Category Archives: European Space Agency

astronomy news, comet, European Space Agency

Rosetta’s Comet Target is a Rotating Two-body Comet

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What do You Do When Your Target Looks like a Rubber Ducky in Space?

A sequence of 36 interpolated images of comet 67P/Churyumov-Gerasimenko each separated by approximately 20 minutes. The images were obtained by OSIRIS on July 14th, 2014 from a distance of approximately 12,000 kilometers (Courtesy: SA/Rosetta/MPS for OSIRIS Team MPS/UPD/LAM/IAA/SSO/INTA/UPM/DASP/IDA)

Imagine designing a mission to land a probe on a comet. You have to make some assumptions about the comet nucleus, such as its shape, rotation rate, velocity, and what kind of ice it’s made of.  That’s what the planners for the European Space Agency’s Rosetta mission had to do, and this week, they’re getting the first up-close images of the target, Comet 67P/Churyumov-Gerasimenko. When the first views came from the spacecraft, I’m sure the scientists were probably incredibly excited to find out that this comet is not just your run-of-the-mill dented-in icy nucleus with a few jets. No way.  Instead, they’ve drawn the cosmic equivalent of a winning lottery ticket: a sort of double-lobed shaped nucleus that someone described as a rotating “rubber ducky” in space.

Check it out for yourself.

Yes, this is not like any other comet astronomers have seen.  And, it’s spurring a LOT of speculation. I used to study comets in grad school, and several questions came to mind immediately. For starters, how did it get to be this shape? Was this nucleus once two chunks of ice that somehow slammed together in the past and are now orbiting wildly in orbit? That would make it the first “contact binary” comet discovered.  Or, was it one huge chunk of ice that somehow got eroded or broken apart, leaving behind this rotating ducky-shaped object?

To answer the question the mission scientists will use the Rosetta spacecraft’s instruments and cameras to study the surface characteristics of the comet. The data they gather will tell them the ice and mineral makeup. If the nucleus came from one body, then the whole thing should show the same mineralogical makeup. If it came from two different bodies, then the studies will show slight (or perhaps not-so-slight) differences in the ices and dust grains on the surface.

When Rosetta gets to the comet (and the scientists decide to deploy it), it will send a small lander called Philae to settle down to the surface to give us some views from the comet, and also give some first-hand information about the surface materials it will be sitting on. Of course, with a two-body comet, now the big question is, WHERE do you land it?  In particular, if this comet turns out to be made of two different chunks of ice and dust, which side do you pick to study?

Stay tuned because Rosetta is supposed to be at its closest approach to the comet on the morning of August 6th, 2014. It will be an exciting morning for another solar system “first”!

comet, European Space Agency, international halley watch

Halley’s Comet

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Remembering the Flybys

The nucleus of Comet Halley as seen by the Giotto spacecraft in March 1986. Courtesy ESA.

It’s hard to believe that 25 years have passed since Comet Halley swung around our way in its 75.3-year orbit. Right about now it’s heading out to the farthest point in its orbit — around 32.6 astronomical units from the Sun.  That’s farther away than Neptune’s average distance. In a few years, it will reach its most distant point (called aphelion) and then start its inward journey to round the Sun again in 2061.

In mid-March of 1986, a small armada of spacecraft flew near and through Comet Halley’s tail. One of those missions was the Giotto probe, which was nearly destroyed by its close passage to the comet.  But, it returned the first images ever seen of a comet’s nucleus and changed how we viewed these dirty snowballs.

The spacecraft was the European Space Agency’s first deep-space mission, and this year the agency has posted a “remembrance” of the night when the spacecraft approached the comet. Giotto was built to a design that drew on the Geos Earth-orbiting research satellites. It was fitted with shielding to protect it from the ‘sand-blasting’ it endured as it sped through the comet’s tail. The mission was originally conceived as a joint project with NASA, the Tempel-2 Rendezvous–Halley Intercept mission. When the United States pulled out after budget cuts, ESA decided to forge on, finding Japan and Russia willing to contribute their own missions. Together, they sent a flotilla, with the Russian missions serving as pathfinders to guide Giotto to its dangerous encounter.

There WAS another mission set to go to the comet — it was called Spartan Halley, or more technically, Spartan 203.  It was equipped with ultraviolet detectors to observe the glowing gases in the plasma tail of the comet. It was set for launch on Space Shuttle Challenger, and was lost when the shuttle was destroyed in the January 28, 1986 accident.

Comet Halley was a milestone of comet science in many ways. I was part of a team that studied the plasma tail of the comet as it traversed our point of view during the months of closest approach to and movement away from the Sun. We used images from the International Halley Watch, a ground-based effort undertaken by hundreds of observers to study the comet throughout the months it was visible to us on Earth. The images we were most interested in stretched from mid-1985 to mid-1986, the months when the plasma tail was “turned on” and we were able to see structure in it. The comet itself had been spotted in an image as early as 1982, but its tail structure had not yet formed, since it was too far from the Sun to do so.

Comet Halley as imaged by Bill Liller from Easter Island on March 8, 1986. The plasma tail is the lower, bluish portion of the tail.

We began studying the images (or at least my part of the project) began in 1988, when I went to work studying those images under the aegis of the Large-Scale Phenomenon Network of the International Halley Watch. My job was to take the images we had selected and pinpoint the exact location of the comet’s nucleus against the backdrop of the sky.  Of course, it was tough to SEE the actual nucleus, so we had to approximate the location very carefully and then use stars to triangulate the position.  From there, we could then figure out the relative position and distance of structures in the plasma tail. That, in turn, told us something about the speed and “loading” of the solar wind, since the solar wind directly affects a comet’s plasma tail.

In the following years after Halley’s appearance, we studied other comets, among them deVico, Borrelly, D’Arrest, Encke, Honda-Mrkos-Padjuakova, Mueller, and others. The goal was to observe the plasma tails as they turned on and were affected by their interactions with the solar wind. It was a great deal of work that added to the comet literature, and I’m pleased to have been a small part of it.  It’s just hard to believe it was 25 years ago!