2 Pallas
Published on Sep 3, 2007 at 7:08 pm.
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Filed under asteroids.
Yesterday, the asteroid 2 Pallas was at opposition. When a planet, asteroid, or comet is at opposition, it generally means that the body in opposition is on the opposite side of Earth from the Sun. In other words, the Earth is passing between the asteroid and the Sun. All of the major planets, and a great many of the asteroids, orbit the Sun in orbits very near the plane of the ecliptic, so this is a good way to remember what the term opposition means. Pallas, though, is a bit different. Its orbit is tilted nearly 35° with respect to the plane of the ecliptic (the plane defined by Earth’s orbit of the Sun). Pallas right now is about 13° north of the ecliptic, at right ascension 22h26m and declination +4°.
Minor planet 2 Pallas was the second asteroid discovered. It was found by Heinrich Wilhelm Olbers in March, 1802, just one year after the discovery of 1 Ceres. Olbers was one of the “celestial police,” a group of two dozen astronomers who had been scanning the sky looking for a hypothetical “missing planet” located between Mars and Jupiter. The celestial police had been searching earnestly for this hypothetical planet when word came that Giuseppe Piazzi had found a tiny body orbiting the Sun between Mars and Jupiter. This body was later named Ceres. Piazzi was quickly invited to join the celestial police after his discovery. The following year, Olbers was searching for Ceres when he found another body nearby. That was Pallas. Olbers was somewhat lucky in that Pallas happened to be near the ecliptic at the time. Most of the time it is some distance away from the ecliptic, as seen above.
The inclination of Pallas’ orbit is not its only unusual property. It orbits the Sun in an elliptical orbit with semi-major axis 2.772 AU (AU stands for Astronomical Unit, the distance between the Earth and the Sun). That part is not so surprising. That is an orbital distance that is right between the 1.524 AU of Mars’ orbit and the 5.203 AU of Jupiter’s orbit. This makes Pallas a main belt asteroid. At this distance from the Sun, Pallas orbits the Sun once every 4.61 years. However, Pallas’ orbit has an eccentricity of 0.231, which means that the orbit is very elliptical. Pallas ranges from 2.13 AU from the Sun all the way out to a maximum distance of 3.41 AU. Pallas isn’t alone in this orbit, though. There are about ten other asteroids in orbits very similar to Pallas’ orbit. Furthermore, these other asteroids have very similar spectral characteristics to Pallas. This suggests that they are all related in some fashion. Perhaps a somewhat larger body was broken apart by a collision in the distant past, leaving Pallas and several smaller bodies orbiting the Sun in similar orbits.
Despite having such an odd orbit, Pallas was the second asteroid discovered. Part of this is serendipity. Pallas was near the ecliptic and near Ceres in the sky in 1802. But, Pallas is also large. It is about 530 kilometers across. This makes it either the second or third largest asteroid found to date (excluding Kuiper Belt objects such as Eris and Pluto). Pallas competes with 4 Vesta, which is similar in size, as the second place asteroid behind Ceres. However, Pallas seems to have less mass than Vesta, making it the third most massive asteroid. Most asteroids are small, so at third most massive, Pallas still contains perhaps 7% of the asteroid belt’s mass. Spectroscopically, Pallas seems to be similar to carbonaceous chondrite meteorites.
Remember that astronomers were looking for a planet when they found Ceres. This is because a mathematical series created by Johann Titius produced a series of numbers that seemed to match the semi-major axes of the planets known in the 18th Century. But, Titius’ relationship also included an extra number between Mars and Jupiter. This relationship, though created by Titius, was widely promoted by Johann Bode, so it is often called Bode’s Law. There is no real physical basis for this relationship, so it does not really predict that a planet should be located between Mars and Jupiter. However, in the two hundred years ago, no one knew that. So, it seemed quite reasonable that there might be a planet between Mars and Jupiter. When Ceres was found, astronomers assumed that it was the missing planet. Then, a year later, Pallas was also found in that gap between Mars and Jupiter. Now, there were two planets between Mars and Jupiter. Then, in 1804, 3 Juno was found with an orbit between Mars and Jupiter. In 1807, 4 Vesta was also found in such an orbit. At the time, astronomers listed all four as planets. Then, astronomers began to find more bodies in the area. Soon, it became apparent that there was a swarm of objects there. Ceres, Pallas, Juno, and Vesta were demoted from planet to simply asteroid. Professional astronomers have long called these bodies minor planets, in recognition of their orbiting the Sun. Now, they are called dwarf planets. Pallas briefly flirted again with planet designation a year ago. The International Astronomical Union had a committee working on the definition of planet. The definition that they came up with made Ceres, Pluto, and Eris planets. But, it also was broad enough that Pallas might have been a planet. The big question was whether or not it had enough mass for gravity to pull it enough into a spherical shape. The exact shape of Pallas is not really known. But, a definition that left nearly a dozen “maybe” planets was not to the IAU’s liking, so they overruled the committee and voted on a definition that left Pluto off of the list (along with Eris, Ceres, Vesta, and Pallas). So, Pallas has twice in history, nearly 200 years apart, been considered for planetary status (along with Ceres, Juno, and Vesta).
-Astroprof
Orbit diagrams courtesy of NASA, JPL’s Near Earth Object Program
