Michelle Thaller
A NEAR perfect landing
On February 12, 2001, the Near Earth Asteroid Rendezvous (NEAR) spacecraft became the first probe to set down on the surface of an asteroid. The landing spawned a jubilant celebration at the Jet Propulsion Laboratory in Pasadena, as well as a few wry jokes amongst the scientists.
The thing was, the little craft was never designed to land on anything; it was just supposed to fly close to the asteroid Eros and take some pictures. NEAR had in fact taken a plethora of wonderful images, and now that the mission was winding down, the scientists wanted to do something a little more risky: actually land on the surface of an asteroid.
And yes, the obvious joke (after the Mars Polar Lander disappointment) was that JPL missions could still successfully land on other worlds, as long as the spacecraft was never designed to do so in the first place. But joking aside, the NEAR landing was a huge victory and a dramatic cap-off to a splendid mission.
To begin with, it's hard to call NEAR's rendezvous with Eros a "landing" at all. Eros, one of the asteroids that passes close to Earth, is only about 20 miles across. Such a small object in space has precious little surface gravity, although it's still enough to hold down small rocks and a thin layer of dust. To give you an idea of the weak gravity, a 200 pound person would weigh about two ounces on Eros, and just throwing a rock into space would give it enough velocity to escape. So, without a strong pull of gravity to worry about, the most complicated part of the whole procedure was slowing NEAR down enough to safely land. Up until then, it had been careening through the inner solar system, and had traveled over 2 billion miles since its launch in 1996.
It seems like a long way to go just to look at a big rock, which, simply put, is all that Eros is. It's low gravity could never pull all the rocky material into a sphere, so it drifts and tumbles through space looking for all the world like a giant baked potato (for a look at the entire asteroid, taken earlier in the NEAR mission, go to antwrp.gsfc.nasa.gov/apod/ap000316.html).
But there are actually several very good reasons for landing on this particular rock in space. Eros certainly isn't the biggest asteroid (that honor goes to Ceres, which clocks in at over 600 miles across), but it is one of the closest to us. Due to some gravitational hiccup many years ago, Eros was flung out of the asteroid belt into an orbit that takes it close to Earth. And a visitor from the asteroid belt is not to be taken lightly.
The asteroid belt is still quite mysterious to us. Most asteroids orbit the sun in a region between the planets Mars and Jupiter, in what we call the asteroid belt. I've often been amused by depictions of asteroid belts in science fiction; they're very dangerous places. In The Empire Strikes Back (I remain a huge Star Wars fan), Han Solo flew through a treacherous obstacle course full of giant, jagged rocks chaotically smashing into each other. The lesser-skilled pilots that pursued Solo didn't stand a chance they were crushed between the massive, tumbling boulders.
I once showed this sequence to an astronomy class I was teaching and asked my students a simple question: how long, in geologic time, could something like this last? If the asteroids were really flying into one another all the time, wouldn't they have ground themselves into dust in a few million years? In reality, the asteroid belt is a much more peaceful, even lonely place. For the most part, everything has settled into stable orbits, and in fact, if you were standing on an asteroid, you wouldn't even be able to see another one in the cold, dark sky.
But the asteroid belt is far from boring. Just how all these giant rocks got there in the first place is quite an enigma. We're pretty sure it's not a coincidence that all this stuff floats through space between Mars and Jupiter. Jupiter is over 300 times as massive as Earth, and packs a substantial gravitational wallop. And here's where the mystery of the asteroid belt takes on an air of poignancy. Scientists believe that the asteroid belt may have been a planet that tried to form too close to Jupiter, and was pulled apart by the gravity of the giant planet.
What evidence do we have of this? Intriguingly, some of the asteroids seem to show differentiation. That is, heavier material like iron and nickel has been somehow separated into distinct layers from the lighter, rocky stuff.
Differentiation happens when a body becomes big enough for gravity to pull the heavier stuff downward, toward the core. Our Earth is highly differentiated, with a solid core of iron in the middle, and lighter rocky material "floating" above and forming our mantle and tectonic plates.
So it seems that some of the asteroids were once part of a larger body that had a chance to form, at least to a some extent, but was then pulled apart by the relentless tugging of Jupiter. It's important to note that whatever it was that tried to form where the asteroid belt is, it never got very far. Jupiter probably pulled it apart before it even solidified, and in fact, there never was a whole lot of material out there anyway. If you put all the asteroids together today, you could make an object about half the size of our moon. Interesting, but not much to work with in the long run.
The NEAR spacecraft was sent out partially to discover whether Eros had been part of such a larger body in the past. Spectrometers and other instruments sampled the chemistry of the asteroid, and looked for the tell-tale layering of heavy and light materials. And, as is so typical in science, the answer to that question just led to a whole bunch of other questions.
It turns out that Eros isn't differentiated at all, which might make it even more important to study. Now, instead of looking at a small piece of some miscarried planet, we might be looking at a true left-over building block of our solar system. Eros is probably the kind of thing that all the planets all formed out of, as gravity slowly pulled the floating debris of the early Solar System into bigger and bigger bodies.
What's even more exciting is that nothing has disturbed the chemistry and composition of Eros for five billion years, making it a pristine example of what the chemistry of the early solar system was like. And once we know what kind of conditions existed so long ago in our own neighborhood, we'll be able to look for similar situations around other stars. Data from NEAR may very well help us to identify likely young solar systems around other stars.
So in February 2001, NEAR drifted to a perfect landing on Eros, taking pictures and data all the way down. We were able to see rocks as small as a human fist sitting on the sterile, thin soil of an asteroid. It might be small and potato-shaped, but it's yet another alien encounter.
Oh well.