The swift and untimely breakup of the doomed Comet Linear is much more than simply a curious celestial fireworks show.

Seeing the "subassembly" of a comet nucleus provides a clear blueprint of the internal structure of comets and helps settle the debate over whether they are flying "gravel piles," or are built up from consecutively smaller pieces, like the Russian Matrushka toy of nested dolls.

Comet together

The comet's solid nucleus was assembled from these mini-comets -- or cometesimals -- at about the time of the birth of the planets in our solar system 4.5 billion years ago. Seeing the comet come apart in such fine detail is like opening a time capsule containing the long-sought relics of the early solar system.

In estimating the number of planets in our galaxy, astronomers need to know how they formed around stars. Astronomers have seen both ends of the planet construction process. They've cataloged over 50 extrasolar planetsand Hubble has surveyed over 100 embryonic disks of fine dust around stars out of which the planets will presumably condense. But the process itself cannot be seen, so astronomers must look for fossil evidence within our own solar system.

Like the planets, comets were built up from micron-sized grains of dust no larger than the thickness of a human hair. Over tens of millions of years, the dust clumped together with ice to form snowball-like frozen bodies measuring dozens of feet (meters) wide.

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These "cometesimals" gently merged, sticking together to build up comet nuclei, which then grew to a few miles (kilometers) across. Some of these nuclei continued to coalesce to form the icy solid cores of the gas-giant planets. Once the giant planets formed, they gravitationally captured or tossed the remaining comet nuclei out of the solar system to create the Oort cloud.

Valuable clues

Knowing how comets are put together offers clues on how to deflect or destroy a wayward comet that might collide with Earth.

Hubble's Comet Linear observations support this idea. The effect of a nuclear bomb on a comet nucleus -- as popularized in science fiction movies -- might be more like punching a pillow rather that shattering a rock with a hammer. The bomb's energy would not be transferred efficiently through such a mushy object.

If a powerful bomb could bust apart a comet's nucleus, it would be expected to fall apart like Comet Linear did -- into a cluster of smaller bodies. If the remaining cometesimals are low-density, or "fluffy," they could be pushed off its collision course with Earth by the solar wind. They might also shoot off jets of gas that would act like rocket motors, nudging them onto a safe trajectory.

If the cometesimals are denser, and less perturbed by non-gravitational forces, they may stay on a trajectory toward Earth like multiple reentry warheads deployed from an intercontinental ballistic missile. This means our home planet would be peppered with many smaller bodies rather than one large body.

Slamming into the atmosphere at 40,000 miles (64,370 kilometers) per hour, many of these falling cometesimals would instantly flatten like pancakes as the atmosphere absorbed their energy in a shockwave. The equivalent of several Hiroshima atomic bombs (100-kilotons of explosive energy) would be unleashed in a burst of light that would be visible for over 1,000 miles (1,610 kilometers), but too far above the ground to do any damage.

The largest cometesimals might penetrate deeper into the atmosphere and explode with enough force to flatten a town. However, even the largest cometesimals would be less than half the size of the asteroid fragment that devastated a Siberian forest in 1908. This fragment had a force equal to a 15-megaton explosion, enough to level an area 10 miles (16 kilometers) wide.

It is important for astronomers to keep an eye on Linear's fragments as they retreat back to the cold fringes of the solar system. It may be a long time before astronomers ever get another close-up look at a comet being dismantled quite like Linear.