What’s Happening at JPL
Cassini/Huygens:
The launch of the Cassini/Huygens mission to Saturn and its unique moon Titan, originally scheduled for the opening of the launch window on 6 October, has had to be delayed. The thermal insulation inside the Huygens Titan probe was damaged accidentally due to a higher than anticipated flow of conditioned air, and it was decided on 3 September to remove the spacecraft from the Titan/Centaur launch vehicle for inspection and cleaning. Repair work was complete 11 September, and the spacecraft is expected to be re-mated with the launch vehicle sometime during the week of 21 September. Managers now expect launch on 13 October at the earliest. The launch window closes in early November.
In August a digital video disk with about 616,400 signatures (including mine!) from 81 countries was installed aboard the spacecraft. No less than three Titan IV launches are scheduled within a few weeks, two classified military missions on Titan IVAs, and Cassini on a Titan IVB. This must present some scheduling complexities for the launch crew! The Titan IVB differs from the IVA in having new, higher performance advanced composite solid rocket boosters, giving about a 25% improvement in total vehicle performance. The total spacecraft mass to be launched on an escape trajectory is nearly 5600 kgm, a record for planetary missions.
What might have been biggest hazard the Cassini faced, the threat of legal action related to its plutonium radioisotope thermoelectric generators (RTGs), may be fading. At the moment it seems that no legal challenges are pending or planned (a suit filed against Galileo's RTGs in 1989 failed). Opponents still hope that President Clinton will intervene to cancel the launch. The RTGs provide 755 W of electric power for the spacecraft.
The problem, of course, is that the light levels that will be encountered, about 100 times lower than near the Earth due to Saturn's 10 AU distance from the Sun, made it very difficult to design the mission to use solar panels. Opponents fear that the 34 kgm of Pu in the generators could be released into the environment in the event of an accident. The alpha radiation emitted by plutonium usually cannot penetrate the skin or a piece of paper, but fine particles might be ingested or inhaled, in which circumstances it is highly dangerous, causing lung and other forms of cancer. Of course the Pu's containment has been designed to be as sure as practicable that no such release will occur, even in the event of a launch or other disaster; yet no reasonable person could claim that it is impossible. This is a typical example of one of those profoundly difficult risk-benefit questions that arise from time to time. Eager as I am to learn about Saturn, I am pretty much useless as an evaluator of such a matter!
Is it even relevant to weigh the chance that 34 kgm of Pu might, against all the odds, somehow be released in the event of a Cassini disaster against the certain knowledge that about 5000 kgm of Pu were actually vaporized and injected into the stratosphere in the course of nuclear weapons tests in the 1950's and 1960's? Or how does one count the small risk of causing conceivably 100 cancers against the perhaps 1,000 human lifetimes of effort that have already been expended in preparing Cassini to fly, which I have to see as a fair measure of the cultural enrichment that will result for all of humankind if it is successful?
Of course, such arguments are not meaningful to the more committed of those trying to stop the mission, some of whom are truly frightened, and to many of whom Saturn must seem cold, distant, and irrelevant. Are those of us who devote our lives to astronomy and space highly qualified to offer an opinion, or particularly unqualified due to prejudice? I imagine most of the astronomically involved readers of Canopus tend to weigh things somewhat as I do, and may even begrudge the space I have devoted to the subject. However, this is the last column before launch, and we will have until Saturn arrival on 1 July 2004 to fill in all the technical details about instruments, science, and mission. Meanwhile, for those of you who simply cannot wait and have access to the World Wide Web, try http://www.jpl.nasa.gov/cassini/.
Mars
Global Surveyor:
As of this writing Mars Global Surveyor is safely in a 45-hour Mars orbit, achieved on 12 September, and managers are about to lower the periapsis to 150 km to begin the atmospheric braking which will get it into a circular mapping orbit in January 1998. Aerobraking should well underway by the time you read this. Scientific data have already been obtained during the previous orbit, by turning on the Mars Observer Camera, the Laser Altimeter, and the Thermal Emission Spectrometer for a short period at periapsis.
Next Time:
As expected, there is much too much happening to discuss in one column. However, the Advanced Composition Explorer ACE was launched successfully, on 25 August 1997, and is currently being maneuvered into a "halo" orbit around the semi-stable L1 point on Earth-Sun line, about 1,500,000 km in towards the Sun. It is devoted to exploring cosmic rays and energetic solar particles, but we will have to leave details for a future column.