Six perpendicular views of Kleopatra, as reconstructed from radar data.
Latitude and longitude contours are spaced by 10°; the scale bar at center is 100 km long.
"Asteroid" means "starlike," which in turn means "an insignificant twinkling dot when viewed by telescope." So it's hard to know what asteroids actually look like, and that makes it hard to understand them scientifically. Four asteroids to date have been viewed up close by spacecraft, and the Hubble Space Telescope has done a decent job with a fifth which is one of the largest and brightest. But we need more! Radar provides one of the best ways to determine asteroids' detailed shapes without the time and expense of spacecraft missions.
Delay-Doppler imaging is a powerful technique in which we keep track not only of the different Doppler frequencies at which different portions of a radar echo return to Earth, but also of the different round-trip times (delays) at which they return. The latter depends on the asteroid's topography, complemeting the rotation information which Doppler shifts provide and enabling us to determine the target's 3-D shape. But this two-category binning of echo power can only be done well for strong echoes, which until now has meant near-Earth asteroids which pass close to us.
In November 1999 a team of nine astronomers from all across the U.S. used the newly upgraded Arecibo Observatory to image 216 Kleopatra, which now is the first main-belt asteroid with a published radar shape model. Our analysis was the cover article for the May 5, 2000 issue of Science, and garnered some press attention as well.
The top row of the figure at left is a sequence of three delay-Doppler images from the first of four observing dates. Pixels closer to the top represent echo power received at shorter delays, that is, reflections from parts of the asteroid's surface closer to Earth. Pixels further to the right within each image represent echo power received at more positive Doppler frequencies, that is, reflections from parts of the surface rotating faster toward Earth. (The left side of each image represents the other end of the asteroid rotating away from Earth.)
The bottom row shows what Kleopatra would actually have looked like from Earth at these three times, based on our 3-D model. The scale bar is 100 km.
How did we construct that model? The middle row shows the images we'd expect to get if our model were correct (and the telescope perfect). We adjusted the size, shape, roughness, and orientation of the model until the top and middle rows - data and theory - agreed as well as possible for all four nights of data.
Kleopatra is 217 km long, give or take 25% -- about the size of New Jersey. Put another way, it's about half again as long as Franklin County here in Maine, and contains enough material to cover all of Maine in a layer of rubble five miles deep.
In asteroid radar experiments we transmit circularly polarized microwave light, meaning that the electric field at a given point in space rapidly and continually shifts direction so as to describe a circle, either clockwise ("right") or counterclockwise ("left") as viewed by someone receiving the light. Kleopatra's echoes are polarized entirely in the opposite circular (OC) sense to the transmission, which implies a smooth surface and uniform subsurface layers at scales from centimeters to meters. This could mean smooth, solid rock or else a deep layer of powder. Which is it?
The figure below is eight views of our 3-D model, this time color-coded by "gravitational slope." Kleopatra's dog-bone shape gives it a complicated gravitational field; this and its rapid rotation make it hard to guess which direction is "downward" at any given point on the surface. But the computer can do this for us, and can tell us just how strongly sloped each part of the surface is relative to this downward direction. Most of the surface is coded purple, blue, and green, indicating gentle gravitational slopes; only a few spots of orange and red indicate possible steep outcrops of solid material jutting forth. This implies that Kleopatra is covered with a thick layer of porous powder which has moved "downhill" to fill in any steep areas.
Kleopatra is highly radar-reflective, which means that its surface layers are dense. We know that these layers are porous powder rather than solid rock (see above), and it's hard to have high average density with all that empty pore space included. Hence the powder must be nickel-iron metal, the densest stuff widely available in the solar system.
The only way to get large chunks of refined metal is for a large object -- a "protoasteroid" which existed long ago -- to get hot enough inside to melt, thus causing the densest material (metal) to sink to the core and the less dense material (rock) to form a surrounding mantle and crust. (This is how Earth is constructed.) Evidently a huge collision then shattered this protoasteroid after it had cooled and solidified, so that large metallic chunks of its core were thrown outward.
Kleopatra's odd shape makes it unlikely that it's a single intact piece of that core, a solid metal dumbbell with a meter-thick powdery veneer. The asteroid belt is pretty empty, but the odds of completely avoiding destructive collisions for several billion years are rather slim. So we instead suspect that two or more core pieces from that huge protoasteroid impact gently collided afterwards, or perhaps went into orbit around each and then gradually got closer and collided. We'd then have an elongated metal "rubble pile," a rotating clump of nickel-iron chunks held together by their mutual gravitational attraction. As the years went by, more objects banged into this clump, gouging material out of the large pieces to produce the metallic powder which now covers them. But Kleopatra is a puzzle, and no explanation of its origin is yet ruled out.
More stuff on Kleopatra -- including the full press release and an MPEG movie of the rotating model -- is available at the Web site of NASA's asteroid radar group. (Imaging results for various near-Earth asteroids can also be viewed there.)
This work was partially supported by National Science Foundation grant AST-9973216
For more information, contact Dr. Chris Magri
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