The European Space Agency's Rosetta mission left earth in 2004 and 10 years later arrived at its object of study, a comet heading inbound from deep in the solar system. Early Thursday morning, Comet 67P/Churyumov-Gerasimenko made its closest approach to the sun (also known as its perihelion), whipped around it, and has begun its long journey back out, deeper into the cold.
Don't expect instantaneous photos that inspire awe across languages and national boundaries, as we've seen this year from spacecraft observing the dwarf planet Ceres, the dwarf planet Pluto, and earth itself. What scientists are looking for may be awe-inspiring; it's just a different kind of awe.
Rosetta is looking at cosmic chemistry. In the wake of 67P's approach, the sun's energy will burn away its ice and dust faster and faster. Scientists expect a kind of heat lag. The comet's warmest temperature, and peak activity, will come after perihelion—just as the hottest day of summer comes a couple of weeks after the summer solstice.
From 115 miles away, instruments have already snapped photos of increased "off-gassing" during the comet's approach to perihelion:
Post-perihelion, Rosetta will be monitoring the comet with its cameras and sniffing for new molecules. Data from the mission's surface probe, Philae, published in July, have already turned up chemicals that scientists hadn't previously seen on a comet. One is acetone, a chemical detected elsewhere in space that, closer to home, is an ingredient in nail polish remover.
The solar system contains pockets of basic organic chemistry—small molecules of carbon, hydrogen, and nitrogen—that we think once gave rise to living things. Watching Comet 67P, Rosetta should return additional evidence to scientists as they try to figure out how this ball of ice and dust—and by extension, the solar system (and by extension, earth and its inhabitants)—all came together.
This story was updated after Comet 67P reached perihelion.