Perspective on four moons

Contributed by
Sep 8, 2010
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The Cassini probe orbiting Saturn returned an interesting picture yesterday. It shows four tiny moons and the rings seen nearly edge-on. Take a look:


[Click to enjovianate.]

From left to right the moons are Epimetheus (113 km/70 miles across), Janus (179 km/111miles), Prometheus (86 km/53 miles) and Atlas (30 km/19 miles). Like I said, tiny.

When I see images like this I like to amuse myself by fiddling in my mind with their perspective. For example, is Epimetheus closer to us (well, to Cassini when the picture was taken) than Janus was? Even more interestingly, are we looking down on, or up at the rings?

Images like this don't give us the clues we usually get here on Earth to figure out distance. Look at the picture: the rings make a tight curve across the field. We know we're seeing a circular ring nearly edge-on... but are we looking down on it, so that the top of the curve is farther away, or are we looking up at it, so that the bottom of the curve is farther away?

For example, take a DVD and hold it so that you're looking at it almost edge-on. Tilt the near edge down a bit so you're looking down on the top side. Now tilt the near side up so you're looking up on the bottom side. See the issue? Without lighting, focus, or other cues, it's hard to tell which way you're seeing an object.

So for the Saturn picture, which is it? I'll tell you below, but see if you can figure it out. Did you guess? I'll tell you a secret: you can't tell from just looking. It so happens that Cassini was above the ring plane looking down on them, so the top part of the curving rings is farther away. But in reality, the picture gives you no clue of this*.

As it happens, too, Epimetheus was slightly closer than the other three moons when this shot was snapped. It was 1.2 million kilometers from Cassini, while the other three were 1.3 million. Since the moons are irregular and lumpy, there are very few distance clues either. One thing that helps is that we're seeing this vista from an angle, so if a moon were a lot farther away than the others, it would look like it's way off the ring plane. So that's a clue that they are all pretty much at the same distance, but other than that there's not much you can determine.

That tells you how important spacecraft telemetry is. We get a constant stream of information from Cassini, telling us where it is, where it's pointing, what filters and camera it's using. Without these bits of data we'd have a hard time actually doing any science with these images.

And while it's very pretty, there's a whole lot more going on here than just an interplanetary tableau. The picture is only one piece of it! Scientists are learning boatloads about the outer solar system with Cassini and other probes, and it's only by diving into the actual data that this can be done.

Image credit: NASA/JPL/Space Science Institute

* As it happens, there might be a way to tell. If the outer ring were not so thin, and actually had a visible width, you'd see the far side as narrow and the near side as wider due to perspective. However, in this small picture and with that narrow a ring, I don't think in this case that's measurable.