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Why Rosetta spacecraft chased after a comet

By Jim Bell
updated 11:43 AM EDT, Fri August 8, 2014
Rosetta's lander Philae is on the surface of Comet 67P/Churyumov-Gerasimenko on Thursday, November 13, and sending back images. One of the lander's three feet can be seen in the foreground. While <a href='http://www.cnn.com/2014/11/12/world/comet-landing-countdown/index.html'>Philae is the first probe to land on a comet</a>, Rosetta is the first to rendezvous with a comet and follow it around the sun. The information collected by Philae at one location on the surface will complement that collected by the Rosetta orbiter for the entire comet. Rosetta's lander Philae is on the surface of Comet 67P/Churyumov-Gerasimenko on Thursday, November 13, and sending back images. One of the lander's three feet can be seen in the foreground. While Philae is the first probe to land on a comet, Rosetta is the first to rendezvous with a comet and follow it around the sun. The information collected by Philae at one location on the surface will complement that collected by the Rosetta orbiter for the entire comet.
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Rosetta: The comet chaser
Rosetta: The comet chaser
Rosetta: The comet chaser
Rosetta: The comet chaser
Rosetta: The comet chaser
Rosetta: The comet chaser
Rosetta: The comet chaser
Rosetta: The comet chaser
Rosetta: The comet chaser
Rosetta: The comet chaser
Rosetta: The comet chaser
Rosetta: The comet chaser
Rosetta: The comet chaser
Rosetta: The comet chaser
Rosetta: The comet chaser
Rosetta: The comet chaser
Rosetta: The comet chaser
Rosetta: The comet chaser
Rosetta: The comet chaser
Rosetta: The comet chaser
Rosetta: The comet chaser
Rosetta: The comet chaser
Rosetta: The comet chaser
Rosetta: The comet chaser
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STORY HIGHLIGHTS
  • Rosetta mission becomes the first spacecraft to go into orbit around a comet
  • Jim Bell: Will it find organic molecules that could help seed a planet like ours for life?
  • He says mission will look for connection between small, icy bodies and large planets
  • Bell: The comet's details are waiting to be deciphered, and that's exciting for science

Editor's note: Jim Bell is an astronomer and planetary scientist in the School of Earth and Space Exploration at Arizona State University. He is president of The Planetary Society and author of "Postcards from Mars," "The Space Book" and most recently, "The Interstellar Age," due out in early 2015. The opinions expressed in this commentary are solely those of the author.

(CNN) -- More than 200 years ago, part of a stone tablet was discovered in Egypt that provided the first reliable way to translate ancient hieroglyphics into a modern language. The Rosetta Stone, as the tablet is called, proved to be the key to unlocking details of the rise and fall of civilizations that flourished on our planet many thousands of years ago.

More recently, a space mission bearing the name Rosetta has begun its quest of unlocking details of the rise and fall of entire planets, including the only one we know that is a safe haven for life.

The European Space Agency's Rosetta mission is a robotic spacecraft designed to get up close and personal with the nucleus of a comet. The comet is called 67P/C-G (short for Churyumov--Gerasimenko, the astrophysicists who discovered it in 1969), and it orbits the sun on a 6½ year elliptical path that takes it from just beyond the orbit of Jupiter to just outside the orbit of Earth.

Rosetta: CNN special coverage

But that orbit is relatively new. 67P/C-G is thought to have originated from much farther away, but one or more close passes by Jupiter altered its orbit, pushing it closer in to the Sun. Thus, 67P/C-G may very well be a piece of primordial, icy debris left over from the original formation of our solar system more than 4½ billion years ago.

It's the Rosetta mission team's job to find out if that is indeed the case, and if there is a specific connection between small, icy bodies like this one and the larger terrestrial planets that they have helped build over the course of solar system history.

Launched a decade ago, the spacecraft -- like the comet it was designed to encounter -- was sent on a long, slow, looping trajectory, passing back by Earth and even Mars for gravity assists, and flying past main belt asteroids Steins and Lutetia along the way.

Rosetta spacecraft first to orbit a comet
Rosetta closing in on comet
Could Rosetta unlock Earth's secrets?

In fact, the cruise to 67P/C-G was so long, and so slow, that the spacecraft was actually put into a state of robotic hibernation for nearly three years to help save money. In a great demonstration of both outstanding engineering and phenomenal patience, scientists and engineers returning to the project this past January were ecstatic when the spacecraft woke successfully from its long interplanetary slumber.

Rosetta team members, including a number of NASA-funded U.S. engineers and planetary scientists, were ecstatic again this week as their plucky little robot successfully nuzzled up to the 2½ mile wide nucleus of 67P/C-G to become the first spacecraft to go into orbit around a comet.

"Nuzzled" is an appropriate word, because with a gravity field more than 10,000 times weaker than Earth's, the pull of this tiny comet is only barely felt by the spacecraft. And not only is it tiny, but it seems to also be just plain weird!

Early Rosetta images show the comet looks like a strange, lumpy, double-lobed peanut covered by cliffs, circular ridges and smooth plains. The "neck" between the two main lobes looks almost fragile from certain angles, suggesting the comet might be about to break into two large chunks. That might perhaps not be a surprising fate, as this small icy world is steadily evaporating, jetting huge amounts of dust, water vapor and other gases into space as it basks in the warmth of the sun.

But the best is still yet to come for Rosetta. The mission's dozen science instruments have only just started to characterize the comet in detail, providing chemical, mineral and geologic clues about its origin and evolution.

In November, the team will attempt to set the oven-sized Philae lander (named after another important Egyptian hieroglyph artifact) down onto the comet to make even more precise direct measurements of the surface. It will be a daring, slow-motion adventure, shared with the world, as Philae descends and then struggles to hang on to the surface under the ultra-weak gravity.

Is this small icy body the kind of world responsible for delivering oceans' worth of water to our planet and others? Are the kinds of organic molecules that Rosetta and Philae might find there the kinds of materials that could help seed a planet like ours for life? What kinds of spectacular sights await as the spacecraft follows the comet even closer to the sun, where the surface is predicted to become even more active?

67P/C-G's planetary hieroglyphics are waiting to be deciphered, and I can't wait to find out what they say!

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