Earthlings, We Landed on a Comet

A decade in the making, Wednesday marks the culmination of the European Space Agency’s Rosetta comet mission—and the seventh world humans have landed robots on.

Up to today, humanity has landed robotic spacecraft on six other worlds: the Moon, Mars, Venus, Saturn’s moon Titan, and the asteroids Eros and Itokawa. Now we’ve added a seventh, as the Philae lander touched down on Comet 67P/Churyumov—Gerasimenko, the first time we’ve ever landed a spacecraft on a comet.

Philae (pronounced “fee-LAY”) is part of the larger European Space Agency (ESA) Rosetta comet mission, which was launched 10 years ago and arrived at Comet 67P in August. In other words, today’s landing is the culmination of more than a decade of research, planning, and—most stressful for those involved—waiting. At the time of writing, we aren’t sure how well Philae survived the touchdown: the evidence is that it bounced slightly upon impact, with signs that the harpoons it should use to anchor it to the comet surface didn’t fire as they should. (We’ll update this article when we have more information.)

UPDATE: Since publication yesterday, the Philae team learned that the probe bounced two times and apparently landed on its side. While on Earth that would certainly end the mission, bouncing on Comet 67P was a slow process, with about two hours passing between subsequent hops. That means it probably flew back up as much as a kilometer before coming back down.

The challenge at the time of this update is to actually figure out where Philae landed and to make sure everything is in working order. The probe appears to be sitting at the bottom of a "cliff" on the comet, but beyond that it's hard to tell. Whether the probe can be tipped upright is another question, and since it isn't anchored to Comet 67P as planned, Philae probably won't be able to drill into the surface. However, the probe stayed in contact with the Rosetta orbiter and has already sent back some photos.

Philae is roughly the size of a kitchen stove, and bristles with scientific instruments to do everything, including take photos, measure the comet’s chemistry, and get a sense of the internal structure. Since this is the first time we’ve landed a probe on a comet, there are a number of questions scientists would like to answer: How solid is it? What is it like on the inside? What is its full chemistry, including the types of organic molecules we’ve detected on other comets?

In that mission, Philae follows the Deep Impact probe, which did not include a lander. Instead, Deep Impact fired a heavy projectile at Comet Tempel 1 and studied both the debris kicked up by the collision, and the material exposed in the crater. This mission, which was named before the movie, finished operations in 2005, shortly after the launch of Rosetta.

While we don’t know if the Philae landed safely, the instruments seem to be in working order, giving scientists on Earth hope that we can still take measurements. The basic approach went as planned: after separating from the Rosetta orbiter, Philae coasted slowly to the comet surface. However, the thrusters intended to hold it in place and prevent bouncing failed to fire, and the anchors to pin it to the ice didn’t deploy properly. That means, with the bouncing, the Philae team isn’t currently sure if the probe is right-side up, and it may not be actually attached to Comet 67P.

For a planet, moon, or other larger body, that wouldn’t be as much of an issue, but Comet 67P is only about 5 kilometers (3 miles) at its widest. Not only that, comets aren’t very dense, being made up of ice (including water ice, carbon dioxide, and other compounds with low melting points) and heavier molecules astronomers call dust. That means Comet 67P has very little gravity: not enough to keep Philae down on its own, which is why it needs harpoons to stick into the ice.

Landing on any other world is hard, but Comet 67P is especially challenging, even apart from the low gravity. The comet is basically shaped like a rubber ducky, but with a much rougher surface. The control room team had to analyze Rosetta’s images, then make an educated guess about where it would be best for Philae to land. Then finally, the probe had to be released at the right moment and on the right course to land at that spot.

Which it did, in an amazing piece of planning and execution. With Philae, as with most space probes, everything must be done automatically. Any command sent to the probe takes 28 minutes to travel from Earth, with another 28 minutes for the return signal indicating the command had been executed properly. No landing of this sort has ever even been attempted before, ramping up both the stress levels upon approach and the palpable relief in the control room when Philae signaled it had successfully touched down.

There was one sour note from the proceedings: Matt Taylor, the scientific head of operations for the Rosetta mission, chose to wear a shirt printed with illustrations of scantily-clad women. That was an unpleasant reminder that for many people, science is still considered a boys club. Someone at the ESA should have considered the image they convey about their science to the general public. And that’s why it’s a problem: we should be able to focus on the science coming from Philae and Rosetta, not on unfortunate clothing choices.

And the science is exciting. Comets are objects left from the Solar System’s early days. As such, they allow us a peek at the chemistry before the planets and moons evolved into what we know them as today. Philae, even if it is not anchored to Comet 67P’s surface, may still be able to fulfill part of its mission. If nothing else, it is the closest we’ve ever gotten to a comet, and anything coming from it will be new.

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Earthlings, we can celebrate the accomplishment of landing a probe on a new world. Let there be many more.