The spacecraft Rosetta, which has been looping around a comet for the past 2 years, made history today in a spectacular swan song, exploring in extreme close-up active jets, sinkholes, and mysterious nodules of its companion before intentionally crashing into the drifting chunk of rock and ice.
Two years ago, the European Space Agency’s Rosetta became the first spacecraft to orbit a comet and, 3 months later, the first to deliver a lander, named Philae, to a comet’s surface—specifically, that of comet 67P/Churyumov-Gerasimenko.
During the final seconds of a 13.5-hour descent that began yesterday at an altitude of about 20 kilometers above the comet, Rosetta took images of 67P that revealed structures smaller than 2 centimeters in width.
What’s more, Rosetta did so in a race against the clock, transmitting those pictures, as well as data on the comet’s dusty shroud, gas, and ionized particles, just before it collided with comet 67P and fell silent forever.
Because the comet, slightly taller than Mount Fuji, now lies 724 million kilometers from Earth, ground engineers had to wait 40 minutes to find out if their receivers had collected those last, precious bits of data. Confirmation that Rosetta flatlined—its carrier signal lost forever—came at 7:19 A.M. EDT on 30 September at the European Space Agency’s mission control center in Darmstadt, Germany, revealing that the crash occurred at 6:39 A.M. EDT.
Public release of all the final data won’t happen for several months, so that the science teams can properly calibrate the information and clarify the location where each data point was obtained, Rosetta scientist Mathieu Choukroun of NASA’s Jet Propulsion Laboratory (JPL) in Pasadena, Calif., told Eos.
Goose Bumps
The crash site, located on the “head” of the rubber duck–shaped comet, lies near a pit, an apparent sinkhole within the Ma’at region, where several of the comet’s dust jets originate.
The walls of this and other pits on the comet feature meter-sized spherical nodules, dubbed goose bumps or dinosaur eggs, that may be some of the primordial chunks of ice that scientists believe clumped together soon after the birth of the solar system to form the two-lobed comet. After close scrutiny, the close-up images may reveal new insights about that formation process.
“Some of us believe that the high-resolution images of the walls of these mysterious holes may carry information, [about] the goose bumps, from the time of formation of this comet,” said Rosetta scientist Eberhard Gruen of the Max Planck Institute for Nuclear Physics in Heidelberg, Germany. “However, I am convinced that it will take a long time, if at all, until we really understand what we will see in those last images.”
Comparing the close-up images taken by Rosetta with those recorded by Philae, the craft’s short-lived lander, will provide further insight into the small structures on the comet, said Rosetta researcher Marcello Fulchignoni of the Paris Observatory in Meudon.
During the first part of its descent, Rosetta imaged several regions of the comet’s nucleus and then focused its gaze on the inside of the Ma’at pit nearest its landing site, said Choukroun.
Some of the final images revealed what appear to be the first close-up views of cracks in the comet’s crust, Rosetta scientist Bonnie Buratti of JPL told Eos. The cracks could indicate where interactions with the solar wind or the activity of jets shooting out from the comet heated the icy surface, she said.
Collecting Data to the End
Among the 8 of Rosetta’s 11 instruments operating during the grand finale, the radio science instrument gathered data to measure the gravity field and mass distribution of the comet at a new level of precision. This may help determine whether the comet’s two lobes have a common origin, providing new insight about how the body assembled.
Data acquired from the craft’s Microwave Instrument for the Rosetta Orbiter (MIRO)—essentially a microwave receiver—are expected to reveal the temperature of the comet’s subsurface around the crash site to a depth of about 4 centimeters, with an unprecedented spatial resolution. Measurements recorded by the craft’s ultraviolet spectrometer, ALICE, are expected to provide new details about the composition of the comet’s surface.
Rosetta has already revealed that the surface of 67P/Churyumov-Gerasimenko is covered in dark, organic compounds and has a surprising lack of highly reflective water ice—entirely unlike the classic “dirty snowball” model of a comet, noted Alan Stern of the Southwest Research Institute in Boulder, Colo. Yet this ice, which evaporates easily, driving out the comet’s vivid jets of gas and dust, must reside within 67P/Churyumov-Gerasimenko, he said. The new ALICE data may reveal where these ices reside just below the surface, Stern added.
Lost Lander
One thing Rosetta couldn’t do during its final hours was to photograph Philae, which until recently had gone AWOL. The lander touched down on 12 November 2014 but ran out of power after just 3 days.
Rosetta spotted Philae on 2 September, wedged into a dark crack on the comet, in images taken by the craft’s high-resolution camera. Pinpointing the location of the lander—about 2 kilometers from Rosetta’s crash site—was critical for comprehending the data transmitted by Philae.
Time to Say Goodbye
The European Space Agency decided to bid farewell to Rosetta with a controlled crash because the agency knew that the comet, and therefore the orbiting craft, would be moving toward Jupiter, away from the Sun, in 2016. Rosetta’s solar panels would produce a dwindling supply of energy. Although engineers could have put Rosetta in hibernation until the comet neared the Sun again in about 4 years, scientists deemed it unlikely that the aging craft, which was launched in 2004, would awaken from slumber.
Because the impact was gentle, occurring at a pace akin to a slow walk, the craft did not disintegrate but will endure on the icy body, not far from the resting place of its defunct lander.
“It’s bittersweet,” said Buratti. “We have all [these] data but we’re never going to be talking to Rosetta again.”
—Ron Cowen, Freelance Science Journalist; email: [email protected]
Correction, 4 October 2016: An earlier version of this article incorrectly included both water ice and nitrogen ice in a statement attributed to a scientist. The statement has been updated to allude to water ice only.