In the ancient days of astronomy, before space probes, we had a notion that asteroids were solid and craggy chunks of rock. Like what you find in a quarry, only the size of a mountain, a city, or a small country. Today we know better.
How do we know better? Density. Most asteroids are not very solid at all.
How do we figure density in space without science lab equipment? All we need is the volume of the body and its weight. Space probes like Galileo and Cassini can image an asteroid (or moon) and determine the size with accuracy. On Earth, scientists can calculate the effect gravity has on the probe’s course, thus determining the overall mass of the asteroid (or moon). Mass (as in grams) divided by volume (as in cubic centimeters) gives the density.
We’ve known the size of the Earth for centuries. We know the effect our home planet’s gravity has on the moon–at least since Sir Isaac Newton. We know, therefore, the Earth has a density of 5 1/2 grams per cubic centimeter. Lots of iron and nickel in the middle covered by a lot of silicate rock. A relatively little bit of water and air–just enough to live on.
Mars is about 3 1/2 grams per cc. Not so much metal, but with lots of rock and some sand. Saturn, in contrast, has a lot of gas, so despite a solid core its overall density is 2/3 g/cc, lighter than liquid water which is 1.0.
There are three important numbers to remember for density with astronomical objects: rock plus metal (Earth) 5.5, rock and little metal (Mars) 3.5; ice a fraction less than 1.0 (ice floats, remember?).
When we find an outer solar system body like Ganymede with a density of 1.9, scientists surmise it is half rock, half ice. Then we have Hyperion (on the left), moon of Saturn coming in at 0.56, which means 56% ice, 44% empty space. Go figure how, unless Hyperion is just a bunch of ice chunks with lots of big caves inside.
Eros (top image, top center) is the most well known asteroid, thanks to the NEAR-Shoemaker mission. It has a density of 2.4. Maybe that means one-third ice and two-thirds rock. Remember that Eros orbits the sun in the vicinity of the Earth. It’s likely that if Eros had any water, some or all might have boiled off into the vacuum of space. Of course, it might have ice hidden deep under the surface. Or it could be mostly rock with some empty space.
Most asteroids seem to be loose piles of rubble with lots of empty space. Densities in the range of 1.5 to 2. Perfect for spelunking. How do we know? Some space probes have flown past them. We measure the effect of their gravity, we map and measure their surfaces. We calculate course deviations. Density is easy, once the modern computer is put to the task. Many asteroids are less dense than Eros. But what might they look like inside?
I thought about how to illustrate this. A perfect example came to mind this morning while fixing hot cereal for breakfast. Right there in my kitchen.
Here’s brown sugar in its usual state in my sugar bowl: chunky and grainy mixed with some air pockets:
I figured about 75% sugar, 25% empty space. This is what we’ll find as we examine more asteroids. Remnant pieces of multiple collisions over billions of years. Big pieces inside with spaces. Little pieces covering the surface. Everything nicely stuck together by gravity.
An asteroid like Eros, for example.
And here’s brown sugar packed down nice and hard:
My estimate is 96% sugar, 4% empty space. This would be one of the parent asteroids, an original remnant of a larger body off which other rock and stuff has been shattered off by collisions. One of my favorite asteroids, Eunomia, has a density similar to the planet Mars, 3.8 give or take. Could be solid rock. Could be chunks of metal with caves inside. Some day we’ll find out.
Meanwhile, all I have is this jar of brown sugar. Anybody for oatmeal and Tang?