Why a Comet?

67P/C-G Comet on November 17, 2014 via ESA NavCam

What on Earth Are We Doing on a Comet?
November 20, 2014

On November 12, 2014 the European Space Agency (ESA) put a man-made object on the surface of a comet for the first time ever. The Rosetta satellite and the Philae lander travelled 10 years and more than 4 billion miles but they did get there.

#CometLanding and related hashtags trending on TwitterBack here on Earth, people were excited. “#CometLanding,” “Philae” and “#Rosetta” were institutions on Twitter for days. With that excitement, however, came a touch of cynicism. Many people used the “WeCanLandOnACometButWeCant” hashtag to express their frustrations about the resources required to accomplish the comet landing when there are so many problems in want of solving right here on Earth.

Their frustration is understandable. The Rosetta mission required 10 years, countless intellectual man-hours and almost 1.75 billion dollars. Why should a comet 50 million miles away take precedence over war, disease or hunger?

The answer is as simple as it is incredible. Comets may hold the secret to the origin of life on Earth.

Many scientists think the earliest life on Earth started as a soup of small organic molecules that slowly began to assemble into larger molecules: proteins, RNA, DNA and eventually the first cells. Where those early organic molecules — called prebiotics — came from, however, is a mystery. One theory is that they were randomly assembled after lighting struck the soup. Another theory is they were brought here by comets.

“During the early days of the solar system, Earth was being hit by all sorts of stuff,” says Michael Castelaz, the science director at the Pisgah Astronomical Research Institute (PARI) in Rosman, N.C.

“Comets carry water and dump off these [prebiotic] molecules into a pretty wild environment. You mix all that together and who knows what is going to happen.”

Scientists who subscribe to this comet seeding theory say life happens. Some even go so far as to claim bacteria rode the comets to Earth, and all life on Earth is descended from those travellers.

67P/C-G Comet on November 17, 2014 via ESA NavCamAs important as comets could be to our origin, however, scientists know very little about them. From Earth, we can see that comets are large balls of rock and ice. They can travel in the vast emptiness that is interstellar space or do wildly elliptical laps around the sun like Halley’s Comet and 67P/C-G — where Philae and Rosetta are. We know when they fly close to the sun, it begins to melt the comet’s icy portion, sending out a bright cloud of gas, dust and very small rocks, which we see as the comet’s tail and coma.

Scientists are limited, however, by what we can see from Earth. Based on how a comet’s tail glows, we can see that they contain water and some organic molecules, but it’s impossible to say exactly which ones are there and how much of them there are.

“If we knew exactly what comets were, there wouldn’t be a spacecraft orbiting one right now,” Castelaz said.

When scientists look at the material in a comet’s coma and tail, what they see is the comet stuff found after it has been pummeled by the sun’s radiation. This shows us the tiniest pieces like water, methane and ammonia. One of the Philae lander’s jobs was to drill into the comet and figure out exactly what is in there: to see any larger molecules, like amino acids, before the sun has a chance to break them down.

Panoramic Lander OrientationPhilae turned its drill, took chemical data and sent that data back to Earth. ESA scientists are currently analyzing what Philae sent back, and with luck, Philae’s drill will have gotten a good scoop of comet dirt and better data.

ESA scientists have already learned quite a bit about the comet’s surface. For instance, comets are much harder than we originally thought, which may explain why Philae bounced twice before settling in. They expected a layer of dust at the first landing sight to act as a cushion for Philae’s landing, but the underlying rock was so hard it sent Philae almost a kilometer into the air. ESA recently released the “thud” Philae made when it first touched down.

Philae didn’t stop there either. While Philae and Rosetta were on opposite sides of the comet, they were able to make a three-dimensional scan of the comet’s core, which will tell scientists exactly how the interior of the comet is put together. Philae also sent plenty of pictures home so scientists can analyze the surface.

Philae’s batteries have run dry so we will not be hearing from the lander again, at least for a while, but ESA says they may get to revive Philae when the comet passes close to the sun next year. Either way, Philae sent back data unlike anything astronomers have ever seen before and captivated millions of people.

Castelaz says missions like Rosetta not only give us potentially crucial information on how the universe works, but also help science. Philae and Rosetta got people excited about science in a time when heading to the International Space Station is “kind of ho-hum,” according to Castelaz.

“It’s too bad we can’t do more like that,” he said. “We’re explorers."

- Daniel Lane

Daniel Lane covers science, engineering, medicine and the environment in North Carolina.

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