Rosetta Team Close To Revised Mission Plan For Comet Explorer
Sacramento - Mar 20, 2003
The European Space Agency, after those initial alarming reports, is now rapidly homing in on a revised mission plan for Rosetta. Last week, several reports in "New Scientist" and other publications quote chief scientist Gerhard Schwehm as saying firmly that the comet Churyumov-Gerasimenko had been firmly selected as the replacement target, with the launch set for Feb. 2004.
Rosetta, as in its original plan, would match orbits with the comet by making one gravity-assist flyby of Mars and two of Earth, but its flight duration would have to be prolonged by one and a half years -- it wouldn't reach the comet (hereafter refered to as "Churyumov", for obvious reasons) until 2014.
Rosetta would have to expend more maneuvering fuel for this mission, and to minimize this it would delay its rendezvous with the comet until it was closer to the Sun -- probably in autumn, when it was perhaps about 3.6 AU from the Sun instead of the 4 AU planned for Wirtanen, which in turn would somewhat reduce its total observation time at the comet.
The added fuel usage would also make it harder for Rosetta to veer slightly off course during the early parts of its mission to make close flybys of one or two asteroids (it was originally supposed to fly by two) -- but Schwehm says that several possible new target asteroids for it have nevertheless been identified.
Schwehm, however, tells SpaceDaily that the reports that this new plan has been firmly chosen are not completely accurate -- the Rosetta team has indeed listed this as its definite favored course, but the ESA won't make the final decision until mid-May. It's not quite impossible that the spacecraft might be launched to Wirtanen after all, next January.
However, a regular Ariane 5 isn't powerful enough to do so, and the more powerful "ECA" model -- which failed so disastrously last December -- is unlikely to be cleared for the mission by January.
The only possible booster is a Russian Proton -- and its standard payload shroud is 40 cm too narrow to contain Rosetta. A new fairing would have to be designed and thoroughly tested, and the overall complexities of switching to this booster make a Wirtanen launch very unlikely.
But a flight to Churyumov has a serious problem of its own: difficulties with Rosetta's small ejectable soft lander. Landing on a small body with such extremely weak gravity (in Wirtanen's case, less than 1/100,000 that of Earth) produces problems entirely different in nature from those of landing on a regular-sized world -- the usual problem is not of having to survive the shock of a high-speed touchdown, but rather of keeping the lander from bouncing back off the surface afterwards.
The "NEAR" spacecraft survived its final soft landing on Eros perfectly; but that was an optional experiment and its post-landing survival really surprised its controllers. By contrast, the Rosetta Lander has to work.
The plan -- once the main spacecraft has located the best landing site during its mapping of the nucleus' surface while slowly orbiting it at close range -- was to have Rosetta change its orbital path to veer down within only about 1 kilometer above the 1.5-km wide nucleus and drop off the Lander there.
The Lander would be stabilized in a fixed attitude by an internal flywheel, and instead of retrorockets it would actually use a 3-kg thrust nitrogen gas jet to push it DOWNWARDS toward the nucleus at about 1 meter per second.
During this time it would unfold its landing gear: a spindly, widely splayed tripod consisting of three nearly-horizontal landing legs, fastened to the Lander's bottom by a short vertical strut doubling as a shock absorber.
We still know virtually nothing about the consistency of a comet nucleus' surface; it could be fluffier than the freshest snowfall, or as hard as asphalt -- and the landing gear must try to cope with either possibility.
When the Lander's footpads sensed surface contact, its gas jet would immediately fire again to keep it pinned to the comet's surface while it fired a small barbed harpoon into the surface and then quickly reeled its line taut to keep itself anchored firmly to the surface.
If the reel's sensor indicated that the harpoon had pulled completely back out of the ground, the Lander would instantly fire a second harpoon. It would also twist a screw fastened to each footpad to further anchor itself into the surface.
Thus anchored, it could then spend anywhere from 3 days to several months studying the surface, swiveling its body around on that vertical strut to aim its cameras in different directions and use its sample-collecting drill at different spots on the surface.
But Churuymov's nucleus is bigger and more massive than Wirtanen's. (Current rough estimates are that its diameter is about 4.5 km, although Schwehm says that very recent observations by the Hubble Telescope indicate that it may be somewhat smaller.)
The result, ironically, is that there really is some worry that if it was dropped from 1 km up as planned, it might hit a hard surface at 2 or 3 meters per second -- fast enough to damage itself, or bounce off the surface too hard for its gas jet to hold it down and then land elsewhere on its side. Alternatively, it might bury itself too deeply in a fluffy surface.
There has been some public worry that this might make the Lander's task impossible for this new comet -- but Schwehm tells this reporter that the Rosetta team are now pretty much satisfied that it can be done, and without even having to modify the Lander's hardware.
The plan would be to have the main spacecraft veer down into an orbit taking it a good deal closer than 1 km to the comet's surface before dropping off the Lander, so it would have to fall less distance.
This, however, produces a problem of its own. An orbit at that lower altitude over a more massive comet nucleus would naturally mean that the spacecraft would be sailing forward faster than was planned at Wirtanen -- and since the Lander can't tilt itself into a new attitude after its release, if it was released at a tilt so that its gas jet's initial firing cancelled out that additional forward velocity, it would hit the surface while still tilted.
The new tentative plan, therefore, is to have the main craft -- just before releasing the Lander -- fire its own thrusters to actually cancel out its horizontal velocity, and then immediately fire them again after the release to resume its forward orbit around the comet. A bit more complex and risky than the planners would like, but it may be the best way to solve the problem.
Even after landing, one of the Lander's experiments might run into trouble on the new comet's nucleus. CONSERT involves the main craft, as it slowly orbits the nucleus, continuously transmitting low-frequency radio pulses -- which can actually travel through the ice of the nucleus to reach a transponder on the lander, which would in turn bounce them back through the nucleus to a receiver on the orbiter.
By constantly monitoring the strength and time delay of these pulses as they travel through the nucleus, a detailed map could be built up of the nucleus' interior structure, layering and rock content.
But Churyumov's nucleus, being perhaps three times wider than Wirtanen's, may be too thick for CONSERT's radio pulses to pierce it successfully. It's too late to modify the instrument; its experimenters will simply have to gamble that it can still obtain usable data at Churyumov.
Even if this happens, though, it still seems likely that Rosetta's new cometary target and flight plan would be highly workable -- it will simply have its arrival delayed by about 2 1/2 years.
If, by bad luck, the regular Ariane 5 has still not been cleared for a resumption of flights by early 2004, there's also a backup launch window to Churyumov a year later, although this would require launch on the more powerful Ariane 5 ECA or a Proton.
Finally, there's the matter of Deep Impact. Last fall, it became apparent that this mission was in danger of busting its cost cap -- not by much, but any overrun whatsoever is unacceptable to NASA, since it would encourage future mission proposers to deliberately underestiamte their costs to get selected, and then come back to NASA rattling their begging bowl for additional money.
NASA is familiar with this -- after all, it's exactly the same technique it used to trick Congress and the White House into initiating and then maintaining the Space Shuttle and the Space Station. But I digress.
The trouble was that there wasn't really very much to cut out of Deep Impact, either -- its science payload had already been cut to the bone, and any further cuts would reduce its science return below the point at which the mission would have been worthwhile at all. To complicate matters further, its "Impactor" turned out to require much more complex software than had originally been planned.
The Impactor is the separate 350-kg cylindrical copper spacecraft which is supposed to separate from the main craft hours before reaching comet Tempel 1 and deliberately steer itself into a collision with the nucleus, while the slightly trailing main craft takes high-resolution photos and IR spectra of both the impact and the resulting 100-meter wide, 20-meter deep crater.
Originally it was supposed to just steer itself toward the center of the nucleus' dayside, using images from its own onboard TV camera (which would also be radioed back to the main craft up until the moment of impact).
But Deep Space 1's photos of nucleus of comet Borrelly in 2001 had revealed that comet nuclei seem to have surfaces much rougher and bumpier than had been expected -- and so the Impactor's software was faced with the additional task of steering the Impactor away from a crash into any shadowed part of Tempel's nucleus where the crater would not be visible to the main craft's eyes.
The Deep Impact team dealt with these problems by making every cut they thought they could get away with in ground testing and post-launch operational procedures -- such as maintaining the ground tests of the craft's ability to endure launch vibrations, while trimming down the separate testing of its ability to endure launch noise to just those components (such as the solar panels) likely to be affected by such noise.
The new software for the Impactor was retained, and in fact became the new benchmark for what was to be retained -- if dumping any aspect of the craft's testing and operations produced less of a likely risk to its success than the new Impactor software, it was dumped, while anything more likely to be necessary to its success was retained.
This trimming resulted in the mission being tentatively retained in NASA's new budget for Fiscal Year 2004 -- but its probable retention wasn't assured until NASA's independent review team reported on March 13 that the cost-cutting changes had indeed resulted in the mission being acceptable both in cost and risk level. This was the final planned review, and Deep Impact's launch is now almost certain.
Deep Impact's principal investigator Mike A'Hearn, however, tells SpaceDaily that one major change has been made: its launch has been delayed a year. It was originally supposed to be launched next Jan. 2, make one complete revolution around the Sun, and then return to Earth a year later to make a gravity-assist flyby that would catapult it into a somewhat different orbit allowing it to intercept Tempel 1 on July 4, 2005.
In the event of schedule delays, however, there was always a backup plan allowing its launch to be delayed until the date of the planned Earth flyby, after which it would simply proceed directly to the comet six months later.
This would have been ruled out if the spacecraft had had significant weight increases preventing its Delta 2 booster from being able to shove it directly to the comet without a gravity assist -- but it had no such weight problems, and the cost reduction from not having to fly it in space an additional year allowed the avoidance of further cuts in its ground testing which might have been dangerously risky. NASA will soon announce that its new launch date is Dec. 30, 2004, with the Tempel intercept still set for the following July 4.
As things now stand, if Deep Impact succeeds it will also provide the first good color closeup photos of a comet nucleus. Stardust's camera has overcome initial fogging problems and should provide photos of the nucleus of comet Wild 2 vastly sharper than ever before when it flies by that nucleus at only 150 km range while collecting its coma dust sample, but those photos will be entirely in black and white -- its camera's color filter wheel has become stuck in the "clear" position, and its controllers don't want to try to correct this and risk getting the filter wheel stuck BETWEEN filters.
Moreover, the failure of CONTOUR has set Deep Impact's managers to more careful consideration of an optional extended mission. After its Tempel 1 mission is over, the main craft should still have several hundred meters/second of maneuvering fuel left, enough to mildly modify its orbit and set up a flyby of a second comet later.
Even without the Impactor, such a flyby would allow extremely detailed and useful photographic and IR compositional mapping of that other comet's nucleus. (By contrast, Stardust, after it has dropped off its sample-return capsule at Earth, will be virtually out of fuel and unable to set up any more comet flybys).
Currently the best target -- in terms of low fuel expenditure and minimum flight time -- is comet Boethin. But there are fully a dozen other possible target comets, including Phaethon and Wilson-Harrington -- two "asteroids" which actually seem to be comet nuclei that have now been completely dried up by the Sun (at least in their surface rinds) and are thus inactive.
Consideration of the various targets will be underway for some time. Unfortunately, no trajectory has yet been found that would allow Deep Impact to fly by a third comet or asteroid as well.
At any rate, notwithstanding the wave of bad luck currently bedevilling comet exploration, it still seems to have a bright near-future -- assuming, of course, that no further problems occur. The next crucial period is early next year, when Stardust flies by Wild 2 and Rosetta likely undergoes its delayed launch.
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