Asteroidal Observations

Miscellaneous ramblings for March, 2001

(occasioned by the momentus landing of NEAR Shoemaker on Eros)

by John Rummel

On the first day of January, 1801, Italian astronomer Giuseppe Piazzi discovered what he thought was a new comet. After the object's orbit was better understood, the news was even better: he believed it was a new planet, located in the large empty gap between the orbits of Mars and Jupiter. Astronomers had long felt that there must be a planet in this wide gap, and now Piazzi had found it. It had to be small, because it was faint and showed up as nothing more than a point of light, even in the most powerful telescopes of the day. It was eventually learned that the object was much smaller than Mercury, and even significantly smaller than Earth's moon. Piazzi named it Ceres, after the Roman goddess of agriculture.

In the next few years, three more small bodies were discovered at approximately the same distance from the sun, with similar orbits. Astronomers began to rethink whether these objects were planets at all. Instead of a planet, maybe this region was occupied by smaller "mini-planets" like Ceres. By the end of the 19th century, several hundred had been found. The objects were reclassified as asteroids (meaning star-like body).

Today, over nine thousand asteroids have been discovered and have orbits that are predictable. Thousands more are known, but their orbits are not yet well enough understood for them to be named and officially cataloged. The majority stay within the gap between the orbits of Mars and Jupiter, but hundreds are known to orbit outside of this belt, and many come closer to the sun than Earth, and many others remain out beyond the orbit of Uranus.

Asteroids that come close to, or cross the orbit of Earth are called NEAs or "near Earth asteroids." They are believed to be fragments of main-belt asteroids that have been bumped off course by collisions with other asteroids, or have had their orbits perturbed by Jupiter's gravity. NEAs are responsible for the large impactors known to have bombarded Earth in the distant past, and for the craters on the moon as well. Today, it is widely accepted that such an object struck Earth 65 million years ago and was responsible for the extinction of many species, including the dinosaurs. Other scientists even suggest that the chemical building blocks of life may have come to the Earth on-board comets or asteroids billions of years ago. If all the known asteroids were lumped together, the resulting body would be only half the size of Earth's moon. It was once thought that the asteroids were leftover material from a planet that was destroyed by a great collision. If true, it couldn't have been much of a planet to begin with. Much more likely, think scientists today, the asteroids are leftover material from the birth of our solar system. Because most are found just inside Jupiter's orbit, it is thought that Jupiter's strong gravity kept this mass of material from ever condensing into a planet.

Scientists planning space missions to the outer planets were once concerned that sending a spacecraft through the asteroid belt would be risky. Popular movies such as Star Wars pictured asteroid belts as regions positively swarming with rocks of all sizes, from pebbles right up through mountain-sized spaceship killers. While such depictions are exaggerated, scientists remained concerned that a spaceship crossing this gulf would run a high risk of being bashed by a space rock and destroyed.

Pioneer I was the first ship to make the trip. In 1973, it crossed the asteroid belt without incident and was the first craft ever to photograph Jupiter from up close. It was followed a year later by Pioneer II, and later by Voyagers I and II, all making it safely across the asteroid belt without incident. Scientists realized that in the vastness of space, a few thousand orbiting chunks of rock posed no threat to spacecraft.

Since the Pioneer and Voyager missions, two more NASA spacecraft have crossed the asteroid belt en route to the outer planets: Galileo and Cassini. Though both missions are aimed at planets (Jupiter and Saturn respectively), Galileo managed to return close-up pictures of two asteroids while crossing the belt, the first such images ever encountered (see below).

The study of asteroids has been revolutionized in the last year by NEAR. Asteroids still pose lots of mysteries to keep scientists busy for decades, but we've come a long way since Piazzi first spied a faint moving star on New Year's Day, 1801.

Picture caption: Galileo image of Ida and its surprise satellite Dactyl. Photo taken in 1993 while en route to Jupiter. Photo courtesy NASA/JPL.

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