Comets are always full of surprises. That’s why we love them so. Take 67P/Churyumov-Gerasimenko, better known as Rosetta’s comet. Back on April 30, the Rosetta spacecraft saw a very active comet with a coma of dust and gas measuring 800 miles across. Even at 4 times the Earth-sun distance, solar heating had begun to vaporize comet ice.
But by June 4, when 67P was 18.6 million miles closer to the sun, activity appears to have shut down. Without its fuzzy coma, the comet looks like one of the many stars in the photo above.
“67P is now almost within our reach – and teaching us to expect the unexpected,” said OSIRIS Principal Investigator Holger Sierks. “After 67P’s onset of activity, our images are currently showing a comet at rest,” he added. OSIRIS is the imaging system Rosetta uses to take photos of its target.
As the sun heats the icy nucleus, cracks develop and expose fresh ice and pockets of trapped gas to the vacuum of outer space. These rapidly vaporize and spew dust and gas like rocket thrusters. Astronomers call them ‘jets’. Jets can turn on and off and new jets can develop as the comet approaches the sun and then returns to deep space. That’s why it’s no surprise to see 67P take a temporary nap.
Usually, once a comet is within Mars distance of the sun, it remains active with an expanding coma and ever growing tail. We have much to look forward to. Currently, Rosetta’s comet is only a pixel across, but in just a few weeks, the OSIRIS camera will discern the shape of the nucleus. Things are heating up – both for the comet and for us watching the show.