In just a few days, the Rosetta orbiter will end its two-year mission by crash landing on Comet 67P/Churyumov–Gerasimenko. The European Space Agency spacecraft will join its ill-fated lander Philae, which was recently located after dying and evading detection for nearly two years after its historic landing on the comet's surface.
As Comet 67P's orbit takes it farther away from the sun, Rosetta's solar panels are exposed to less and less light to use as energy. The mission team considered putting Rosetta back into hibernation until the comet swings back around in four or five years – the spacecraft slept for part of its journey to the comet, waking back up as it came close to the lump of primordial dirt and ice – but have decided that the risk of Rosetta failing to wake up is too great to justify extending the mission.
“There’s no point putting an old experiment with old [principal investigators] into hibernation,” mission scientist Kathrin Altwegg joked to Nature in 2015.
But Rosetta will be doing science until her last moments: On Sept. 30, the robot will descend to a mysterious cometary region known as Ma’at. Located on the smaller of 67P's two lobes (the comet is shaped sort of like a rubber ducky), Ma'at is home to several "active pits."
Comets like 67P are mainly studied because of the materials locked away in their icy cores: These objects formed at the dawn of the solar system, and their far-flung orbits have kept the sun from degrading those original building blocks.
Scientists hope that studying these pristine materials can help unlock the secrets of the early solar system. When comets are warmed by proximity to the sun, they become "active" as parts of their surface begin to spew gas into space. The Ma'at region contains several pits that are hundreds of feet wide and more than 100 feet deep, some of which have been seen to spew jets of gas and dust.
“The heat of the sun, spreading through the comet, warms underground deposits of carbon dioxide and carbon monoxide ice,” Mark McCaughrean, a senior science adviser at ESA, told the Guardian. “These deposits then evaporate, leaving caverns whose roofs collapse. Inside them, you can see features we call goosebumps or dragon’s eggs that could be primordial objects from which the comet formed. So we’re taking Rosetta down to study these.”
After finishing up one final flyover on Sept. 24, Rosetta will begin a series of course corrections designed to send her down into a pit dubbed Deir el-Medina. She'll go slowly – not much faster than a casual amble down the street, and slowly enough to send home pictures of the mysterious pit as she approaches it – but the impact is still expected to damage the spacecraft enough to end communications with Earth.
“That will be the end of Rosetta,” McCaughrean told the Guardian. “It will be an emotional moment. This has been a wonderful mission and a great team effort.”
Philae's premature death notwithstanding, Rosetta has been an incredibly successful mission. Researchers have used the orbiter's data to help disprove a long-held hypothesis that Earth's water might have been carried to the newborn planet by comets. But the discovery of certain life-giving organic molecules suggests that comets such as 67P may have seeded our planet in other ways. Even after Rosetta dies, researchers will continue poring over the orbiter's data for months and years.
But while Rosetta's days are numbered, the spacecraft's work isn't over. Scientists say the orbiter's toughest navigational challenges will come in these final days.
“Although we’ve been flying Rosetta around the comet for two years now, keeping it operating safely for the final weeks of the mission in the unpredictable environment of this comet and so far from the Sun and Earth, will be our biggest challenge yet,” Sylvain Lodiot, ESA’s spacecraft operations manager, said in a statement.
“We are already feeling the difference in gravitational pull of the comet as we fly closer and closer: it is increasing the spacecraft’s orbital period, which has to be corrected by small manoeuvres. But this is why we have these flyovers, stepping down in small increments to be robust against these issues when we make the final approach.”