Chips of Pallas grace meteorite collections around the world. See where they all came from when the asteroid reaches opposition this spring.
With all the excitement over the dwarf planets Ceres and Pluto, it would be easy to overlook the asteroid 2 Pallas. But if you did, you'd miss a fine opportunity to observe an asteroid that, like Vesta, has delivered pieces of itself to Earth as meteorites.
Pallas, discovered in March 1802, was originally considered a planet, as were all the early asteroid discoveries. But by the mid-1800s, with 15 "new planets" on the roster and more in the offing, astronomers thought it prudent to reclassify these small bodies as minor planets or asteroids. Sound familiar?
With the discovery of large, Pluto-sized asteroids in the distant Kuiper Belt beginning in 1992, Pluto suddenly had company, stirring the International Astronomical Union to re-brand the planet and its ilk as dwarf planets in August 2006.
Pallas remains a traditional asteroid but with unique qualities. It's larger than Vesta (338 miles vs. 326 miles in diameter) and estimated to comprise 7% of the mass of the main asteroid belt. Astronomers studying Pallas's reflectance spectrum have found a good match between the minerals on its surface and the CR chondrite clan. Reflectance spectra have been a key diagnostic when tracking down the parent asteroids of at least some of the meteorites found on Earth.
When sunlight bounces off a rock, specific wavelengths of light are absorbed or reflected, depending on its mineral composition. By carefully analyzing the sunlight reflected off an asteroid, we can get a good idea of its surface composition and then compare it with meteorite reflectance spectra to find a mate: mineralogical matchmaking.
So what are CR chondrites? They occupy a rare subset of the already sparse carbonaceous chondrite class, dark-colored stony meteorites that contain small amounts of carbon. Specifically, CRs contain clay-like minerals and show signs of having been altered by contact with water; they're also rich in metal, like the weird and wonderful CB chondrites, a closely related group.
Ceres also shows similarities to carbonaceous chondrites, making it a relative of sorts to Pallas. But Vesta is quite different. This rocky asteroid, target of the first half of the Dawn mission, appears to have far less water/ice than either Ceres or Pallas. Vestan meteorites belong to the "HED" group, named for howardites, eucrites, and diogenites, all highly-processed igneous rocks that find their match on heavily cratered Vesta located more than 220 million miles from your front door.
Now that we have something of a feel for what Pallas might look like up close, let's find it in our telescopes. Pallas comes to opposition on June 11 and shines modestly at magnitude +9.5 now through July, making it very easy to spot on a pleasant May night with the smallest of telescopes or even a pair of large binoculars. As oppositions go, this isn't a bright one, but if you missed last year's close pass (magnitude +7), this is that second chance you were hoping for. The detailed map below is also available in reverse with black stars on white.
Pallas moves at a snail's pace across the constellation Hercules, so you'll always know where it's hanging out. From early to mid-May, Pallas climbs high enough in the east for a good view beginning around 11 o'clock local time. By June, it's nicely placed at nightfall.
Look for a star-like point of light slowly moving across your star field night by night. Even the orbiting Hubble Space Telescope, located far above the blurring effects of Earth’s turbulent atmosphere, sees our featured asteroid only as a pixelated object flattened sphere with a large, darker “depression” that might be a crater.
Pallas’ highly tilted orbit of 34.8° makes it extremely difficult, energy and time-wise, to dispatch a probe for in-depth study and photo-taking. For now, we're left with tantalizing meteorites and vague textures.
That's just enough to visualize a cratered world with a crust rich in water-soaked rocks, spherules of pure metal and seed-like chondrules that began their life so long ago as molten droplets in the solar nebula.