by Bruce Moomaw
Cameron Park - April 19, 2000 - During the current fuss over the radical revision of NASA's Mars program, another, quieter series of changes in another planetary exploration program has gone mostly unnoticed.
But this reporter has just acquired some important information on some probable revisions -- with more to come soon -- in the plans for the Europa Orbiter and the Pluto Express mission. Dr. Jay Bergstralh, Program Scientist for the Outer Planets Program, was indispensable as an information source.
First: there have been rumors for several months that the launch of the Europa Orbiter -- which will orbit Jupiter's moon Europa and try to settle conclusively the important question of whether it currently has a subsurface ocean (or smaller water bodies) underneath its ice crust -- will be delayed for up to two years.
There have also been stories claiming that this delay would be due to the continuing technical problems of NASA's Gravity Probe B -- a sophisticated and expensive satellite designed to conduct the most thorough test yet of Einstein's General Theory of Relativity -- which now seems likely to require an additional $70 million in funding.
Dr. Bergstralh told SpaceDaily that the Europa/Pluto project is formulating "a number of options... that will be presented to NASA Headquarters toward the end of April", and that one of the recommendations will indeed "likely" be a 26-month postponement of the Europa Orbiter launch to January 2006 -- which would delay its arrival at Jupiter to fall 2008, and its final arrival in orbit around Europa itself to summer 2010.
However, he emphasized that "this is NOT driven by Gravity Probe B's funding problems" [italics his]. NASA space science chief Ed Weiler has previously stated that a delay in Europa Orbiter and/or a delay in the next two Mid-sized Explorer (MIDEX) satellites would probably be necessary to find the additional funds --which raises the possibility (which I haven't been able to confirm) that NASA has followed a stated possible alternative course and decided to cancel the troubled Gravity Probe B, despite the fact that $450 million has already been spent on it. However, it's also possible that NASA has managed to find the extra funds somewhere else.
Why was Europa Orbiter delayed? Bergstralh says that it was because the probe requires more sophisticated technologies than its cousin, the Pluto Express flyby probe (which was originally scheduled to fly a year later in 2004). The Europa probe's electronics must resist very high radiation levels from Jupiter's powerful radiation belts -- a total dose of 4 megarads over the entire mission, much more than the "Galileo" spacecraft -- and it must also carry out a series of large trajectory maneuvers (totalling 2.5 km/sec), while remaining lightweight.
Launching the Pluto Express in Nov. 2003 rather than Dec. 2004 has three other major advantages. First, Pluto is currently moving away from the Sun in its eccentric orbit -- which means that launching the probe a year earlier would make it much easier for it to reach Pluto. Its flight could be shortened by as much as six months -- or, alternatively, the probe's weight could be increased by as much as 50 kg if needed.
More importantly, it's scientifically important to get the probe to Pluto as soon as possible. As Pluto moves further from the Sun, its temperature is dropping -- and its faint but interesting atmosphere (made mostly of nitrogen and carbon monoxide, with a surface pressure only a few hundred-thousandths that of Earth's) is likely to freeze completely out onto its surface over the next decade.
Finally, to reach Pluto the probe must make a gravity-assist flyby of Jupiter -- but if its launch is accidentally delayed beyond 2004, Jupiter will move out of position for this maneuver for ten years. To launch the probe to Pluto during that period, gravity-assist flybys of Venus and perhaps Earth itself would be needed, making the mission more complex and much longer.
The original recommendation from NASA's outer Solar System science advisory group was to launch the Pluto probe first, but NASA surprised everyone by announcing in its FY 1999 budget request that it had decided to launch the Europa probe first (possibly because of its astrobiological interest).
However, Bergstralh and Pluto Express scientist Richard Terrile told SpaceDaily that at this point, this is only one optional possibility, and that the Pluto Express will indeed be moved to a launch in Nov. 2003 -- which would allow it to reach Pluto as early as summer 2011. This change in launch date would also mean that the Pluto probe will have to fly much closer to Jupiter than previously planned -- within only about 150,000 km, which would make it possible for the probe to make an optional close flyby of Io. However, Bergstralh said that as far as he knows, this is not being planned. In any case, the additional radiation dose from the Pluto probe's closer Jupiter flyby is very small compared to the Europa Orbiter's dose, and should require no major design changes.
Bergstralh added that the Europa Orbiter delay is unlikely to delay the launch of the next spacecraft in the program -- the Solar Probe, which is scheduled to use a Jupiter flyby to hook itself back into the inner Solar System for an astonishing flight within only 2.1 million km of the Sun's surface, actually flying through the corona, and hiding behind a carbon fiber shield resistant to temperatures of 2100 deg C while making by far the best observations yet of the Sun.
This craft -- which, despite its radically different mission, is similar in many ways to the Europa and Pluto probes -- is still scheduled for launch in Feb. 2007, for its kiss and tell date with the Sun in Oct. 2010." The Europa Orbiter's launch vehicle will also probably be changed. The original plan was to launch both the Europa and Pluto probes on Space Shuttles equipped with IUS solid-fueled upper stages.
But (despite NASA's continuing efforts to conceal the fact with creative bookkeeping), it would be far cheaper to launch them on expendable boosters of proper size -- and such a smaller booster, in the event of a launch explosion, would be distinctly less likely to scatter any plutonium from the spacecrafts' RTG electrical power generators.
(Each craft, by the way, will contain only about one-tenth the plutonium carried by the Cassini spacecraft -- both because the craft are much smaller and because NASA has now developed an improved RTG design that converts radioactive heat into electricity three times more efficiently.)
In December it was officially decided to move the Pluto Express onto one of the new "EELV" expendable boosters that should be developed by then -- either the Delta 4 or the Atlas 5 -- and now Bergstralh tells me that the project will probably recommend that the Europa Orbiter also be moved to such a booster. (Even given Boeing's continuing troubles with the Delta 3, these boosters are first scheduled to fly in 2001 -- which should provide an adequate time margin to get the bugs out of them.)
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