A rendezvous with a comet — and our origins

The historic landing of the European Space Agency’s Philae probe on a comet’s surface is a stunning achievement that has rightly generated tremendous interest.

Images from Philae and its mother ship, Rosetta, reveal Comet Churyumov-Gerasimenko’s bizarre and alien beauty in detail never before seen of a comet. Even if Philae proves to be short-lived following its accidental bouncing across the comet’s surface into the frigid shadows of an icy cliff, it will always remain an amazing triumph. But the science of comets is so outside ordinary experience, and the scales of time and distance involved so mind-bogglingly great, that it may be difficult to put these feats in any comprehensible context.

First, we must recognize that comets are special. These icy bodies were among the first objects to form in our solar system 4.6 billion years ago. Originating in the cold region of the giant planets, trillions of comets were flung away from the sun by those planets during naturally occurring gravitational “slingshot” maneuvers. Those comets that were thrown almost but not completely out of our solar system remained exiled in deep-freeze conditions for billions of years.

Given this history, it has long been thought that comets are treasure troves of information about our solar system’s origin. Their icy compositions, primarily frozen water enriched with complex hydrocarbons, were likely the source of some of Earth’s oceans and atmosphere and may have seeded our planet with the fundamental building blocks of life. But even the best telescopes have only seen comets as blurry points of light, and we have only been able to make informed guesses as to their detailed makeup. Even past space probes like NASA’s Deep Impact comet collision or its Stardust sample-return mission, which brought back microscopic bits of comet dust, have given us only an incomplete view. This is why a close-up, prolonged visit is so important.

But it’s incredibly difficult to rendezvous with comets. Because they fall toward the sun from great distances, comets travel very quickly as they pass through the inner solar system, as fast as 84,000 mph in the case of Churyumov-Gerasimenko. Matching this speed would have required more fuel than any launch system could have carried. Instead, Rosetta was sent on a vast, looping decadelong journey through our solar system, making three close passes by Earth and one past Mars to get successive gravity-assist speed boosts. The journey was precise, too. After traveling 4 billion miles, Rosetta entered orbit around the 2.5-mile-wide comet in August 2014. These numbers confound rational analogies: One could say that Rosetta’s targeting was the equivalent of throwing a basketball twice the distance to the moon and hitting nothing but net.

Finally, the wonderful investigations being performed by Rosetta and Philae are being carried out by robots. The discoveries are made by humans — but the remote explorers are machines. The tremendous excitement surrounding Philae’s landing, and the Curiosity Rover on Mars a couple years ago, show that human explorers are not required to generate interest in space. Human missions may make sense when they support a path forward to a sustained human presence in space, but they are required neither for productive cutting-edge science nor exciting public relations.

As alien as comets may seem, they are part of our human origin. Explorers like Philae may be mechanical, but they advance human exploration. And these amazing scientific advances, no matter how far they are made from home, are still expansions of human knowledge that let us better know who we are and where we came from.

Mark Hammergren is an astronomer and director of the Astro-Science Workshop at the Adler Planetarium in Chicago.