Saturday, March 7, 2015

Right Where I Left It

NASA scientists are pretty pumped because the space probe Dawn has entered its parking orbit around the dwarf planet Ceres.

Ceres is between Mars and Jupiter in what's commonly called the asteroid belt. It was found in 1801 by using a theory that actually doesn't work, even though it more or less predicted Ceres or something like it would be found about where it was.

The Titius-Bode Law, which is more commonly called Bode's Law even though Johann Daniel Titius published it first, was a theorem that suggested where planets would be found. It said that the planetary orbits were mathematically related to each other, and that planets beyond Mars should be found at roughly twice the distance of their nearest sunside neighbor.

Well, even with the earliest telescopes scientists could see that there was a bigger gap between Mars and Jupiter than there was supposed to be. Mars was, according to the ratio, 1.6 times as far from the sun as the Earth. That makes it 1.6 "AUs" from the sun, where "AU" is not a way of getting someone's attention when they're on the telescope scaffold but an Astronomical Unit. Since we're the ones doing the measuring, we get to set the Earth exactly one AU from the sun.

So Johann Elert Bode said there should be a planet at about 2.8 AUs from the sun, in between Mars at 1.6 AUs and Jupiter at 5.2 AUs. But nobody had ever seen one, until Giuseppe Piazzi spotted it on the very first day of the 19th century: January 1, 1801. He was looking for stars and saw it but thought it was a slow comet. Other astronomers confirmed his findings, which were published in September 1801. Ceres was 2.77 AUs from the sun, about where Bode's Law said it should be.

The rest of the world's astronomers got their telescopes out to observe Ceres and confirm Piazzi's sighting, but the change in the Earth's position relative to Ceres meant that it was too close to the sun to be seen and folks weren't sure where it might pop up again. Enter the brilliant mathematician Carl Friedrich Gauss, who whipped up a new formula for determining orbital positioning in a few weeks and handed it off to Franz Xaver von Zach, the scientist heading up a group of astronomers searching the gap between Mars and Jupiter. And there it was, found again on the last day of 1801 by von Zach and Heinrich W.M. Olbers.

That made things look pretty good for Bode's Law. It had already predicted the location of Uranus (stop it!) some 20 years earlier, and now it helped find Ceres and a small fleet of similar objects about where there was supposed to be something in orbit. Alas, the discovery of Neptune in 1846, which was nowhere near the place Bode's Law said it should be, followed by the discovery of Pluto in 1930, the Kuiper Belt in 1992 and Eris in 2005 -- none of which were in locations that matched Bode's Law predictions -- put it to rest.

Fortunately, Dawn could rely on some more up-to-date methods to find its destination, and now we just have to hang around and scarf whatever data it feeds us.

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