Wednesday, January 24, 2007

Minor Planets


The lines between asteroids, comets and minor planets become even more blurred. Astronomer Mike Brown discovered the Kuiper Belt Object Eris, formerly called Buffy. Discovery of these objects has changed definitions such that Pluto has been demoted to a "minor planet." At AAS earlier this month, Brown announced calculations that Object 2003 EL61 may someday become a comet.

The object is the size of Pluto, but is shaped like an American football spinning end over end. You can see animations of this spin at Mike Brown's 2003 EL61 page. This asteroid has two known satellites and orbits outside Neptune's orbit. Interactions with Neptune may someday alter 2003 EL61's orbit, turning a minor planet into a comet.

Above is a Hubble Space Telescope photo of Vesta. We are fortunate to have samples from this asteroid. In October 1960 two fence workers in Millbillie, Australia saw a fireball in the sky. Western Australia is a big place, and it was ten years before the meteorites were found. Comparing their makeup with spectrographic data showed that the meteorites came from Vesta.

The meteorites are mostly composed of pyroxene, a mineral common in our Hawaiian lava. Vesta's surface is mostly basaltic rock, the product of lava flows. Evidence indicates that Vesta once had a molten core, like Earth. Old ideas of radioactive decay would have difficulty accounting for this heat. Samples from Vesta and other meteorites show no sign of radioactive materiels.

On the right is a computer reconstruction. The bottom of the asteroid is one enormous impact crater, including a high central peak. An impact this big could have torn Vesta to pieces. An internal singularity would have kept things together, and explained Vesta's internal heat. Vesta will be the first asteroid destination of the DAWN probe.

Vesta is pictured below with Ceres, the second asteroid to be explored by DAWN. As much as 25% of Ceres may be water, possibly more water than Earth. Ceres' spherical shape indicates that it too has internal heat. The inner core has been interpreted as rock, but could contain something else. As minor planets, these worlds will be excellent places to search for singularities.

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