Somewhere dark and icy on a comet 320 million miles away, the history-making, comet-bouncing Philae spacecraft is sleeping. Its batteries are depleted and there isn’t enough sunlight to recharge. But while the lander finished its primary job, collecting invaluable data on the surface of comet 67P/Churyumov-Gerasimenko, the Rosetta mission is far from over. For many scientists, the excitement is just beginning.
Philae’s landing two weeks ago was a wild one. The washing-machine sized spacecraft dropped right onto its intended landing site, but the harpoons designed to anchor it into the ground didn’t fire. Without anything to latch onto the surface, the spacecraft bounced back up a kilometer into space, soaring for nearly two hours before returning to the ground. After another smaller bounce, Philae settled somewhere in the shadow of a cliff, at least 1 kilometer from where it was supposed to be.
Mission engineers are now scouring the comet for signs of the lander. They’re using the OSIRIS camera onboard the Rosetta spacecraft that’s orbiting the comet to look for any glint of brightness reflected by Philae, says planetary scientist Sebastien Besse, a member of the OSIRIS team. They’re also using data from the CONSERT instruments on Rosetta and Philae, which send radio signals between the two spacecraft, to triangulate the lander’s location.
Once they find Philae, mission engineers can better assess the chances that it will eventually receive enough sunlight to recharge, wake up, and do more science. “The chances seem reasonably good,” said Mike A’Hearn, a planetary scientist from the University of Maryland and a member of the Rosetta science team. So far, the team has narrowed down Philae’s location to a sliver about 100 feet wide and 1,150 feet long near a depression on the south side of the comet, where it’s now winter.
But summer is coming. Over the next few months, the changing seasons will bring more direct sunlight onto Philae. The comet is also moving toward the sun, and the hope is that in the next few months, both the coming summer and increasing proximity to the sun will give Philae the warmth and power it needs to wake up. Mission controllers have done everything possible to give Philae a chance, Besse says. Before Philae shut down, they rotated the lander 35 degrees to orient its solar panels toward the sun.
For now, all we can do is wait. “I’m very confident that Philae will resume contact with us and that we will be able to operate instruments again,” said lander team leader Stephan Ulamec in a statement on Nov. 17.
Of course, there’s a lot of uncertainty, and Philae will need quite a bit of luck, Besse says. Still, the team is hopeful. “In this business, you have to be optimistic,” he said.
Despite the unexpected triple landing and its current MIA status, Philae did the science it was supposed to do. It ran its preprogrammed sequence of commands and started collecting data with its suite of instruments, sniffing, hammering, drilling, and even listening to the comet. Powered by only 60 hours of battery life and with its ability to recharge in doubt, Philae beat the clock and sent all the data back to Earth before its battery petered out.
Scientists are still busy analyzing the bounty of data, but they’ve already released some preliminary results. Philae has detected organic molecules, which are necessary for life. One of the reasons scientists want to study comets is that the icy bodies could have delivered the organics and complex molecules needed for life when they slammed into the planet early in its history.
Past comet missions and ground-based telescopes have seen dozens of molecules on comets, including organics. For example, this summer, the Atacama Large Millimeter/submillimeter Array telescope in Chile found organic molecules in the atmosphere of comets ISON and Lemmon. But what kind of molecules did Philae detect? That remains to be seen.
One instrument enabled Philae to hammer into the ground and find that the surface underneath the lander is surprisingly hard, likely made of ice. Philae also measured vibrations created when the feet of the lander hit the ground on the first landing, producing the first-ever audio recording of a comet touchdown. An analysis of the recording suggests a layer of soft dust sits on top of the hard, icy surface.
The lander was also supposed to drill into the comet and deliver samples into its ovens, which would analyze the chemical composition of samples. Both the drill and the ovens worked perfectly, but it’s unclear whether Philae was able to drill anything at all. The team is still analyzing the data to see what, if anything, the ovens measured. Initial indications don’t look good, however. According to a tweet on Nov. 17 from Science magazine’s Eric Hand, Fred Goesmann of the Max Planck Institute for Solar System Research and leader of the COSAC instrument said that the drill didn’t deliver any samples into the ovens. “There’s nothing in it,” Goesmann was quoted as saying.
Indeed, the botched landing did compromise some of the science. For instance, accelerometers and thermometers on the harpoons never deployed, so couldn’t gather any data. “It’s unfortunate the lander didn’t do exactly what it was supposed to do,” said planetary scientist Anita Cochran of the University of Texas at Austin, who is not a part of the Rosetta mission. Still, she says, Philae got loads of important information. “Whatever they get is way more than we had,” she said.
In the days surrounding Philae’s landing, the Rosetta spacecraft collected scientific data from afar. Much of those results have been submitted into scientific journals and will likely be published in the next couple weeks, Besse says. But Rosetta’s main job has been to support Philae, scouting out possible landing sites, and watching over the lander as it settled onto the comet. Now, the real science begins for Rosetta.
As part of a maneuver to adjust its orbit, the spacecraft will fire its thrusters to lift it to 19 miles from the comet. On Dec. 3, it will move closer until it’s 12.5 miles away. Rosetta will remain in orbit, watching the comet come to life as it approaches the sun, reaching its closest point in mid-August. The ices on the comet will heat up, sublimating into gases that are ejected into space. The rubber-duck-shaped chunk of ice and dust will be enshrouded in a haze of dust and gas called the coma. The sunlight will push the dust and gases away and form the comet’s tail.
And Rosetta will be right there watching the action.
Previously, spacecraft have visited seven different comets, but nearly all missions were quick flybys. In 2005, the Deep Impact mission fired an impactor into comet Tempel 1, blasting a cloud of debris that could then be analyzed. The Stardust spacecraft, which went to comet Wild 2 in 1999 and grabbed a sample of its tail to return to Earth, swung by comet Tempel 1 in 2011 for a closer look at the crater created by Deep Impact.
All these missions studied a comet at a single point in time, capturing just a snapshot. But comets are dynamic objects; their characteristics are defined by change. They suddenly appear in the sky, growing brighter and brighter, its tail stretching longer and longer. Then, just as sudden as they appeared, they shrink and fade. Now, for the first time, Rosetta will be able to observe what is actually happening on the comet up close.
For example, Rosetta will be able to see how exactly dust and gas escapes the comet and how this varies from place to place, says A’Hearn. In doing so, scientists can distinguish features that are due to the comet’s evolution over time from primordial features that were a part of the comet since its formation. Pinpointing those properties, A’Hearn says, is essential for understanding how comets form, the history of the solar system, and whether comets could have delivered the chemicals needed for life on Earth.
Rosetta will also probe the interior of the comet, mapping the different layers of ice and dust and how its density varies. Another question, Cochran says, is how comet 67P’s shape will change over time. Will the neck connecting the two lobes whittle away? Will the comet eventually split apart? Is the comet the result of two pieces that stuck together?
Rosetta’s mission will continue until December 2015, following comet 67P as it swings back away from the sun. The hope is that the spacecraft will keep working deep into 2016. But that will depend on the unpredictable volatility of the comet, Besse says. Dust particles expelled by the comet could damage the spacecraft. The comet could belch out gas that blows Rosetta off course. Or, Rosetta could just wear down. It is, after all, already 10 years old. In that time, many of us have already gone through several computers and phones. But so far, Besse says, Rosetta seems to be in great shape.
So we’ve landed on a comet. And we’re now orbiting one for the first time. What’s next? Scientists are already planning future comet missions, which most likely will involve another lander at the very least. One idea is for a spacecraft to hop from place to place on a comet—this time, on purpose—and study differences on the surface. One such proposed mission, the Comet Hopper, made it to the final rounds of NASA's selection process in 2012 before losing out to a Mars lander called InSight, which is slated for launch in 2016.
Missions like Comet Hopper and Deep Impact were NASA Discovery missions, which are intended to be faster and cheaper projects. For the next Discovery mission, there are at least three proposals for sending a spacecraft to a comet, says A’Hearn, who led the Deep Impact mission and was a part of the Comet Hopper proposal.
But what comet scientists would really want is a sample return mission: Send a spacecraft that could grab a chunk of comet and send it back to Earth. The kinds of experiments you can do in Earth-bound labs are much more sophisticated than anything onboard a spacecraft, Cochran says. But such a mission would be difficult and expensive. For example, Besse adds, you would have to build a cryogenic capsule to keep your comet stuff cold. And comets are cold. In August, Rosetta measured comet 67P’s average temperature to be -94 degrees Fahrenheit.
Because of the added complexity and expense, a sample return mission would have to be one of NASA’s higher-cost New Frontiers missions. One of those missions, New Horizons, will begin exploring Pluto and its moons in 2015. Another, Juno, will arrive at Jupiter in 2016. “I expect that there will be at least two separate proposals for a comet surface sample return mission in the next round of New Frontiers,” A’Hearn said. These future missions will be needed, he says. Rosetta will answer many questions about comets, but it will also raise many more.
Until then, Rosetta has center stage. And the show is about to begin.
