August 13 marks an important milestone in comet chaser Rosetta's mission: perihelion. The spacecraft's target comet, 67P/Churyumov-Gerasimenko, made its closest pass by the Sun in the early hours of the morning (European time).
That means more light cast on its mysterious surface, and an uptick in comet activity such as outbursts of dust and gas.
"The perihelion by itself is more a symbolic date," Nicolas Altobelli, deputy project scientist for Rosetta, said in a phone call. "I don't expect really everything to explode on the perihelion day."
Though if that did happen, he said, "It would be quite interesting actually because you would really see the interior material."
Perihelion is the single moment in 67P's 6.5-year orbit that it's closest to the Sun, at 186 million km distance (which is still further from the Sun than Earth is). But activity on the comet generally increases across the stretch of time before and after it.
The most dramatic thing that could happen would be if the comet splits.
On Tuesday, the European Space Agency reported a particularly dramatic jet bursting out of the rubber ducky-shaped comet's "neck" at the end of July. These jets are caused by icy parts of the comet warming in the Sun and turning to gas, and they're what cause the trademark "tail" you picture when you imagine a comet streaking across the sky.
The maximum activity is expected a few weeks after perihelion, as the heat takes a while to penetrate beneath the comet's surface, to where it may be more volatile.
The most dramatic thing that could happen would be if the comet splits—perhaps along that "neck," which is cracked across the middle.
"This is unlikely, because the comet has survived eight perihelions of this distance since its last encounter with Jupiter in 1959," Altobelli said. That encounter with Jupiter set 67P in the orbit it's more or less still in, with a closer perihelion distance than it had before.
"What is more likely, is that you get more of the outbursts like the one we have seen recently: collimated jets of dust and gas that we have been expecting, actually, since the comet woke up," he said.
One of the most exciting things about the perihelion period is that parts of the comet that are normally in darkness will be illuminated. This could help scientists discover more about the comet's history.
Altobelli explained that one reason we're so interested in comets is because they can provide a relatively "pristine" record of the early solar system. "But what is unclear is how this original state was modified by the comet history," he said.
It's unclear, for example, how 67P got its shape. By studying the comet activity from a more dormant period through perihelion and onwards, researchers hope to recover more information about how this comet, and comets in general, works.
Of course, while Rosetta's instruments observe from afar, little Philae, the lander that got lost, is still somewhere aboard 67P. ESA is still attempting to reestablish contact with Philae when it can, having last heard from it in July.
But, Altobelli assured, Philae "will not be baked," despite the increasing temperatures on the comet. As the lander is known to be in the shadow, it should be OK. Stay safe, Philae.