JPL and NASA News

Bill Wheaton, IPAC

1997 November

I confess I have heard skeptical comments in the rank and file about NASA head Dan Goldin's "Faster, Better, Cheaper" slogan. However that may be, the contrast between the last year, and the time when I arrived at JPL in 1979 leading on into the bleak 1980's that followed -- there were no interplanetary launches from Voyager in 1977 until Galileo in 1989 (eleven years in development, the last three due to the Challenger accident), -- is simply wonderful to anyone who believes in this enterprise.


The Cassini/Huygens launch, after all the media attention due to anxiety about the plutonium Radio- Thermal isotopic Generators (RTGs), was memorably smooth. I confess my sleep was disturbed, as I went in early Monday (launch window opened at 0155 local time) on 13 October to hang out with the crowd gathered to watch in JPL's auditorium. I arrived just in time to hear the flight director announce that they had decided to scrub for the night, due to out-of-limits high altitude winds. But two days later everything went with easy grace, until the huge spacecraft, the largest US payload ever sent beyond the Moon, separated from the spent Centaur upper stage. Cassini is now receding at about 4.2 km/sec on the dusk terminator side of the Earth, roughly towards that point on the Ecliptic, 90 degrees from the Sun, from which we have lately come. Soon its reduced speed will cause it to fall inwards. As it falls it will begin to speed up again, until it passes ahead of us, to meet Venus (which will then also be in the morning sky) on 21 April 1998, where it will whip by on the dusk side of the planet, picking up enough speed to carry it far out, beyond the orbit of Mars. About two Venus-years later they will meet again, on 20 June 1999. And then yet two more rendezvous will still be needed, with Earth (16 Aug 1999) and finally with Jupiter (30 Dec 2000), which is a sort of record for circuitousness. Voyager 1 required only a little more than 3 years to get to Saturn, and was launched on an earlier, less-capable Titan. The reason is that with the huge Cassini spacecraft mass (which is mostly rocket propellant needed to slow down and be captured by Saturn), even the biggest Titan IV cannot manage anything like the 10 km/sec needed for the direct trip. So, as Cassini floats in the lux æterna of its natural habitat, we may wish it well in its travels, and return to explore further details of the mission during the ample time available between now and arrival.

News From the Mars Front:

Despite great successes, all is not well on Mars. Mars Pathfinder has had difficulties recently which seem to be related to its battery and the fact that the mission has now run well beyond its nominal life. It is still not clear if useful control of the lander and rover will be regained. More worrisome is a problem that has developed during MGS's (Mars Global Surveyor) aerobraking. Shortly after launch, many will recall, it was discovered that the -Y solar panel had not fully deployed and latched. The panel was determined to be about 20 degrees short of the fully-extended position, and after some study, the most likely explanation seemed to be that a damping strut had broken during launch, and then lodged itself in such a way as to block full extension. Since 20 degrees is not too far short, it seemed that the impact on the mission was likely to be small. It was decided to rotate the panel in such a way that during aerobraking, the drag force would tend to force it in the direction of full extension, against the hypothetical broken strut.

Aerobraking proceeded very well with the periapsis (lowest point of the orbit) at about 110 km, so that by early October the period had been reduced from the initial 45 hours down to 35 hours. On 1 October, at periapsis number 12, it was observed that the -Y panel had moved 14 degrees closer to full extension. This was generally regarded as a hopeful sign, until periapsis 15 on 6 October, when it moved somewhat beyond where the Project believed it should latch. On that particular orbit, the atmospheric density had suddenly jumped by about a factor of two above its value on previous orbits, so that the drag force was proportionately greater. Such density variations, while unexpected, are not considered extraordinary for the season on Mars. However, there were indications that the panel was moving during drag passes in ways that were not understood, so it was decided to raise the periapsis to 121 km and think things over. During the two following passes the atmospheric density was as expected, but the panel continued to show unexpected motion. It is a worry, because if the panel should actually break or cease to function in its power-gathering capacity, the effect on the mission would obviously be serious. Thus on 11 October it was decided to raise the periapsis completely out of the atmosphere to 172 km pending further study.

A decision is expected shortly, but the hiatus in braking has already had a permanent effect, in that the planned orbit can no longer be achieved. MGS was placed in a near-polar orbit, so that the entire planet can be mapped. The exact inclination has been chosen so that the orbit, once lowered fully, will precess synchronously with the Sun and always cross the equator at the same local time, which had been planned to be 2 PM. Then the illumination angle will always be the same, aiding in the effort to achieve uniformity of coverage and easing interpretation of the images. However the precession will not really begin until the orbit is lowered, and meanwhile Mars is continuing about its orbit. The upshot is that by the time MGS can get to low obit, the the local 2 PM meridian will have passed. That is hardly disaster, as the 2 PM orbit was itself a compromise among the various scientific objectives. The worst possibility is that the Project may decide that aerobraking all the way down to low orbit entails excessive risk to the panel, in which case gentler aerobraking might get us into a lower, but still elliptical orbit, so the mapping would yield correspondingly poorer resolution under the high point.

On the other hand, the first pictures released are very impressive. They are of the north polar region where the periapsis is located, and a big step beyond the previous generation taken by the Viking orbiter.


I will have to leave a fuller report to future issues, but I should at least mention a few other developments. First, Lunar Prospector is scheduled to be launched on 23 November, on one of the new Lockheed-Martin Launch Vehicles (LMLV). It will map the geochemical composition of the Moon, and explore the polar regions for water ice that has been suggested to exist in permanently shadowed lunar craters. Second, Deep Space 1, the first mission of NASA's New Millennium Program, recently completed an 8,000 hour ( = about one year) qualification test of its ion drive. Its launch is due in July 1998.