Tuesday, 20 January 2009

Mooning over creationists (sequel)

In a post last year, I noted an intriguing question asked on the website of the local Edinburgh Creation Group (ECG).

They asked how the moon got into orbit.

The standard answer emerging from science is that some large object about the size of Mars (sometimes called “Theia”) hit the Earth very early on in the formation of the Solar System. That impact threw up debris which coalesced into the Moon. This hypothesis explains many otherwise-odd facts about the Earth-Moon system, such as their relative composition and their angular momentum.

However, the ECG folks claim to have found a problem. It’s very simply and persuasively illustrated by this simulation of orbital mechanics, which was linked from the ECG's website*. Play with it a bit yourself, to get a feel for the principle involved. Basically, once other forces (rocket thrust, planetary impact, atmospheric friction, etc) have stopped acting, only gravity is left. And when two objects (say, a fragment of debris and the Earth) are interacting only by gravity and are not substantially affected by external forces, there are three possible outcomes:
  1. The smaller object already orbits the larger (or they orbit each other), and will continue to do so indefinitely.
  2. They are on a collision course, and will impact (probably not parting again).
  3. The smaller object meets or exceeds escape velocity, and leaves.
There is no trickery here – the simulation linked to is a faithful representation of two bodies interacting gravitationally. Now, I am not a creationist. I tend to distrust claims from people with a vested interest, unless they can demonstrate that they've tried to set aside their own bias (for example, by doing actual science). I therefore put more trust in the scientists who report in a peer-reviewed forum, suggesting they've worked out the bugs in their hypothesis. But I do like to understand things. The creationist question is a valid one, a question I wanted to know the answer to. So in my earlier post, I asked my readers to help me out and let me know what the simulation is missing. One of them pointed to a researcher who has some animations on her website. However, the details of exactly how the creationist argument falls down were not obvious there. So (after a thesis-induced hiatus) I finally decided to look it up myself. Here is what I found.

First, I learned that scientists occasionally call it the Giant Splash rather than the Giant Impact, because at that scale even bodies of solid rock behaves more like a liquid when thumped together.

Second, when a bunch of debris is flying through space above the Earth's surface after such an impact, there is a very obvious force present that is adjusting the orbit of the fragments: the other fragments! Collisions and gravitational interaction between the bits of debris (which, remember, must together have massed at least as much as the moon) caused some of the fragments to settle into a stable orbit, where over time they accumulated into the single satellite we know today.

All this is a very abstract description of the answer. The details take massive computing power for simulations of all the particles thrown up by a big splash. Researchers have simulated many different types of impact, with different impactor sizes, different angles of impact (from head-on to glancing blow), and so on. Not all of the questions are resolved, but the problem highlighted by the ECG is certainly a non-issue.

If all that is too cerebral for you, I'll repeat something I said in the earlier post: I love games that use orbital mechanics. KSpaceDuel is one that models only a single centre of gravity, so if you don't use thrust you'll just stay in your initial orbit (until your opponent blows you up, anyway). [Note that it's a Linux-only game - yet another reason to start shedding that Windows or Mac addiction.] And for those of you who haven't yet converted to Linux, Orbit is an online flash game that very neatly illustrates how complex the gravitation effects become when you’re dealing with more than a couple of objects.

Notes for an undergraduate course at UCL (detailed)
New Scientist, 2006 (summary)

* I am currently unable to find the link - they seem to have redesigned their site and left it out. I therefore cannot be certain whether or not the claim is supported by the members of the ECG.

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