A map of the slopes on the surface of the asteroid Kleopatra show it’s smooth; most of the hills have gentle slopes (most up to around 12°). Credit: Shepard et al.
More info i
A map of the slopes on the surface of the asteroid Kleopatra show it’s smooth; most of the hills have gentle slopes (most up to around 12°). Credit: Shepard et al.

The asteroid Kleopatra: Less of a dog bone and more of a dumbbell

Contributed by
Apr 26, 2018

I don't suppose everyone has a favorite asteroid. I'm not saying they shouldn't, just that most folks haven't devoted that much mental energy to the topic itself.

I think about asteroids a lot, so in my case it makes sense to have an attachment to some over others. If I had to pick, though, my top spot would go to 216 Kleopatra*. Why? Because it looks like… a dog bone. Yes, seriously.

New analysis of a whole bunch of observations of Kleopatra has refined the shape somewhat, and while it's lost some of the obvious resemblance, the canis os-tiness of it is still there.

Comparison of three models of Kleopatra’s shape. The original (top) is the iconic dog bone, a better model (middle) is much flatter, and the most recent (bottom) is flatter than the original but still pinched in the middle. Credit: Shepard et al.

Comparison of three models of Kleopatra’s shape. The original (top) is the iconic dog bone, a better model (middle) is much flatter, and the most recent (bottom) is flatter than the original but still pinched in the middle. Credit: Shepard et al.

I suppose it looks more like a dumbbell now, but still.

The analysis relied on two major types of observations: radar and occultations. The radar observations used the mammoth Arecibo radio telescope in Puerto Rico to ping the asteroid, and these pulses of radio waves are reflected back and picked up by the dish. A ping hitting the near side of the asteroid takes a little bit less time than one hitting the far side, so a map of the shape can be constructed from this. Also, the wavelengths of the radio waves are subtly changed if the asteroid is spinning, so its rotation rate can be deduced as well (for Kleopatra, that's one rotation every 5.385280 hours with an accuracy of 0.000001 hours — an absurdly precise 0.0036 seconds!).

Occultations are when an asteroid passes in front of a more distant star. The size and shape can be found this way as well.

What they found is that Kleopatra is very elongated, which we already knew. But the new work gives the best numbers yet. Kleopatra is 276 kilometers long and shaped like two ellipsoids stuck together end to end, with a neck or bridge connecting them. Each lobe is like a mildly flattened football. One is 140 x 79 x 69 km, the other is 134 x 90 x 69, and the connection is 60 km long and 45 thick.

Mind you, this is pretty big. If you took all that stuff and instead shaped it into a sphere of equal volume, it would be 122 or so kilometers across.

Older high resolution images of Kleopatra (top) are compared to the predicted orientation of the asteroid using the new model. The numbers are the date of the observation, the distance in AU (1 AU = 150 million km), and the resolution of the observation.

Older high resolution images of Kleopatra (top) are compared to the predicted orientation of the asteroid using the new model. The numbers are the date of the observation, the distance in AU (1 AU = 150 million km), and the resolution of the observation. Credit: Shepard et al.

That shape has been surprisingly difficult to pin down. One big problem is how far away it is; it's a main belt asteroid, which means it orbits the Sun between Mars and Jupiter, so it's never closer than about 200 million kilometers away. Plus, the long double-lobed shape makes it hard to get a single model to explain all the observations. You can get two similar shapes that differ in detail that still produce the same data we see (as a very rough example, an American football seen on end would look similar to a circle or sphere), so nailing this all down can be hard.

But this gets better. Kleopatra has two moons, called Alexhelios and Cleoselena. They orbit the main asteroid due to its gravity, and that depends on its mass. That means we can get the mass of the main object, which turns out to be 4.6 x 1018 kg — 4.6 quadrillion tons. Now that we know its size, that means we can get its density, and that’s critical! Ice, rock, and metal — three things asteroids tend to be made of — have very different densities, so measuring the density means understanding something about the composition.

The new work indicates the density of Kleopatra is about 4.9 grams per cubic centimeter, or 4.9 times the density of water. Rock is about 2 g/cc, so this means Kleopatra clearly has a lot of metal in it.

That matches other observations! Spectra taken of it also indicate it's predominantly metal, making it the second largest metal asteroid in the solar system. Also, the radar observations of one of the lobes indicate it's metallic.

However, the waist between the two lobes is not. The radar indicates that region is mostly rocky. Why?

I love this part. It's due to gravity. On a perfect sphere, the gravity you'd feel pulling you down would be the same no matter where you were on its surface. But Kleopatra is most decidedly not a sphere, so the gravity you'd feel changes as you change position. At the poles gravity is weakest, and at the bridge between the two lobes it's strongest. So pretty much wherever you stand on Kleopatra, the direction toward the bridge is downhill.

A map of the geopotential of Kleopatra — think of it as a gravity map. The speeds given show the velocity of an object if it moved from the ends of the asteroid to a given location, accelerated by gravity. The neck has the highest gravity.

A map of the geopotential of Kleopatra — think of it as a gravity map. The speeds given show the velocity of an object if it moved from the ends of the asteroid to a given location, accelerated by gravity. The neck has the highest gravity. Credit: Shepard et al.

And that explains the odd radar observation. Any loose rocky material on the surface would feel that pull and roll downhill, piling up in the center. That probably happens mostly when it gets hit by much smaller asteroids, which can shake the surface like an earthquake, dislodging the looser stuff.

This means that if you wanted to escape the asteroid's gravity, you'd have to jump at 165 meters per second off the bridge, but only 49 m/s at the ends of the lobes. That's still pretty fast (faster than a car drives on the highway) but a lot less than Earth's 11 kilometers per second. Getting off that asteroid isn't too hard.

Interestingly, the radar observations also indicate the surface is relaxed. Not like chill (though it's cold that far from the Sun), but that there are very few steep hills. Most of the sloping is very gentle, around 12°. On Earth that would feel steep, but on an asteroid with weak gravity that's pretty shallow. Grainy material there would naturally tend to pile up with slopes around 35° (this is called the angle of repose, a phrase that is somehow pleasing to me), so Kleopatra is clearly less pointy overall than you'd expect.

In fact, the astronomers found a huge flat area on one pole that's about 2,500 square kilometers (roughly 50 km on a side), and a second one about half that size. I wonder what those are! My first thought was fine-grained material building up there to make it smooth, but that's where gravity is lower, so I wouldn't expect too much stuff like that in that area.

Here's an idea: Let's go there! I'd love to see a probe to Kleopatra to map it out and examine it as the Rosetta spacecraft did the comet 67/P Chuyurmov-Gerasimenko… which is also a double-lobed object. Such shapes are common among asteroids and comets, and we're still not sure why; there are several possible scenarios where this can occur.

I'll note NASA just okayed a mission to Psyche, another metal asteroid. But what I'm rather hoping is that the drop in launch costs thanks to SpaceX (and soon Blue Origin) will mean we get more astronomy and planetary science missions in the future. We'll see. That takes a while, so in the meantime, we'll have to keep being clever about what we have right here and now.

*No offense intended to what may be a more obvious choice.

Tip o' the YORP effect to Jason Perry, aka VolcanoPele on Twitter (via Emily Lakdawalla)