion necessary for capture into Mars orbit. On August 15 it was 6.75 million km out, approaching at 242,500 km per day. It will arrive shortly after 0100 UTC on September 12, when a 22-min rocket motor burn will cause it to slow by about 1 km/s, and be caught into a long elliptical orbit. Without this burn, which must reduce MGS's speed below the local Mars escape velocity, it would simply fly past into space. It was at just this point that the ill-fated Mars Observer Mission (MOM) came to grief, vanishing in August 1993, just as it was about to execute a (much larger) maneuver to put it into a low Mars orbit. In order to minimize the mass of fuel which must be carried, the slowing for MGS is the bare minimum necessary: after the burn it will still fly far out into space, ( 56,000 km ) , before finally turning back in a 48-hr Mars orbit.

And here is where MGS is smarter than MOM:  the propellant carried by the current mission is completely inadequate to slow the spacecraft enough to put it into the low-altitude mapping orbit desired for the science objectives.  It is the despair of mission designers, how a kilogram of propellant needed at Mars translates into tons of rocket power at liftoff from Earth. The omission of that propellant meant that MGS could be flown with a relatively low-cost Delta launch, rather than a more expensive rocket, probably a Titan.  The savings meant the mission could be launched in 1996, instead of in 1998, or 2001, or maybe not at all.

The difference will be made up by aerobraking in the thin martian atmosphere, roughly 100 km above the surface. When MGS is safe in Mars orbit, the controllers will delicately lower the low point of the orbit until it begins to encounter significant drag. The exact altitude must be between about 105 and 110 km. Too low, and the spacecraft burns up like a meteor; too high, the air is too thin, and not enough slowing is obtained. The entire operation will have to be adjusted carefully as it progresses, because the exact density of the martian atmosphere at 100 km altitude is not known, and also because it depends to some extent on the unpredictable weather. Then, orbit by orbit, easy as she goes, the high point will start to descend, as energy is dissipated during the few moments of closest approach. Eventually, in January 1998, a close circular orbit will be reached, whose final altitude will be adjusted upwards a little to keep it clear of further unwanted drag.  Then at last mapping can begin.

Bill Wheaton
Waw@ipac.caltech.edu

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Sign in an optometrist's office:
     "If you don't see what you're looking for, you've come to the right place."

Sign on a scientist's door:
     "Gone fission."

Sign in a science teacher's room:
     "If it moves, it's biology.
      If it stinks, it's chemistry.
      If it doesn't work, it's physics."

Sign at a computer store:
     "Out for a quick byte."

Sign on restaurant window:
     "Don't stand there and be hungry.  Come in and get fed up."
 
 

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