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8.14 Asteroid Shapes

Asteroids smaller than about 100 km are not perfectly round. Why? The answer involves gravity and the idea of equilibrium. Thermal equilibrium is the tendency of thermal energy to flow so that temperature differences are equalized. Equilibrium applies to gravity, too. Every part of a planet’s surface has a gravitational potential energy. This quantity can be expressed as the product of the object’s mass (m), the acceleration of gravity (g), and the height of the object (h):

PEGravity = m × g × h

So potential energy increases with the mass of the planet (because a more massive planet will have a larger value of g), and with the distance from the planet’s center. Gravity pulls all the parts of an object toward its center of mass. That’s why the cereal in a box settles when you shake it! The gravitational potential energy of the cereal is minimized when it moves toward the center of the Earth. Gravity always works to minimize the potential energy of all parts of a planet. High places have a larger gravitational potential energy, and are pulled downward more strongly than low places. Take a large tray and cover it with sand or gravel heaped into "mountains." If you shake the tray, the sand will quickly settle to a flat surface where the gravitational potential energy is the same everywhere. This is an example of equilibrium.

Ceres, the largest asteroid in the asteroid belt. Click here for original source URL.

What does the idea of equilibrium have to do with asteroids? Large bodies in the solar system have enough mass so that their strong gravity forces their surfaces to have the same potential energy everywhere. The result is the most symmetric shape possible: a sphere. The Earth is as round and smooth as a billiard ball. Small asteroids, however, do not have enough gravity to overcome the strength of the rock they are made of. So they have irregular shapes.

Asteroid Ida with its tiny moonlet Dactyl. Click here for original source URL.

Not only are asteroids shaped oddly, sometimes they have companions. The second close-up photo ever taken of an asteroid, that of the 52 km long asteroid 243 Ida taken by the Galileo spacecraft, revealed a second small body orbiting Ida. Such satellites had been suspected among other asteroids, but never confirmed. Their existence explains an older discovery: Earth, the Moon, and Mars show numerous pairs of adjacent impact craters of the same age. These must have formed when an asteroid and its satellite hit simultaneously. Studies suggest that perhaps 10 to 20% of asteroids have sizable “moonlets” moving around them.

A related discovery came from radar images of asteroids. Large radio telescopes bounce radar signals off nearby asteroids as they pass close to Earth, and the product is a different type of image than a regular photograph. Results of this technique have astonished astronomers. In 1989, when the 1 km by 1.5 km asteroid 4769 Castalia passed by Earth, a fuzzy radar image showed a two-lobed, dumbbell shape rotating end over end. Such images, together with other techniques, show that some asteroids are no more than loosely bonded clumps of several large fragments. These “compound” asteroids are within reach of our spaceships. It would be a strange experience for an almost weightless astronaut to float among the rocks of an asteroid built like two rounded mountains, just touching each other!

Compound asteroids and asteroid satellites may be part of the same type of phenomena. Imagine an asteroid hitting another body and blowing it apart in a colossal collision. Now picture an asteroid passing too close to a planet and being pulled apart by the larger body’s tidal forces. In both cases, a jumble of fragments races outward. Adjacent fragments may bump into each other. Some may fall together as pairs and make dumb-bell shaped compound objects, and others may go into orbit around each other. Thus, the seemingly disconnected discoveries of asteroid moons and compound shapes may both be clues to asteroids’ violent histories.