Surprise: The Comet We Hit Wasn't What We Expected

By James Donahue

September 2005

NASA’s $240 million space vehicle that purposefully collided July 4 with Tempel 1, a five-mile long potato-shaped comet, has yielded some surprising information about these mysterious space travelers.

Comets, or at least Tempel 1, is neither ice nor rock, as scientists once theorized. Instead, this one is a large collection of brittle dust, organic material, ice and gas that appears to be about as fragile as a snowball.

The entire package appears to be faintly held together by the gravity created by its own mass.

That such a thing can hold together for hundreds of thousands of years as it hurls at supersonic speeds on wide sweeping orbits in and out of our solar system, is but one of the many mysteries surrounding these strange objects that sweep across our night skies, sometimes once in decades.

NASA’s Deep Impact probe was deliberately crashed into the passing comet on July 4 while cameras from Earth and the Hubble space telescopes recorded it from many angles. The information gleaned to date suggests a pristine interior of organic material that might date the solar system’s formation to as long as 4.5 billion years, a report in the journal Science says.

Michael A’Hearn, an astronomer at University of Maryland and a co-author of the report, said the large amount of organic material ejected following the collision agrees with a hypothesis that comets like this may have delivered the building blocks for primordial life on Earth.

The comet itself is filled with craters from being hit by other celestial objects during its adventures through space and time.

A’Hearn believes the comet’s porous outer surface is so something like a snowy dirt ball that reacts to the heat of sunlight when passing through out solar system. It thus ejects millions of tiny fragments that reflect sunlight in passing, creating what appears as a long tail to observers on Earth.

Karen Meech, from the University of Hawaii Institute for Astronomy, said the project allowed scientists to directly measure a comet’s density. It appears they aren’t as threatening to the Earth if they collide as once feared.

That is if all comets are made of the same stuff.

German astronomer at Max Planck Institute said Tempel 1 might be compared to an ancient block of Swiss cheese. “The whole thing is brittle and crumbles and has lots of holes. The surface is dark because light gets trapped everywhere,” he said.

In case you wonder, the probe, which was about the size of a coffee table, left a hole the size of a football field and about 30 feet deep.