In three of them, Galileo would make a gravity-assist flyby of Callisto in May 2000 which would shorten the period of its orbit -- allowing it to make three Io flybys in August and October 2001 and January 2002.

According to those earlier rumors, the first two would be very low-altitude flybys of Io's north and south poles, at a height of only 100 km (since JPL now has confidence in its ability to navigate the craft past Io at such close range).

The north polar flyby in August would also allow Galileo to make very close observations of the Tvashtar caldera, where it observed a fiery lava fountain -- 25 km long, one and a half km high, and at a temperature of 700 to 1300 deg C. -- at longer range last November, which had still not completely cooled down by its third Io flyby last February.

The third and last Io flyby would be more flexible, allowing NASA to use the exact flyby trajectory to select one of three final missions for it. In one, the Io flyby would swing Galileo into a final orbit with a very low periapsis over Jupiter but a period of 10 months -- and the natural effects of Jupiter's motion around the Sun and the gravitational tuggings of the four Galilean moons would guarantee that Galileo's next periapsis would be automatically further lowered, so that it would plunge into Jupiter's atmosphere and burn up on its next low pass in December 2002.

In the second, it would be put into an orbit with a slightly higher periapsis, allowing it to instead make a close flyby of Amalthea during that next close approach in November 2002, then transmit back the data from that flyby during one last orbit of Jupiter before burning up on its next low pass in September 2003.

And in the third, it would make a somewhat different third Io flyby to put itself into an orbit with a period of eight months -- making an Amalthea flyby on its next close pass of jupiter in September 2002, then making two more orbits around Jupiter (and thus more observations of the planet itself) before finally making its automatic suicide plunge in January 2004. Which of these three scenarios is chosen would depend on how much money NASA had to extend the mission.

These missions are all scientifically desirable -- but they all involve keeping Galileo going and under control until that third Io flyby in January 2002. If NASA decides instead to further cut any risk that the craft might eventually hit Europa, it may instead choose a fourth, more conservative final mission -- in which Galileo's Ganymede flyby this December will be designed to immediately put it into an orbit with a very low periapsis and an eight-month period.

This will allow Galileo to make an Amalthea flyby on its next close approach to Jupiter in August 2001, and then complete two more orbits around Jupiter before finally plunging into the planet in December 2002 without getting the chance to make any more Io flybys. That way, Galileo need only be kept under control until this December to rule out any chance that it might later hit Europa (or Ganymede or Callisto).

Is the scientific gain from the extra Io flybys worth the added risk of contaminating another world? NASA asked the COMPLEX committee for advice on that. But the Committee concludes in the new report that it cannot yet give a firm answer, for no detailed calculations have yet been done to estimate just how likely it is that Galileo will break down during 2001, or how likely it is that an uncontrolled Galileo would eventually hit Europa as opposed to meeting some other final fate.

COMPLEX urges NASA to carry out such analyses -- but in the meantime, COMPLEX has reached its own preliminary conclusion: "Based on the information supplied to the committee, an extra year of operations can be expected to increase the burden of radiation absorbed by Galileo by only approximately 20%.

This estimate, plus the fact that Galileo retains full redundancy in all its major subsystems and that the radiation effects sustained thus far has not handicapped control of the spacecraft, suggests to COMPLEX that the probability of total loss of control during this extra year is relatively small.

"Moreover, the chances of total failure can be mitigated by prudent monitoring of the spacecraft's health and by a commitment on the part of NASA to retarget Galileo onto a Jupiter-bound trajectory following the loss of redundancy in any major command and control subsystem." (That is, if the multiple Io flyby option is chosen, either of the first two Io flybys -- or the gravity-assist flyby of Callisto preceding them -- could also be modified to immediately put Galileo into an automatic self-destruct orbit around Jupiter if signs of trouble appeared before those flybys.)

So -- pending the further calculations that COMPELX wants NASA to make -- "The committee reached a consensus that an appropriate interim course of action is to defer the destruction of Galileo until after the completion of the Io polar flybys, in order to obtain as much science as possible from the mission."

At any rate, it is now certain that Galileo's long, strange saga will not finally come to an end until sometime between December 2002 and January 2004 -- and that in the meantime, it will make anywhere from two to six more scientifically valuable flybys of Jupiter's moons.

GALILEO'S JOVIAN VERDICT - PART ONE - PART TWO - PART THREE
EXO WORLDS
Are Martians Dangerous
Cameron Park - December 2, 1999 - In his Oct. 5 piece here, Barry E. DiGregorio made an impassioned argument against NASA's current plans for returning one kilogram of Martian surface samples directly to Earth in 2008 -- on the grounds that the chances that "extant" (that is, still-living) microbes still exist on Mars are higher than NASA is making out, and that there is a genuine and serious chance that such microbes might prove harmful to Earth's biosphere -- and perhaps to human beings themselves. How accurate is this?

Life on Mars: Will It Survive First Contact
by Bruce Moomaw
Cameron Park - October 7, 1999 - A new era is about to begin in space exploration: an era in which samples of material from worlds more distant than the Moon are returned to Earth by unmanned spacecraft.