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astronomy news, cassini, comet

Back from a Week Away

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Exploring an Ice World

An unprocessed image of the moon Helene, orbiting Saturn. Taken by the Cassini spacecraft, June 18, 2011. Courtesy NASA/Cassini Solstice Mission.

I took a week off from writing (everybody  needs a short vacation, right?) and am now back in front of the screen, going through the latest astro-news.

What caught my eye first thing today was the ongoing Cassini Solstice mission. It’s the little spacecraft that just keeps going and going. While her older sisters Voyager and Pioneer are out exploring the outer limits of our solar system, and her little sister New Horizons is headed to Pluto, Cassini keeps sending back images and data about the Saturnian system. The latest views are of a little moon called Helene.

It’s an icy world, which is why I tuned into the story.  Today, June 20, we woke up to snow (which has since turned to rain).  Seems rather incongruous the day before summer solstice (for the northern hemisphere). But, the snow outside reminded me of the snow “out there” – orbiting Saturn like a lopsided iceberg.

It doesn’t take a very close inspection of the image to see the mottled, serrated-looking surface of this little world and to figure out that it looks just plain cold. As to be expected, since temperatures of ice worlds are far, far colder than the conditions we experience here on Earth. Helene is really an irregularly shaped chunk of ice that orbits Saturn in the same orbital path as another moon, Dione. It appears to have been beat up by collisions with other debris in Saturn’s orbit.

Spotting Another Ice Chunk in Space

Animation showing the comet moving against the background of stars. Images taken at the Pan-STARRS 1 Telescope on the night of June 5-6, 2011. Hawaii time is 10 hours earlier than Universal Time (UT). Credit: Henry Hsieh, PS1SC

The distant solar system contains many icy bodies, including these chunky worlds orbiting the gas and ice giant planets. But, there are icy chunks out there that aren’t gravitationally bound to any planets — and astronomers using the Pan-STARRS telescope on Haleakala in Hawai’i spotted one of them just outside the orbit of Jupiter. It’s a comet, called C/2001 L4 (PANSTARRS).  A preliminary orbit computed by the Minor Planet Center in Cambridge, Mass., shows that the comet will come within about 30 million miles (50 million km) of the sun in early 2013, about the same distance as Mercury. The comet will pose no danger to Earth.

The good news about this find is that as it gets closer to the Sun, astronomers have excellent chances to study this comet and figure out just how bright it will appear to be in our skies. If conditions are right, once the comet gets close enough to Earth (say, within the orbit of Mars), it should start to sprout a plasma tail. That’s the tail of ionized gases that streams out from a comet when it gets close enough to the Sun for solar radiation to heat up the gases (and cause them to glow).

It’s tough to know right now just what the composition of the comet is — certainly it’s made of ice. But, how much dust is embedded in that ice?  What kind of ice is it?  Astronomers should be able to tell as they study the comet’s tail with spectrographs (instruments that break up the light from an object into its component wavelengths — and each gas gives off a specific “fingerprint” in the spectrum).

Astronomers think that this comet could be on its first trip around the Sun.  It mostly like comes from the Oort Cloud, that collection of icy objects left over from the formation of the Sun and planets, some 4.5 billion years ago. This cloud lies at the very fringes of the solar system and is a treasure trove of objects that can tell us what conditions were like ‘way back when.

Keep an eye out for this comet in a couple of years. It probably won’t be easy to spot, but if you do see it, you’ll be seeing an object that harks back to a time when our Sun was still forming and the planets were still a work in progress.

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!