It might seem like a tautology — and that’s because it is — but sometimes the only word you can use to describe an image from the Cassini Saturn probe is otherwordly:

[Click to engasgiantize.]

This otherworldy picture was taken on March 24, 2010. The big moon is Rhea, seen from 1.2 million kilometers (750,000 miles) away, and the little one below it is Epimetheus, from 1.6 million km (990,000 miles) away. Perspective makes them look right next to each other, but in reality the distance between them is the same as the Moon from the Earth! Saturn and its rings provide the backdrop for this stunning alien portrait.
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This is totally cool: an animated simulator that lets you make model solar systems! It’s put together by the PhET Interactive Simulations group at — hey! — the University of Colorado at Boulder.

All you have to do is put in the masses, locations, and initial velocities of the objects (up to four) and then hit "go". What you’ll probably find is that for almost any parameters you use, you won’t get a stable system. You’ll fling off the tiny moon, or drop a planet into the star, or collide two planets (when you do, one survives after a brief comical flash). There are preset conditions that will put together a stable simulation, so I suggest you start there and then tweak the numbers. The most fun thing is to fiddle with the mass and see what happens.

You’ll note a slider that says Accurate vs. Fast. That has to do with bin size. Basically, a simulation like this calculates the force of gravity of each object on every other object using Newton’s law. But it needs a time interval to do this: where will all the objects be after some period of time? You can pick that time step, but the smaller the time step the more accurate it will be. That’s because gravity works continuously. If you take the Earth’s current position and velocity and ask where it will be a year from now by just adding a year to the program, it’ll extrapolate the Earth’s current velocity direction! The program will take that velocity (about 30 km/sec) and multiply it by one year, and get a distance of about a billion kilometers. It’ll then place the Earth there. But that’s not right, because the Earth orbits the Sun; the Sun’s gravity is continuously changing the direction of Earth’s motion. So the smaller the time step, the more accurate the program will be.

At least, I think that’s what’s going on here. I’ve fiddled with programs like this before, and that’s what I’ve found. Roundoff error can be bad too; because the program can’t do the calculations exactly — the decimal value has to cut off somewhere — every step has a little bit of error in it. That adds up, and after a few orbits things can go wonky. This one does a pretty good job of that, it looks like.

Anyway, go play god with your very own cosmic erector set. It’s fun, and before you know it a long time will have passed… but you might get a feel for orbital mechanics. It’s worth it.

The picture above shows a cosmic bulls-eye of epic alignment. But before I can tell you about it, I have to tell you about how the dart got thrown.

One of the more amazing aspects of looking into deep, deep space is that the path there is tortured and twisted. Space itself can be distorted by mass; it gets bent, like a road curves as it goes around a hill. And like a truck that must follow that road and steer around the hill, a photon must follow the curve of space.

Imagine a distant galaxy, billions of light years away. It emits light in all directions. One particular photon happens to be emitted almost — but not quite — in our direction. Left on its own, we’d never see it because it would miss the Earth by thousands or millions of light years.

But on its travels, it passes by another massive galaxy. This galaxy warps space, and the photon does what it must do: it follows that curve in pace, and changes direction… and it just so happens that the curve is just right to send it our way.

The intervening galaxy is essentially acting like a lens, bending the light. If the more distant galaxy is exactly behind the lensing galaxy, we see the light from that more distant galaxy distorted into a perfect ring, a circle of light surrounding the lens. We call this an Einstein Ring. If the farther galaxy is off to the side a bit, we see an arc instead of a complete ring. Gravitationally lensed arcs and rings are seen all over the sky, and they can be used to determine the mass of the intervening galaxy! The more mass, the more distorted the light from the farther galaxy. So the Universe has given us a nice method to let us weigh it.

In a surprising twist, astronomers have found a new type of lensed galaxy: a double ring! In a rare alignment, there are two distant galaxies aligned behind an intervening lensing galaxy. They’re like beads on a wire, lined up just right such that both more distant galaxies are lensed by the nearer one. In this case, the lens is about 3 billion light years away, and the other two are 6 and 11 billion light years away, an incredible distance.

This image is amazing, but it is also a powerful scientific tool. It allows us to measure not just the mass of the lensing galaxy, but also the amount of mysterious dark matter nearby. We cannot see the dark matter, but it too bends light, and contributes to the lensings. By observing lenses like this, we can take a sample of dark matter in the Universe, and that’s a crucial first step in understanding it. Even better, these double rings allows us to measure the amount of total mass not just in the nearest galaxy, as is usual, but also in the middle galaxy as well, since it distorts the light from the galaxy behind it (turns out it’s a rather lightweight one billion solar masses; our own Galaxy has more than 100 times that mass, so the middle galaxy is considered a dwarf).

This is a beautiful happenstance; it gives us a measure of the Universe at two points, with one being for free. In fact, Tommaso Treu, the astronomer at U.C. Santa Barbara who investigated this lens, points out that if we can find as few as 50 of these double rings, we can get a much better idea of the distribution of not just dark matter, but also the even more mysterious dark energy in the Universe. That’s one of the biggest goals of modern astronomy… and we may get a handle on it due to a coincidental ring toss.