Dawn, Go Away I’m No Good For You

I was watching the local news the other day — OK, to be honest, I was trying to summon enough consciousness to start the coffee machine while the local news played somewhere within the truncated reach of my senses — and I heard the anchor say something about an asteroid approaching Earth and a NASA probe making contact with it. At first I assumed he was talking about 2011 MD, the asteroid that passed a scant 7500 miles beneath Antarctica — which I guess would actually have been above Antarctica to anyone standing on Antarctica when it happened.

But then I realized that 2011 MD had already passed us by, and that NASA had not sent any probes to meet it. (That would have been tough to do, since humanity only learned of its existence five days before its flyover.)

Curious now, I tried to shake off the somnolence (my head actually made that ratchety sound that Warner Brothers cartoon character’s heads make when they shake them) and pay attention to the story. I eventually figured out that he was talking about NASA’s Dawn probe, which was going into orbit around Vesta. The reason I didn’t get that right away is, of course, that Vesta isn’t approaching Earth. It’s right where it’s always been, in the Asteroid Belt, between the orbits of Mars and Jupiter. (There’s a reason why my wife and I refer to that anchor as “Ted Baxter.”)

Anyway, it occurs to me there’s a buttload wealth of explanationizing that can be done about asteroids, Vesta, and Dawn.

Asteroids

First of all, forget about any scene in any movie in which Our Heroes rocket through an asteroid field. Asteroids do not move that quickly, and are not that close together. In fact, if you were standing on Vesta — well, you’d probably die from, y’know, cold and no air and stuff. But just before you died, you’d look up and realize that you couldn’t see any other asteroids. They’re all too small and too far away.

When I was a kid, back before the telescope was invented, the consensus was that the asteroids in the Asteroid Belt were the remnants of a planet that had been hit by something really big and smashed to smithereens. Now the consensus is just the opposite.

Oh, wait. The opposite would be fragments that got hit my something really big and joined up to form a planet. That’s not what the asteroids are. Duh. Doesn’t even make sense.

Let’s start over.

The current thinking is that the solar system started as a swirling disk of dust and gas. Hydrogen atoms in the center of the disk were attracted to each other by gravity. More and more of them pressed closer and closer together until nuclear fusion started and they became the Sun. (Yes, Virginia, this is the short version of the story.)

The Sun was still surrounded by a disk of dust, and gravity was still at work. Dust pieces stuck together to form dirt pieces, which stuck together to form rocks, which stuck together to form planets.

(Short version, remember? And, obviously, I’m talking about Mercury, Venus, Earth, and Mars here. Jupiter, Saturn, Uranus, and Neptune are made of gas and ice, not dirt and rocks.)

The one place this didn’t happen was near Jupiter. Mars formed OK, but Jupiter is so big, so really, really huge, that its gravity messed with the pieces of rock in the orbit beyond Mars. They were never able to form up into a planet. (I wonder if they resent Jupiter for that. I know I would.)

Vesta

I’m sure you all recognize the name Vesta, and remembered right away that it’s an asteroid, because of …

Anyone? Anyone?

No?

Isaac Asimov’s first published science fiction story was called “Marooned Off Vesta.” It’s about three guys who were — well, you can figure it out.

There is, as near as I can tell, exactly one other
interesting thing about Vesta, but to me it’s pretty darned interesting. Y’know how Earth has a crust, a mantle, and a core? Well, so does Vesta. It really does seem to have started down that path toward becoming a planet, at least until Jupiter came along and ruined everything. As it is, it’s only 360 miles (578 km) in diameter, and still it has a core, a mantle, and a crust.

(A lot of the news stories I’ve read say that Vesta is “360 miles across,” which is “about the size of Arizona.” That may be true — I’m really not going to bother looking up Arizona’s size — but it occurs to me that Arizona is flat and Vesta is three-dimensional. A sphere with a diameter of 360 miles would have a surface area of 407,150 square miles, and Arizona only measures 114,000 square miles (OK, so I looked it up — it’s roughly 340 mi x 395 mi). So the popular media are getting the math/science wrong. What a surprise.)

Anyway, so the cool thing about Vesta is that it’s a protoplanet. The prefix proto– means “first,” or “original” or “primitive.” Think of words like prototype, protohuman, and protozoan.

Oh, yeah. Vesta is also the one of the oldest objects in the Solar System, so studying it will give us a pretty good idea about what was going on on Earth when it was forming. Also, pieces of Vesta have been knocked off over the millennia. Some of them litter the Asteroid Belt. Many of them have fallen to Earth as meteorites. OK, that another cool thing.

Dawn

There are some pretty interesting things about Dawn, too, and not just the fact that it was named by a NASA administrator who’s also the head of the national Tony Orlando Fan Club. (I made that up.) For example:

• It’s the largest probe NASA has ever launched.

• It’s the first thing they’ve sent into the Asteroid Belt (as opposed to through it).

• It’s the first probe to orbit something that’s not a planet or the Moon.

• It’s the first probe that’s going to orbit something for a couple years, then blast off and orbit something else (the dwarf planet Ceres, also in the Asteroid Belt).

• It uses ion propulsion.

Kirk: Interesting design.

Scotty: I’ve never seen anything like it. And ion propulsion at that. Ach, they could teach us a thing or two.

Dawn is actually the second probe to use ion propulsion. All the fuel for the entire mission (not including the rocket that actually put Dawn in Earth orbit) consists of a 937-pound lump of xenon. Well, not a lump, because xenon is a gas. Although it freezes at –170˚ F, so maybe it is a solid out in space.

Anyway, those big solar panels produce electricity, which is used to make xenon ions jump out of the xenon tank and out the back of the craft. Hocus-pocus, ipso-facto, badda bing-badda boom, action-reaction, the craft moves forward. Slowly, at first. Really, really slowly. See, ions shooting out the back of a spacecraft don’t provide a lot of force. Hold your hand palm-up and set a sheet of printer paper on it. Can you feel the force with which that sheet of paper is pressing down on your hand? That’s about how much force the ion propulsion provides. But it’s enough to take Dawn on its multi-million-mile journey.