JPL and NASA News

Bill Wheaton, IPAC

Perils of Space: Mars Global Surveyor Antenna

The effects of a blockage in the pointing mechanism for the high-gain antenna on the Mars-orbiting photographic mapping mission, MGS, may have been largely overcome. As a result there is hope the problem will not much affect the data returned during Mars mapping. MGS is designed to allow science data to be collected and stored on-board, and then transmitted to Earth at high rate through the 1.5 m dia. parabolic antenna, without needing to interrupt picture taking in order to reorient the spacecraft. Having a steerable antenna approximately doubles the data returnable.

MGS has had previous mechanical problems. You may recall from our earlier reports that its mapping mission was delayed for a year due to a broken solar panel attachment, which forced interruption of areobraking back in October 1997. MGS finally reached its desired 2-hr. mapping orbit earlier this year. In early March it carried out a 3-week mapping sequence with its high-rate antenna stowed, in order to ensure fulfillment of minimum mission science requirements, even in the event of some unexpected problem with its deployment. The antenna was finally deployed without incident on March 28. Normal mapping then began, with 12 orbits per day, including some high-rate, real-time orbits each day, and a 10-hr DSN (Deep Space Network) pass. However on April 15 mapping was interrupted and the spacecraft was found to be in contingency mode, with science instruments off. The reason was some problem in the antenna azimuth drive, which blocked its motion before it could reach its intended position for playback.

Further study has shown that normal operations can actually continue through February 2000 without exceeding the current limitations on the range of the antenna's motion. Since the basic mapping mission is just one year, this should be almost enough to complete it without significant loss. If the problem is not resolved by next February, the geometry between the Earth and Mars again becomes unfavorable, and the spacecraft may have to be operated in a mode in which it periodically breaks off observations of Mars and turns to point the antenna towards Earth. The range of azimuth motion appears to be stable and the consensus seems to be that it is unlikely to get worse. Mapping operations accordingly resumed on 29 April and are proceeding well so far.

MGS Science

Meanwhile, MGS science continues to pour in. MGS magnetometer data have recently indicated Mars has magnetic striping similar to that observed on the sea-floors of the Earth. On Earth, the striped pattern is produced by the combination of sea-floor spreading, the periodic reversals of the Earth's magnetic field, and the cooling of magma from the mid-ocean ridges in such a way that the ancient magnetization is locked into and "remembered" by the rocks. Discovery of the Earth's magnetic stripes was the key factor that convinced the geophysics community of the reality of the motion of the Earth's continents, sea-floor spreading, and the "New Global Tectonics" in the 1960's, producing a real conceptual revolution. The new results seem to indicate that Mars has experienced plate tectonics also, although the details are evidently somewhat different: Mars, after all, has only a weak magnetic field and no present oceans. With any luck at all, the differences will be interesting and instructive for planetary physics in general.

In May 1999, as I write, the wide-field, low-resolution camera is being used to image the entire planet at 300 m resolution. Similar images were obtained by the Viking Orbiters, but it required 3 years and the MGS sequence will be much more homogeneous in such things as lighting and camera angle. The May images will be used to build a precision reference grid tying data from the narrow angle camera and all the other instruments into one system.

Yet it is simply impossible to adequately cover the riches of images and scientific data from a spacecraft in the space available here. I urge the reader to give themselves the excitement and pleasure that only examining the new results first hand can afford. Fortunately numerous mirror WWW sites are available, including one for South Africa:

http://www.southafrica.co.za/mars/mgs

Cassini / Huygens

For many months the enormous Cassini / Huygens spacecraft, the joint NASA / ESA mission to Saturn and Titan, has been moving slowly through the inner fringes of the Asteroid Belt beyond Mars. It has recently come back inside the orbit of the Earth on the way to a 24 June flyby of Venus. Then, shortly thereafter on 18 August, it will pass close by Earth at high speed, and leave the inner Solar System forever (see figure). At each of these Venus-Venus-Earth encounters it gains some energy as it bounces from one world to the next. Just now we are about halfway from launch to the passage by Jupiter late next year; which itself almost bisects the nearly 7-year journey to Saturn. So far the health of the spacecraft and instruments seems to be good.

Bill Wheaton
Waw@ipac.caltech.edu