How do you land a spaceship on a comet?

Getting There Is Half the Hair-pulling
Artist's impression of Rosetta arriving at comet 67P/Churyumov-Gerasimenko in August 2014
Image by C. Carreau, courtesy of ESA

Catching a speeding comet, let alone landing on one, requires trick-shot billiards on an astronomical scale. Imagine whipping a ball bearing in a circle on the end of a string. Now picture trying to hit the bearing with another string and ball bearing. Now try this on for size: If that string measures 1 yard (0.9 meters) long, then the two ball bearings would measure a scale equivalent of 10 nanometers and 4 picometers, smaller than an antibody molecule and a hydrogen atom.

Now let's talk speed and power. Rosetta is an aluminum box measuring 9.2 x 6.9 x 6.6 feet (2.8 x 2.1 x 2.0 meters) and weighing about 6,600 pounds (3,000 kilograms) at launch. Flight scientists needed the craft to catch comet 67P/Churyumov-Gerasimenko, a lopsided lump measuring roughly 2 x 3 miles (3 x 5 kilometers) and traveling at a clip of up to 83,885 mph (135,000 kph) [sources: ESA; ESA].

There's just one problem: We don't make spacecraft capable of doing this. Instead, Rosetta first launched into a parking orbit around Earth on an Ariane 5 rocket. Next it departed on a 10-year loop-the-loop mission through the solar system, borrowing speed from gravitational slingshots past Mars (in 2007) and Earth (in 2005, 2007, 2009). While traversing the main asteroid belt, Rosetta also made observations of asteroids 2867 Steins (Sept. 5, 2008) and 21 Lutetia (July 10, 2010) [sources: ESA; ESA; ESA; ESA].

Screaming along on a curving intercept course, the hibernating Rosetta awakened as it approached its rendezvous around 3.5 AU from the sun. Because it came in hot, it spent January to May 2014 periodically tapping its braking thrusters, ramping down to a relative speed of 6.6 feet per second (2 meters per second). By August, when it inserted itself into orbit, that velocity dropped further, to a few centimeters per second [sources: ESA; ESA].

Then, like a wedding photographer, the craft spent some time dodging around, snapping photos and looking for the best illumination conditions. European Space Agency mission control used these shots to calculate the comet's position, size, shape and rotation. Once in orbit, Rosetta mapped the comet and observed spin-axis orientation, angular velocity, major landmarks and other basic characteristics -- everything necessary for plotting five potential landing sites [sources: ESA; NASA].

In November, Rosetta released its lander from about 0.6 mile (1 kilometer) over the comet. Philae was slated to touch down at human walking speed, using its flexible legs to dampen its rebound and a harpoon to anchor it against the comet's low gravity, but the landing didn't go quite as planned. From there, it will ride the comet in and around the sun, carrying out observations as long as it can. The mission is scheduled to complete in December 2015 [sources: ESA; ESA; NASA].