October 29 – Gaspra Gets a Close-Up

nPostednOctober 29, 2016

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Gaspra’s colors are
exaggerated in this photo.

nOnnthis date in 1991, the American Galileo spacecraft became thenfirst probe to visit an asteroid.

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nGalileonmade its closest approach to 951 Gaspra by passing it fewer thann1,600 kilometers away (about 994 miles away); it took 57 photos thatnimaged about 80% of the asteroid.

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

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nSincenthe early 1990s, there have been other space probe flyby successes,nplus a few that orbited, landed on, and even returned samples fromnasteroids! Check out the asteroid probes listed in Wikipedia.

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nWhynflyby asteroids?

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nAsteroidsnare minor planets or hunks of rock that circle the Sun, as opposed toncircling a planet like a moon or satellite. n

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nAlthoughnthe eight planets of the Solar System are always in different spotsnin their orbits, the orbits seem to be spaced pretty nicely, with theninner planets spaced closer together than the outer planets. Justnwhere it seems that there SHOULD be a planet, between Mars andnJupiter, instead there is a ring of small worlds and rocks andnrubble. The first asteroid was discovered way back in 1801.

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nTherenare enough asteroids in that region, between the two planets, thatnthe term asteroid belt began to be used in the 1850s. About anthousand asteroids had been discovered by 1921, and by now we cannestimate that the asteroid belt includes between one and two millionnasteroids larger than 1 km (0.6 mile) in diameter, along withnmillions of smaller ones!

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nTherenare asteroids located elsewhere, including some that are near Earthnand some that accompany Jupiter in its orbit, located in clumpsnbefore 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?

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Thendiagram of the location of the various asteroids, above, and diagramsnlike it cannot show both the location and the size of asteroids innthe same scale. It looks as if traveling through the asteroid beltnwould look like this:

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nButninstead it would look like this:

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nInnother words, it would look like you were traveling through emptynspace rather than through a field of rubble. However, you would bentraveling quickly, and asteroids travel quickly, so even a tinynimpact could be dangerous. That’s why it would be important to tracknall the known and viewable asteroids and make sure that there wouldnbe no impact.

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nBynthe way…

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nTherenare other smaller-than-planet bodies that circle the Sun at a muchngreater distance. We don’t call them asteroids if they orbit the Sunnamong the outermost of the planets or beyond; instead, we commonlyncall them Kuiper Belt Objects, plutoids or dwarf planets. KBOs tendnto be icier and some become comets with long, eliptical orbits,nburning off the icy elements as they approach the Sun. n

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nBacknto 951 Gaspra

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nLikenmost asteroids, Gaspra isn’t large enough to have a spherical shape.n(If a body is large, gravity pulls hard enough that even rock isnpulled into a sphere.)

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Likenother asteroids, Gaspra has many small craters that speak to the factnthat it was born out of collision (it was likely was once part of anlarger body, called a parent asteroid) and continues to suffer fromncollisions.

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nBecausenof its irregular shape, Gaspra looks like it changes shape as itnrotates. It has very weak gravity, of course, since it is teeny (thenMoon has only one-sixth of the Earth’s gravity, and Gaspra is maybe anmillionth the size of the Moon!), but the gravitational field is alsonlopsided…because the asteroid is lopsided!

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nAlsonon this date:

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