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Planetary News: Asteroids and Comets (2008)Rosetta Unearths a "Jewel of the Solar System"by Emily LakdawallaSeptember 9, 2008
"Europe has had its first encounter with an asteroid" was the triumphant announcement made Saturday by Rita Schultz, project scientist for ESA's Rosetta mission. "We can now increase the number of asteroids that have been visited from seven to eight." The newly observed asteroid is (2867) Šteins. At roughly 5 kilometers (3 miles) in diameter, Šteins is a relatively small body. In the first images that resolved its shape, it appeared like a brilliant-cut gem, prompting the camera team's principal investigator, H. Uwe Keller, to pronounce it "a jewel of the solar system." The flyby saw many other "firsts" for ESA. Of particular
importance was the first use of autonomous tracking of a target using optical
navigation, in which a spacecraft analyzes its own images to determine
how to fine-tune its pointing for subsequent images. "It was
a big success, from our point of view," reported Andrea Accomazzo,
Rosetta's operations manager. "We were observing the asteroid,
autonomously, in a closed loop." The autonomous tracking was
necessary, he said, because "we wanted to fly as close as possible
to an object whose orbit was not very well known." He said that
control was handed over to the spacecraft just three hours before closest
approach, and that the spacecraft performed the tracking flawlessly. Autonomous tracking is usually only performed with spacecraft when events unfold too quickly for Earth controllers to be involved. It's necessary in situations where targets are small, encounters are fast, and orbital information imprecise -- all of which are common in missions to small bodies. The first spacecraft to demonstrate autonomous navigation was the technology demonstration mission Deep Space 1, which used it to target asteroid Braille and comet Borrelly. More recently, Deep Impact used autonomous tracking to ensure that its impactor would hit Tempel 1. A total of 15 science instruments was active at different times during the
flyby, 14 of them on Rosetta itself and one on Philae, its piggybacked
lander. These
included four "optical remote sensing" instruments, which investigated
the appearance and surface properties of Šteins from afar, 10 "fields
and particles" instruments that examine the magnetic field, radiation
environment, gas, dust, and charged particles in the space near the asteroid;
and a radio science experiment. In addition, a navigational camera,
considered an engineering instrument, not a science one, also captured
images.
Struggling to Get Science at ŠteinsAccomazzo explained that originally, no instruments were planned to be turned on at Šteins at all. The original plan was to reproduce the flyby dynamics of a future encounter with a much larger asteroid, (21) Lutetia, but then to stop all science activities before closest approach. The reason: the geometry of the flyby risked exposing sensitive areas of the spacecraft -- called "cold faces" -- to direct radiation from the Sun, resulting in unsafe thermal conditions. However, the science team expressed great interest in the possibility of acquiring data during the flyby. The operations team devised a new plan for the flyby that involved pushing the spacecraft to its limits. In order to track the asteroid during the fast flyby (at 31,000 kilometers per hour or 19,000 miles per hour), the spacecraft would have to rotate at the highest speed its reaction wheels would allow. And in order to keep the asteroid within the fields of view of the optical remote sensing instruments, the accuracy of the spacecraft's pointing would have to be improved, despite relatively poor knowledge of Šteins' exact orbit. Yet, at the same time, it would have to keep as little sunlight as possible from striking the cold faces. In order to improve the tracking, a two-pronged approach was required from Rosetta. First, it needed to study faint Šteins from a great distance, picking it out against the background of stars, so that workers on the ground could reduce the uncertainty in Šteins' orbital position. And second, Rosetta needed to be taught to track the asteroid itself during its closest approach. Šteins was first spotted by the high-resolution narrow-angle camera of the Optical, Spectroscopic, and Infrared Remote Imaging System (OSIRIS) instrument from a distance of 26 million kilometers on August 4. The ensuing month permitted mission managers to reduce the uncertainty in Šteins' position greatly, which in turn allowed them to take advantage of two opportunities to correct Rosetta's trajectory, bringing it within two kilometers of its desired flyby point. After two trajectory correction maneuvers, it was judged that Rosetta's path was close enough to the desired target to call off the third and final possible maneuver.
That brought Rosetta to its desired trajectory, but did not solve the problem of sunlight striking the spacecraft's cold faces. To deal with that, Rosetta was instructed to wait until just 40 minutes before its close approach to Šteins, then perform a flip in its attitude that would permit it to roll and follow the asteroid throughout the flyby. The maneuver took 20 minutes to complete, after which control of the spacecraft's pointing was handed over from the sequence written on Earth to the spacecraft's own "brain." The temperature of the spacecraft did rise rapidly as a result of the attitude flip. Most of the instruments continued to take data. One notable instrument found the extreme conditions of the flyby to be out of its set safety parameters, and put itself into a protective "safe mode" nine minutes before closest approach, recovering a few hours later. That instrument was the narrow-angle camera on OSIRIS, which would have produced the highest-resolution images of Šteins. OSIRIS principal investigator H. Uwe Keller explained that the safety parameters had been set conservatively on OSIRIS because, as interesting as Šteins is, it is not Rosetta's primary science target. Unfortunately, the loss of the potential highest-resolution image data resulted in news reports focused on the loss of these data rather than the overall success of the flyby. In fact, Rosetta acted as it was instructed to do when OSIRIS' narrow-angle camera safed, preserving its capability for its much more important future target. Results of the FlybyIf the narrow-angle camera didn't work as planned, the wide-angle camera certainly produced results. It snapped photos of Šteins through many different colored filters throughout the flyby, returning images of a surprisingly brilliant, jewel-shaped body. The narrow-angle camera would have produced images with a resolution approximately five times finer than these.
In fact, the shape of Šteins was not that much of a surprise, Keller explained during the briefing. Ground-based and OSIRIS studies of how much light Šteins reflected as it rotated had produced a shape model that showed "remarkable agreement between the predictions and the observations." However, its symmetry was a bit of a surprise, and Keller was evidently struck by its pretty appearance.
Keller said that the asteroid turned out to be "slightly larger than assumed" at 5.9 kilometers (3.7 miles) east-west by 4.0 kilometers (2.5 miles) north-south, so "the reflectivity has been overestimated" in the past. He put its albedo -- a measure of how much incoming light its surface reflects -- at 35%. Keller also pointed out the very large crater near its north pole. "This crater has a diameter of two kilometers [1.2 miles]. This crater is big enough, together with a crater on the shadowed side, to have shattered the whole body. So we have to assume that the whole body is fractured." Keller pointed to the terminator (day-night boundary) and said, "You also see a chain of craters, about seven craters in a line. Those are a phenomenon we observe on the Moon, they are either produced by a multiple impact or by ejecta [from one impact]. On a small body like this we really have to think about what the explanation of this is." However, this may be an overinterpretation; the craters are suspiciously aligned with the terminator, and may just be a chance alignment of features. The wide-angle camera is capable of color imaging, and below is the highest-resolution color image of Šteins. "It is essentially gray," Keller remarked, though he also said that more color information would be forthcoming from the narrow-angle camera images that were taken up to the time that the instrument safed.
Although the OSIRIS data was all that had been planned for release on Sunday, an exultant Angioletta Coradini, principal investigator for the Visible and Infrared Thermal Imaging Spectrometer (VIRTIS), squeezed a few slides into the press briefing. "We didn't expect to present data today. But we got part of the data and they are really exciting. I am very proud to say that this is the first time that in real time we have calibrated the data. In all previous flybys these came several months later." Coradini continued, "We have a movie mode, where we can take pictures of the asteroid every two seconds. We pushed the camera as much as we could to take pictures as fast as possible. The asteroid was in the center [of our field of view], without any wobbling." VIRTIS has a lower spatial resolution than OSIRIS, so Šteins was never more than a dot. Coradini said, "People were making fun of me because I was excited about that, this small spot in the center of nothing." But the small spot contained plenty of spectral detail, as she showed in a pair of slides. Although she was not prepared to interpret what the VIRTIS spectra meant for the composition of Šteins, she pointed out that the shape was a familiar one, with lower reflectivity at short wavelengths and higher reflectivity in infrared wavelengths, a so-called "red slope" that results from space weathering. Reassuringly, the spectrum contains dips and wiggles that the team will eventually be able to use to interpret Šteins' surface composition.
One other instrument, the Grain Impact Analyser and Dust Accumulator (GIADA), a dust counter, also produced early results. Its principal investigator, Luigi Colangeli, "confirmed that environment around the asteroid is clean, from the concern of very small particles." What's Next for RosettaThe next stop for Rosetta is a return visit to Earth, which it will make on November 13, 2009. It will pass by another asteroid, (21) Lutetia, on July 10, 2010. At 100 kilometers (60 miles) in diameter, Lutetia is much larger than Šteins, affording the possibility of a much longer science timeline during its encounter. Finally, Rosetta will arrive into orbit at its destination, comet 67P/Churyumov-Gerasimenko, in May of 2014, where it will drop its lander, Philae. Its mission there is planned to last at least until December of 2015.
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