Update: May 30, 2007

A DIFFERENT KIND OF ‘SECOND LIFE’

By Pam Eastlick for THE DEEP on line

Welcome to The Deep science and technology column where we cover topics from the deep sea to deep space and beyond.

Most of you have probably heard about the website Second Life.  That’s the virtual reality website where you create your own avatar and live a ‘second life’.  But there’s an older meaning to the term.  It’s when you have completed your basic tasks and goals and then get a second chance to try something totally new.  That’s about to happen to one of our robot spacecraft.

Almost two years ago, on the 4th of July 2005 something remarkable happened.  A comet ran over a washing machine-sized man-made object at 24,000 mph.  The resulting blast threw bits of comet dust and ice into space for many of the world’s astronomers to see.  And what they saw, wasn’t what they expected. Scientists have long wanted to study comets up close and personal because they think they’re unchanged remnants of the early solar system that can tell us much about how Earth and all the other planets formed.  The Deep Impact mission was designed to do just that.  It launched a probe directly into the path of Comet Tempel-Tuttle 1 and the much larger comet ran over it.  One scientist likened the impact to a mosquito hitting the windshield of a 747. This picture was taken by a camera onboard the impacting probe 90 seconds before impact.

The resulting explosion was more violent than astronomers expected.  They figured they would get a crater the size of a house and got one the size of a football field.  They figured what would be blown out would be mostly ice but was, in fact, mostly dust.  They never expected to find water ice on the comet’s surface but they did.  There was also a large amount of olivine, which is produced on Earth in the great heat in the throats of volcanoes.  It’s obvious that Comet Tempel-Tuttle 1 has been a lot nearer to the Sun than Earth!

The material launched from the comet was mostly very fine dust and not ice.  Tempel-Tuttle 1 has a density 2/3 that of water ice and if it’s made mostly of solid dirt-like material and not ice, then 50-70% of the comet has to be empty space to account for the reported density.

The close-up photos of the comet, taken by both the mother ship and the doomed impactor show that it has a surprising amount of surface diversity for an object that was thought to be made mostly of ice that would be smoothed by the Sun’s heat on its periodic trips close to our star.  There are many layers on its surface, smooth patches, craters, filled-in craters and ridges and cliffs.

Two years later, scientists are still studying the data returned by the Deep Impact mission.  We all know what happened to the probe, but what happened to the mother ship?  And that’s where the ‘Second Life’ theme comes into play.

For the last two years, the Deep Impact robot spaceship has cruised silently through space.  All its scientific instruments and telescopes still work and the scientists who manage the mission realized that with the spacecraft already built and launched, extra discoveries could be made at very little cost, a bonus for an already successful mission.

The first proposal involves aiming Deep Impact at another comet called Comet Boethin (comets are named for the people who discover them).  When it arrives it will take pictures of the comet (no more impacts; there was only one impactor probe aboard).  But the most intriguing part of this proposal is what Deep Impact will do along the way to the comet.  It will investigate alien worlds.

More than 200 alien (extrasolar) planets have been discovered to date.  Most of them are found indirectly, by measuring the gravitational pull they exert on their parent star.  Actually seeing extrasolar planets is very difficult, because their stars are extremely bright when compared to the much smaller planet’s reflected starlight.  Planets simply get lost in the glare, like fireflies near a headlight.
Occasionally, however, astronomers find an extrasolar planet with an orbit that’s aligned so that it passes directly between its star and the Earth.  In these rare cases, reflected light from the extrasolar world can be seen directly.  If you analyze the star’s light and then measure the combined light of the star and planet, you can subtract the star’s light and what remains is the star’s light that’s been reflected from the planet.  You can analyze that light to discover what the atmosphere of the planet is like.

Deep Impact will observe three nearby stars with "transiting extrasolar planets," so named because the planet transits, or passes in front of, its star.  The planets were discovered earlier and are giant planets with massive atmospheres, like Jupiter in our solar system.  They orbit their stars much closer than Earth orbits the sun, so they are hot and belong to the class of extrasolar planets nicknamed "Hot Jupiters" These giant planets may not be alone.  If there are other worlds around these stars, they might also transit the star and be discovered by the Deep Impact robot.  Even if they don't transit, Deep Impact could find them indirectly.

An artist's concept of a "Hot Jupiter" extrasolar planet. (Credit: NASA/JPL-Caltech)

Michael A'Hearn, the principal investigator of the Deep Impact mission said, "It's exciting that we can combine two totally independent science investigations into a single project.  However, both relate to understanding how solar systems form and evolve."

Comets and their asteroid kin are the leftover building blocks of planets, and might have contributed water and organic material to the ancient Earth, aiding the start of life.  By observing extrasolar planets, scientists can compare them to our own and discover what we have in common, what we don't, and perhaps why.

So here’s to the ‘second life’ of Deep Impact as it explores our deepest roots both here in the solar system and in worlds beyond.