On May 8, the Hubble Space Telescope snapped a series of short exposures of Comet ISON. Cut together, these frames capture ISON as it hurtles into the inner solar system. But when we released this video, our astute friends on Facebook noticed something important: the comet appears to be moving in the wrong direction.
So why isn’t ISON streaking across the stars into the lower-left, in the direction we all intuitively assume it’s supposed to? To answer, let’s go behind the scenes to understand how the Hubble Heritage Team made this GIF in the first place. There’s a whole system of moving parts to consider – only one of which is ISON itself.
ISON is moving in the direction you think it is...
For a comet, traveling through the solar system isn’t always smooth sailing. Radiation from the Sun and the charged particles that make up the solar wind tear away at a comet’s cold, cloudy coma like thickets of brambles. As a result, many of the more volatile ingredients that make up comets – like frozen water and more exotic ices – sublimate off into large plumes of gas.
ISON is pretty much in free fall now, falling faster and faster into the lower left of the image, toward a close brush with the Sun and a flambé-style roasting. So, if we had looked from a sun-fixed reference point against the backdrop of distant stars, our movie would show ISON moving with its tail fluttering in the solar wind like a cape.
...and yet the Earth moves...
The basics of ISON’s orbit: over the next six months, it’s going to slingshot around the Sun and then loop back for a close flyby past the Earth. To see this path in three dimensions – which is the only way to really understand it – check out this brief animation of the orbit from NASA. What’s obvious from the video (Spoiler alert, if you didn’t watch it yet, or aren’t hip to the Copernican Revolution) is that the Earth moves a lot during this span, too. So as ISON is approaching and then whipping around the Sun, we on supposed terra firma are making good time of our own.
...and Hubble moves.
Here’s where things get complicated. The biggest factor behind ISON’s apparent winding path is Hubble’s orbit around the Earth. When we say Hubble Space Telescope, we’re serious about the “space” part -- the Hubble Telescope soars around the planet every 97 minutes. For just about anything in the sky, including ISON, the Earth is in the way for a large part of that orbit, blocking Hubble’s view. So when Hubble observes ISON for a single orbit, we have an hour or less to capture as many pictures as we can. For each exposure making up the GIF, we chose to keep the telescope trained on Comet ISON so that we’d get the crispest resolution possible on the comet.
Since we’re following ISON, it’s the stars that appear to move. In each exposure that’s been added into the above image, stars are in a different position. From frame to frame, they move like the comet moves in the GIF. Moreover, the images of single stars are smeared out instead of round. Even during the short individual exposures, stars are visibly drifting relative to comet ISON.
From this image, we can also simulate what we would have seen if the telescope stayed on the stars instead of ISON. By aligning each image so that the stars are in the same place, we’ve made the image below, which is basically a static version of the GIF.
So why does ISON meander in these images? Well, ISON is heading toward the Sun, sure. At the same time, the Earth is swinging through its own orbit. And in the roughly hour-long span during which these images were taken, Hubble is crossing from one side of Earth to the other.
Astronomers call the effect parallax -- it’s what happens when you hold your thumb in front of your face and blink back and forth from eye to eye. Closer objects (your thumb, ISON) shift when your perspective changes, much more than objects farther away like the stars do. So if you find ISON’s movement in the GIF confusing, rest assured: it’s not ISON. It’s us.