The European Space Agency's $1.7 billion Rosetta probe, flying in close formation with a slowly tumbling comet, is poised to release a small lander Wednesday that will attempt an unprecedented touchdown on the boulder-strewn, debris-spewing nucleus, using ice screws and harpoons to keep from bouncing off in the comet's feeble gravity.
Assuming final clearance to proceed, the seven-hour descent is expected to begin with the Philae lander's release from Rosetta at 3:35 a.m. EST (GMT-5) Wednesday. Touchdown somewhere within a 1,600-foot-wide landing zone is expected about 40 minutes to either side of 10:34 a.m. At a distance of 317 million miles from Earth, it will take 28 minutes for radio signals confirming touchdown to reach anxious flight controllers in Europe.
"You won't gain anything without taking risk. exploration is all about going to the limits, exploring the envelope," said Mark McCaughrean, senior advisor with ESA's Directorate of Science and Robotic Exploration. "We've been past comets before, but nobody's ever actually dared to stop next to one, orbit around it, and of course naturally, you're going to want to land on it.
"But we know how difficult that is. Everybody's nervous, everybody's on tenterhooks, but we know the risk is worth taking, the rewards are enormous. But we also know we have a mission which is working fantastically and that we'll go into 2015 in great shape to do brilliant science next year as the comet lights up as it gets closer to the sun."
Philae's touchdown, he said, is "part of a fantastic mission" that will continue regardless of the lander's success or failure.
"I think it's one of the major achievements in the history of spaceflight, no matter how it goes tomorrow," said Andrea Accomazzo, Rosetta spacecraft operations manager. "We've done everything we could to prepare for tomorrow, but exploration implies risk. ... If you're not ready to take the risk, then you shouldn't do exploration."
Philae weighs just 220 pounds and is shaped like a cube with each side about one yard across. It is equipped with 10 compact instruments and cameras that will document the spacecraft's approach to the nucleus and collect data about the immediate environment. Once anchored to the surface, a 360-degree 3D panorama will give scientists a detailed look at the landing site, along with microscope views of the soil directly beneath the lander.
"We're then going to have a number of little laboratory experiments that will measure the gas and the dust, the organic material and the plasma coming away," McCaughrean said earlier. "And then we're actually going to drill beneath the surface to look at material just under the surface and melt some of that material, we'll dig (it) up and bring into the spacecraft."
But first, Philae has to get there. And despite the best efforts of the engineers and scientists who designed the spacecraft, it will need a bit of luck.
Comet 67P/Churyumov-Gerasimenko measures about two-and-a-half miles across and its gravity is roughly 100,000 times less than Earth's. Philae, weighing about the same as a sheet of paper at the comet's surface, will hit the nucleus at a walking pace, or a little more than 2 mph.
To keep it from bouncing off, ice screws in its three landing legs will drive into the dusty soil while two harpoon-like snares fire downward to get a firmer grip. A cold-gas thruster atop the lander will fire at the same time to counteract any recoil.
But a major unknown is the nature of the landing site. Philae is not equipped with maneuvering thrusters or any sort of hazard avoidance system. Once released from Rosetta, it will fall to the comet along a trajectory that cannot be adjusted to avoid boulders, cliffs or other terrain that might pose a threat.
The 1,600-foot-wide landing zone is defined by how accurately flight controllers know Rosetta's location and velocity at the moment of Philae's release. Given the comet is slowly tumbling and Rosetta itself is moving along a complex trajectory, it's a bit like a quarterback running out of the pocket and trying to hit a receiver in the end zone who's running in a different direction at a different pace. Once the ball is thrown, its trajectory cannot be changed.
Rosetta will be about 14 miles away from the nucleus at the moment of Philae's release and errors in the mothership's position and velocity will translate directly into where in the landing zone -- or beyond it -- Philae might land.
And 67P/Churyumov-Gerasimenko is not a garden-variety potato-shaped comet with broad expanses of smooth terrain and plenty of margin for error. In a major surprise, pictures taken during Rosetta's approach earlier this year revealed a bizarre body vaguely resembled the outline of a rubber duck, with a thin neck connecting two bulging lobes with steep 500-foot-high cliffs, boulder fields and deeply cratered terrain.
Finding a suitable landing site for Philae was a major challenge and mission managers selected the best choice out of five candidates. None of them was ideal, but mission managers said Tuesday the lander had a much better than even chance of getting down and anchoring itself in place for long-term observations.
"If you decide you're going to land on an object you know nothing about 10 years, 20 years ago, then you run risk," Fred Jansen, the Rosetta mission manager, told reporters. "It's not a nice, round potato, it's rough, it's more difficult. But we've analyzed the terrain, we've analyzed the comet, and we're confident that the risks we have are still in the area of 75 percent success."
Putting the landing in perspective, he said Rosetta's observation will amount to about 80 percent of the expected science return for the overall mission while Philae's contribution, if successful, will be about 20 percent.
"Of course, we are here for the landing," he said. "But we shouldn't forget that Rosetta has a mission. It's already done a vast amount of science. ... The landing will be the cherry on the cake, connecting what we see in orbit with what we see on the ground."
Discovered in 1969, 67P/Churyumov-Gerasimenko circles the sun in an elliptical orbit extending nearly 500 million miles from the sun at its far point -- beyond the orbit of Jupiter -- to a point between the orbits of Earth and Mars some 115 million miles from the sun. The comet measures 2.5 miles across and rotates every 12.4 hours.
Like all comets, 67P/Churyumov-Gerasimenko is a frozen remnant of the primordial material used to form the sun and planets 4.6 billion years ago.
"When the solar system was forming out of gas and dust, it formed the planets, the one we live on today, it formed asteroids and it formed the comets," said McCaughrean. "And the comets are a remnant, therefore, something we can investigate about the very earliest phases of the evolution and the birth of our own solar system."
The ice found in comets can "give us great clues to the origin not only of our own solar system, but potentially even life," he said. "Because we know that comets also contain organic molecules, the building blocks of even DNA and RNA. We know that there are amino acids in comets, for example. So comets play a key role in our understanding of the cycle of star formation, planet formation, perhaps life formation."
Rosetta is the most ambitious comet research mission ever attempted. Launched on March 2, 2004, Rosetta had to carry out four planetary flybys, using the gravity of Earth and Mars in repeated passes to pump up the spacecraft's velocity enough to move out into deep space and catch up with 67P/Churyumov-Gerasimenko.
On the way, Rosetta flew past two asteroids, sending back high-resolution pictures and other observations, and spent two-and-a-half years in electronic hibernation while it closed the gap with its target. On Jan. 20, Rosetta woke itself up for the final leg of its journey, matching 67P's orbit on Aug. 6.
Koen Geurts, Rosetta lander technical manager with the German space agency DLR, said getting Philae to the surface is "a one-shot opportunity."
"Once landed, Philae will start to conduct non-stop science activities for 48 hours," he said earlier. "After this 48 hours, the Philae batteries will be depleted. But in these 48 hours, for example, a drill to take comet surface samples will have the opportunity to drill roughly 30 centimeters (one foot) below the comet's surface, extract tiny particles of this pristine comet material, take it up on board the lander, deploy it in one of the on-board ovens ... for analyzing the comet material."
In addition, a thermometer will be "hammered into the surface. This is in order to evaluate the comet's thermal characteristics during its rotation around its axis." Other instruments will characterize the dust environment and even the comet's interior by measuring radio signals sent back and fourth between Rosetta and the lander.
Scientists hope Philae will remain operational for three to four months of close-range observations as the comet warms up and becomes more and more active. But that will depend in large part on how much debris jets away from the comet and whether Philae's solar panels can get enough sunlight to keep its instruments powered up.
Eventually, the comet will warm up enough to cause problems and "at some point, the systems will overheat and this will give a natural end to the Philae mission, Geurts said. "At this point it is, of course, hoped that Philae has contributed significantly to the understanding of comets and their evolution on their path around the sun having conducted this unprecedented scientific in situ measurements."