How do we measure density in the solar system?

Editor's note: This story was originally posted 5 Feb 2014, but it's now graduated and become part of Astronomy 101!  Please be aware that any references to events that seem current may not actually reflect events happening right now.

You may have seen the recent news that astronomers have just "peered inside" an asteroid for the first time.  You might be surprised to find out that measuring something as seemingly basic as density is actually rather tricky in astronomy.  Why is this?  Let's take a look.

Early scientists determined that the Earth and Jupiter have quite different densities. (Image credit: NASA)

Early scientists determined that the Earth and Jupiter have quite different densities. (Image credit: NASA)

For centuries, astronomers were basically limited to making two measurements about objects in the sky: their position and their brightness.  Even with this limited amount of information, early scientists were able to determine a basic property of solar system bodies: their mass.  They needed one more tool - physics - and its use in determining the makeup of our solar system was one of the first instances of modern science.  Two basic ideas were important. The first was Johannes Kepler's observation that the time it took a planet to circle the Sun was directly related to how far away it was.  The second idea came from Isaac Newton: any two bodies in the Universe exert a gravitational force on each other proportional to their mass and inversely proportional to the square of the their distance.

These ideas allowed Kepler to estimate the distances of the planets and Newton to estimate their masses.  There was only one catch - all these values were relative to the Earth.  So Jupiter was 5 times farther than the Earth from the Sun and the Sun weighed about 170,000 times more than the Earth - but we had no idea how much that was!

The critical experiment wouldn't come until 1797, when British scientist Henry Cavendish would measure the bulk density (and thus the mass) of the Earth.  In one of the most remarkable experiments in the history of science, Cavendish measured the mass of the Earth to within one percent of its modern value.  When divided by the volume of the planet, a density could be determined.  This quickly distinguished the inner, rocky planets from the outer, gaseous worlds.

These ideas worked for planets, but smaller objects like asteroids and moons were just too minor for their gravitational effects to be determined.  To measure their density, we'd have to get closer.

With the advent of interplanetary spacecraft, we could do just that.  Space probes opened up whole new methods for nailing down the density of objects both large and small.  The easiest way was to use the ship itself.  With a known mass, changes in a spacecraft's orbit due to an asteroid it was flying by could be directly related to the mass of that body.  By measuring its size from closeup images, a density could be derived.

These measurements provided unprecedented accuracy.  They were so accurate, in fact, that scientists could now ask harder questions.  Up until now, the density we've been talking about is called the "bulk density." This is just another word for the average.  But objects aren't the same the whole way through - we know the Earth has a core of iron and a mantle of rock and oceans of water on the top.  Other worlds are similar.

So, today, astronomers are interested in the density distributions of objects.  This sheds light on the different materials stuff in the solar system is made of.  The most common way to do this is to make many repeated orbits of the same body and compare the density measurements made at different places around the body.

But what about objects, like most asteroids, which are too small to orbit?  That's where this new technique comes in.  It's works by using a curious phenomenon called the Yarkovsky Effect.  It works like this: as the Sun shines on a small object like an asteroid, it heats it up.  The asteroid then radiates that heat back into space.  But, if the asteroid isn't round (and they pretty much never are!), then it won't radiate the heat equally in all directions.  This can cause the asteroid to spin. The builds up extremely slowly, but it's something we can often measure.

By using a unique technique, astronomers were able to determine that this asteroid doesn't have the same density everywhere.  (Image credit: ESA/JAXA)

By using a unique technique, astronomers were able to determine that this asteroid doesn't have the same density everywhere.  (Image credit: ESA/JAXA)

This team of astronomers, led by Stephen Lowry at the University of Kent, looked at images of an asteroid and from its shape determined how the Yarkovsky Effect ought to make it spin.  When observations from the Earth determined that the rate of spin was different, they were able to relate this difference to irregularities in density.  This works because materials of different density radiate heat at different rates.

Efforts like this remind us how great planetary science can still still happen from here on Earth.  And, with NASA's planetary exploration budget likely to remain small for years to come, that's a great thing.

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