Hubble's new image of Comet ISON shows the comet streaking across space on its course toward the Sun. It's a beautiful picture, and it lets us know that a.) ISON doesn't appear to be shattering and b.) ISON's jet seems to have vanished. But how do we get that information from the picture you see below?
Comet expert (and blog guest contributer!) Dr. Jian-Yang Li created a mathematical computer model of Comet ISON's coma — the fuzzy, spherical cloud of material that forms around a comet's nucleus when it nears the Sun — using the latest Hubble data. By subtracting that model from the reality of the data, we can see where the two differentiate.
It's best explained with a metaphor. Imagine you have a hill. The hill has an overall shape, but it also has bumps that may be difficult to see.
Now imagine you could create a hill that is the same shape, but without the bumps. If you could use that model of the hill to subtract the overall shape of the hill, all that would be left behind are the bumps, which are then quite obvious. We then exaggerate the bumps — or scale up the data — to make the difference more obvious.
This is essentially what we do with the computer model and Hubble data. We're looking for differentiations, places where the comet's coma deviates from the model coma — one in which no radiation pressure from the Sun or solar wind plays a role. We're looking for the "bumps" that would indicate shattering or a jet once we subtract the model from the reality.
But what we see here is nothing out of the ordinary. There's no jet, no fragmentation.
Let's compare it with our previously modeled image from May.
Credit: NASA, ESA, the Hubble Heritage Team (AURA/STScI) and Jian-Yang Li (Planetary Science Institute)
In this image, we do see a jet. The jet might have shut itself off, or it could have been a bit of icy, volatile frosting left over from the comet's Oort Cloud days that flared briefly and then spent itself.
What else can we see in our most recent October image? Let's take a look.
This is a single exposure and a raw image. So you see the flecks caused by cosmic rays striking Hubble's detectors, and stars that are turned into streaks by the motion of the telescope as it tracks a moving foreground target.
Finally, let's look at this current image of Comet ISON, and our last image of Comet ISON. What's changed?
Check out that significant difference in size. In the Oct. 9 image of ISON, the comet is much bigger. That's because it's physically closer — 177 million miles from Earth on Oct. 9 versus 386 million miles from the Sun on May 8 — but also because its increasing proximity to the Sun has made it much bigger. The coma is expanding and the tail is lengthening as it warms in the Sun's glare.
Comet ISON remains a subject of fascination for astronomers, but will it still be an impressive comet for the casual viewer? It's hard to say, but many astronomers are now saying they suspect the comet will require binoculars and not become a naked-eye object. It just hasn't brightened as expected.