September 4, 2002
Contours of the Abyss
Comet Exploration Beyond Contour
by Bruce Moomaw
Los Angeles - Sep 04, 2002
As the remaining wreckage of NASA's latest comet explorer Contour fades into the abyss of deep space, the question of what happened grows more frustrating as yet another theory is discredited as to why Contour's STAR-30BP solid motor apparently exploded destroying Contour and NASA's plans to zoom by Comet Encke November 12, 2003 at a mere 100 km from the comet's nucleus.
Another theory proposed by an outside observer as to the cause of the apparent STAR-30BP solid motor explosion that wrecked the CONTOUR comet probe has apparently been disproven.
As announced in SpaceDaily's last article on the subject, one theory -- that the motor's lining might have been thermally stressed by repeated passages of the spacecraft through Earth's shadow during the six weeks it spent in Earth orbit before the motor was fired -- has already been disproven by the fact that the orbits were explicitly designed so that CONTOUR would never pass through that shadow. But the possibility has since been raised that, while the motor's temperature was at a stable level while in orbit, that level might have been too cold, with such protracted cold damaging the motor.
However, CONTOUR Project Manager Ed Reynolds tells SpaceDaily that the motor casing was in fact equipped with heaters controlled by two sets of thermostats to keep its temperature between 16 and 26 deg C -- that is, room temperature -- and that once-per-second telemetry measurements confirm that the thermostats switched the heaters on and off throughout CONTOUR's six weeks in orbit at precisely the expected rate. So it seems that all theories that the STAR failure might have been due to exposure of the motor to improper temperatures can be ruled out.
This leaves the failure a puzzle, which the newly appointed Failure Review Board must probe at length -- but it also increases the odds that the failure was due to a long-shot, entirely random and undetected flaw in the manufacture of the STAR-30 BP (although it has failed only twice in 88 earlier flights). It thus somewhat reduces the chances that the failure was due to negligence by the Applied Physics Laboratory in the design and testing of CONTOUR -- which in turn increases the chances that Congress (despite the opposition of both NASA Headquarters and the White House) will approve funding later this year for the proposed 2006 "New Horizons" Pluto probe, which APL would base to a large degree on CONTOUR's design.
It also improves the chances that a replacement "CONTOUR 2" mission may be launched some time before the end of this decade. There are quite a few other comet probes scheduled for this decade. Deep Space 1 has already made a highly successful flyby of Comet Borrelly during its optional "extended mission" last year; the Stardust probe is well on its way toward a 2004 flyby of Wild 2 and the return of a tiny sample of dust and gas from it to Earth in 2006; the European Space Agency's very ambitious Rosetta mission is set for launch next January on a nine-year trip to actually rendezvous with Comet Wirtanen and land a small spacecraft on its nucleus; and NASA's "Deep Impact" probe is scheduled for launch in 2004 to crash a secondary spacecraft into the nucleus of Tempel 1 at high speed and blast away a section of its Sun-modified outer crust to examine the comet's internal material.
It might seem that a second CONTOUR is thus superfluous. But planetary scientists regard comets as one of their highest-priority targets in Solar System exploration, because they seem to be the last well-preserved relic of the original nebular cloud of dust and gas out of which the Sun and planets condensed, and can thus enlighten us as to just what its original composition and physical consistency actually was -- which in turn will give us a much better understanding of the exact nature of the processes that modified that cloud to form the planets at various distances from the Sun.
And since Earth-based measurements have already confirmed that comets differ significantly in composition -- probably depending to some degree on the distance from the Sun at which they first condensed -- we need comparative analyses of the dust and gas composition (as well as the overall physical structure) of as many comets as possible.
CONTOUR was intended to provide just such a comparative survey by making repeated gravity-assist flybys of Earth to set up flybys of two comets during its 5-year primary mission, and as many as two or three more during a likely extended mission that might have run another 5 or more years. Moreover, it was a very flexible mission. If Earth-based telescopes identified a suitable incoming "new comet" -- that is, one which had not been identified before because it had an orbital period of centuries or longer, which would also mean that that its outer surface had been less chemically modified by being baked during repeated returns to the Sun -- there was an excellent chance that CONTOUR could have broken off its originally planned mission at any point after its first comet flyby in 2003 and be rediverted to visit the newcomer, after which it might have been able to resume visiting a set of known short-period comets.
The Discovery Program of low-cost Solar System missions (chosen from competitive proposals by different scientific teams), of which CONTOUR was a part, has already had its next four missions selected. Should a repeat of CONTOUR or a similar mission be picked in the next round of mission selections, the earliest it could be launched is about 2009. CONTOUR principal investigator Joseph Veverka of Cornell has already expressed the hope that NASA might be willing to provide special funding for a second CONTOUR launch earlier.
The original CONTOUR would have flown by Encke -- the comet which has currently has the shortest-period orbit around the Sun -- in November 2003, and then past Schwassmann-Wachmann 3 ("SW3") in June 2006. The latter is a particularly interesting target because it actually split apart (as comet nuclei often do) during its return visit to the Sun's vicinity only seven years ago -- and so, by flying past one of the three fragments, CONTOUR might have gotten a cross-section view of the internal structure and layering of a comet before repeated exposures to the Sun modified its freshly exposed surface again.
If CONTOUR 2 was launched in early 2006, it would still be able to fly by both comets -- but in reverse order. It would make a flyby of SW3 only a few months after leaving Earth, then fly by Comet d'Arrest in 2008 (as CONTOUR 1 would probably have done during its extended mission), and then fly by Encke in 2013 -- with, again, the ability to extend its mission to more comets, or modify it early on if a tempting new-comet target turned up.
The trouble, however, is getting the funding for such an early reflight of the mission. NASA remains seriously strapped for cash, thanks mostly to the ever-increasing demands of the Space Station -- and while the US Senate has been willing to add $105 million to its fiscal-year 2003 budget to fund the 2006 Pluto probe because the latter mission will seriously suffer scientifically from any delay, it's a good deal less likely that it would shell out another serving of extra money just to ensure a flyby of Schwassmann-Wachmann 3.
Since it would be a near-duplicate craft, CONTOUR 2 would not have the design costs needed for the first CONTOUR -- but its booster would cost more. The first CONTOUR's protracted six-week stay in Earth orbit was to allow it to be launched by a smaller, cheaper "Medlite" version of the Delta booster. During any "launch window" period to a Solar System target, the beginning and ending launch dates during the period are the least favorable ones (otherwise it wouldn't be a "window"), while a launch in the middle of the window is best. But, given the serious danger of prelaunch problems delaying the launch completely beyond the window, such missions are nevertheless launched at the earliest possible date in each window -- which reduces the maximum payload weight that any particular model of rocket can send to that Solar System target.
CONTOUR's mission design was intended to circumvent this problem for the very first time, by having the spacecraft safely launched as early in the window as possible -- after which it could dawdle in high Earth orbit until the date of the middle of the launch window, and only then fire a relatively small rocket to break out of Earth orbit. But -- while it would still probably be regarded as safe to use another copy of a STAR motor to boost CONTOUR 2 out of Earth orbit in the same way -- the trouble is that the Boeing Company has already discontinued its manufacture of Medlite Deltas. CONTOUR 2 will have to use a full-sized Delta 2, costing $20 million more, in any case.
All this raises another possibility: might it be possible to extend the missions of some of NASA's already funded spacecraft to visit additional comets and thus "pinch-hit" for the lost CONTOUR? The Stardust spacecraft -- after dropping off its return capsule, with a sample of Wild 2's dust and gas, at Earth in January 2006 -- will itself fly by Earth and continue in solar orbit. By adjusting the precise track of that flyby, it might use Earth's gravity to redivert itself to a flyby of a second comet -- and, while it could not return a sample of that comet, it would still be able to return closeup photos of its nucleus and use its "CIDA" instrument (identical to the one on CONTOUR) to analyze the most important compounds in its dust particles.
However, Stardust scientific team leader Donald Brownlee tells SpaceDaily that not only does the team lack the money for any such extended mission, but Stardust itself will have very little maneuvering fuel left after its Earth flyby to set up such a second flyby -- probably ruling it out.
Another possibility is the "Dawn" spacecraft set for launch in 2007, which will use a powerful ion drive to tour the Asteroid Belt -- spending 11 months in orbit around Vesta before departing and proceeding to orbit Ceres, the biggest asteroid. During its protracted flights between Earth and Vesta, and between Vesta and Ceres, it may also make high-speed flybys of as many as a dozen other asteroids. It might be possible for it to fly by a suitably located comet during this time -- although such a flyby (like Deep Space 1's flyby of Borrelly) would have to be at several thousand kilometers distance, since Dawn lacks any shield to protect it from high-speed impacts with cometary dust particles.
However, Dawn's team leader Christopher Russell says that no comet in a suitable orbit has been located. The possibility can't be ruled out that Dawn will have enough ion-drive propellant and hydrazine chemical fuel left after its lengthy examination of Ceres to leave that asteroid as well in 2015 and resume its flybys of other asteroids, and it might be able to fly by a comet then. But it's very uncertain that this extended mission will be possible -- and even if it is, Dawn's controllers might very well decide to simply continue flying the craft by as many additional interesting asteroids as possible, rather than expose it to even mild danger from a comet's dust cloud.
Finally, there's the Deep Impact spacecraft -- and here things look more hopeful. After its spectacular flyby of Tempel 1 in July 2005 -- dropping off its piggyback "impactor" spacecraft, and observing the impact and the subsurface layering inside the big resulting crater -- it will probably have enough leftover maneuvering fuel to slightly modify its solar orbit and set up another flyby of Earth in 2006 or 2007, allowing it to redivert itself to a second comet flyby.
Team leader Michael A'Hearn says that it would probably be possible to set up a flyby of Comet Hartley 2, or one of either Wilson-Harrington or Phaethon -- two strange little objects which seem to be comet nuclei completely dried up and drained of ices by their repeated close passes by the Sun. While Deep Impact lacks any mass spectrometers to directly analyze a comet's dust and gases, it could take extremely high-resolution color photos of the nucleus (showing objects only a meter or two across) and use its near-infrared spectrometer to provide a good deal of data on the nucleus' surface composition (and to measure water vapor and carbon dioxide and monoxide in its "coma" cloud).
Unfortunately, it doesn't seem practical to redivert Deep Impact to a flyby of Schwassman-Wachmann 3 -- and any extended mission for the craft at all depends on finding the funds for it. So it still seems likely that at some point a replacement for CONTOUR will be flown -- but the year in which this will happen is very uncertain, and indeed that replacement may end up being a different kind of spacecraft, perhaps even made by a different organization.