Today we answer some of the Comet ISON questions you've posed on Facebook, Twitter and Google+. What do you need to know about ISON? Tell us and we'll try to find out.
I saw your post on how the last Hubble image of Comet ISON was made. Why don’t we see those kind of weird streaks in other Hubble images?
The short answer is that we see them all the time. For instance, here’s one:
In the box at the lower right is an image of an asteroid that Hubble happened to capture while imaging the distant galaxy in the main picture. Basically, this is what happens when Hubble takes in a moving target, like an asteroid or, in ISON’s case, a comet, while focused steadily on the background galaxies.
People are rarely exposed to Hubble’s raw images, but their features help explain how Hubble functions. So let’s do what the picture tells us, and see that closeup. Hello, asteriod.
This is a stacked image of several exposures, which is how all of Hubble’s images are made.
Hubble always takes multiple exposures, because that’s how it creates a picture. The multiple exposures allow Hubble to look at an object through different filters, which supply varied information about the object. It also allows scientists to weed out artifacts caused by the telescope and cosmic rays – the stuff that appears in one exposure but not the next is junk that needs to be subtracted from an image, not real celestial objects.
Hubble is also moving as it takes its images. In this case the telescope is moving and so is the asteroid, so in this image you’re seeing the effects of both motions.
See the dot? This is a short exposure of the asteroid, about 10 seconds long. Because it's so brief, it looks like you would expect a snapshot of an asteroid to look – a little blob.
But these long streaks are also the asteroid. These are extended exposures of the same asteroid, around 1,000 seconds long. We see these kind of long-exposure blurred effects all the time in pictures on Earth. All you have to do is focus your camera on a background object, while something else moves in the foreground. In this case, the asteriod is the moving foreground object, and the giant galaxy is the background object. These streaks are curved due to Hubble’s orbit around the Earth.
Let's look at that closeup again.
Check out the gaps between the streaks and the dots, and the way they seem to be spaced above one another. The gaps are places where Hubble stopped imaging to do things like change filters, or save files. The spaces between indicate the points where Hubble was on the other side of the Earth, unable to take images. Remember, Hubble spins around the Earth at a speed of five miles (8 km) per second. It completes this trip every 97 minutes.
But it's probably easier to just show you.
There’s also a lot of garbage in this raw, unprocessed image – see all those flecks and lines?
Those are cosmic rays – radiation we’re lucky enough to be protected from by Earth’s atmosphere. They penetrate the telescope and leave images on its detectors. The specks are cosmic rays that hit the detectors head on. The streaks are rays that entered at a glancing angle.
Pictures like this asteroid image are extremely common. We see them when Hubble captures pictures of asteroids, and when it captures man-made satellites. In fact about one out of every 20 Hubble frames has a satellite in it somewhere. They’re hard to avoid.
So. Kind of fascinating, no? Here's that image of the asteroid again, with the features called out.
So why don’t all of Hubble's comet pictures look like this? Because while this Hubble image focuses on Comet ISON at the expense of the background galaxies, here we made the decision to focus on the background galaxies and let the comet in the foreground blur. We felt it helped convey how ISON is mere hundreds of millions of miles away, while the galaxies are many millions of light-years distant.
Only Hubble is capable of taking an image like this of Comet ISON, that gives this sense of the immense depth of space.
But this is what it looks like before it's been processed.
See those streaks that indicate the comet nucleus? Look familiar?
And here's your comet as a series of exposures.
Will the Solar and Heliospheric Observatory (SOHO) observe Comet ISON?
SOHO expects to get a view of the comet from early Nov. 27 to the end of Nov. 30.
If you want to see what kind of comet observations SOHO has made in the past, check out its comet video gallery.
How big is ISON's nucleus?
According to our expert, Dr. Jian-Yang Li, "We've put an upper limit of 2 km on the radius of the nucleus. But we are still working on the images to refine the results." You can read more about how we discover the size of ISON's nucleus in these previous posts.