October 29 – Gaspra Gets a Close-Up

Posted October 29, 2016

Gaspra's colors are
exaggerated in this photo.
On this date in 1991, the American Galileo spacecraft became the first probe to visit an asteroid.

Galileo made its closest approach to 951 Gaspra by passing it fewer than 1,600 kilometers away (about 994 miles away); it took 57 photos that imaged about 80% of the asteroid.

Of course, Galileo was on its way to bigger and better things – Jupiter, to be exact. And it did a slightly more distant flyby of 243 Ida, in 1993, so it was also the second probe to visit an asteroid!

Since the early 1990s, there have been other space probe flyby successes, plus a few that orbited, landed on, and even returned samples from asteroids! Check out the asteroid probes listed in Wikipedia

Why flyby asteroids?

Asteroids are minor planets or hunks of rock that circle the Sun, as opposed to circling a planet like a moon or satellite.

Although the eight planets of the Solar System are always in different spots in their orbits, the orbits seem to be spaced pretty nicely, with the inner planets spaced closer together than the outer planets. Just where it seems that there SHOULD be a planet, between Mars and Jupiter, instead there is a ring of small worlds and rocks and rubble. The first asteroid was discovered way back in 1801.

There are enough asteroids in that region, between the two planets, that the term asteroid belt began to be used in the 1850s. About a thousand asteroids had been discovered by 1921, and by now we can estimate that the asteroid belt includes between one and two million asteroids larger than 1 km (0.6 mile) in diameter, along with millions of smaller ones!

There are asteroids located elsewhere, including some that are near Earth and some that accompany Jupiter in its orbit, located in clumps before and after the huge planet. The latter are named the Trojan and Greek asteroids.

Asteroids in the asteroid belt appear here in white.
Can you see the scattering of near-Earth asteroids,
 inside of Mars's orbit, shown here as colored dots? 

The diagram of the location of the various asteroids, above, and diagrams like it cannot show both the location and the size of asteroids in the same scale. It looks as if traveling through the asteroid belt would look like this:

But instead it would look like this:

In other words, it would look like you were traveling through empty space rather than through a field of rubble. However, you would be traveling quickly, and asteroids travel quickly, so even a tiny impact could be dangerous. That's why it would be important to track all the known and viewable asteroids and make sure that there would be no impact.

By the way...

There are other smaller-than-planet bodies that circle the Sun at a much greater distance. We don't call them asteroids if they orbit the Sun among the outermost of the planets or beyond; instead, we commonly call them Kuiper Belt Objects, plutoids or dwarf planets. KBOs tend to be icier and some become comets with long, eliptical orbits, burning off the icy elements as they approach the Sun.

Back to 951 Gaspra

Like most asteroids, Gaspra isn't large enough to have a spherical shape. (If a body is large, gravity pulls hard enough that even rock is pulled into a sphere.)

Like other asteroids, Gaspra has many small craters that speak to the fact that it was born out of collision (it was likely was once part of a larger body, called a parent asteroid) and continues to suffer from collisions.

Because of its irregular shape, Gaspra looks like it changes shape as it rotates. It has very weak gravity, of course, since it is teeny (the Moon has only one-sixth of the Earth's gravity, and Gaspra is maybe a millionth the size of the Moon!), but the gravitational field is also lopsided...because the asteroid is lopsided!

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