Asteroids are small and dim, but over time astronomers have managed to calculate the orbits of thousands of them. What is less well known, however, is their size and shape. They are just too far away for even the largest telescopes to get a good picture of any but the very largest ones.
However, once in a while, an asteroid will pass directly in front of a star (as visible from Earth) and for a brief few seconds the star will wink out, then reappear as it emerges from its eclipse. An event like this is called an "occultation" (because the asteroid temporarily occults, or blocks, the light from the star).
Timing the length of the occultation to a fraction of a second can actually give us an estimate of the size of the asteroid. Since we know its orbit and speed, knowing how long it occults a star makes it easy to calculate its size.
But what if the asteroid is oddly shaped, like say a peanut? Then an observer on earth who saw the fat part of the peanut occult the star might measure the star winking out for say 10 seconds, while an observer on earth a small distance away might see the star wink out for only 6 seconds, depending on whether the fat or the skinny part of the asteroid passed between their eye and the star. But if there were many observers, and they each had accurate clocks, you could actually use these observations to sketch an outline of the asteroid!
This is exactly what the members of the International Occultation Timing Association (known as IOTA) do. They coordinate their locations across the path of the asteroid's shadow on earth as well as along its "ground track" and send their data to the organizations central database. Over time, these observations build up and provide information about these objects size and shape. In addition, the actual times of arrival of the shadows lets astronomers improve their estimates of the orbit of the asteroid. All of this knowledge from a few frames on a video camera where a star disappears!
Well, this morning I made an attempt to add to this database by observing and recording the occultation of star TYC 5559-00693-1 (a very dim, 10th magnitude star) by the asteroid Hygiea from Hamton Beach State Park. This asteroid is pretty big, about 470 km across, so the occultation was expected to last about 10 seconds or so and cover a wide swath of ground across the Northeast. The occultation was scheduled to happen at 5:07am, so I set up my equipment (telescope and video camera) at 4am to record the event.
However, as I zoomed in on the area of the star that was going to be occulted, all the stars around it matched up with my sky map of the area except one. There was an extra, dim star very close to the target star, and this dim star did not appear on the star map. For a moment I wondered if I had mistakenly pointed my telescope at the wrong area of the sky. Then I realized with a rush of excitement that this "extra" star was actually the asteroid itself, on its way to pass in front of the target star. I actually saw the asteroid and the star close to each other!
Although I was now sure I had the target star clearly identified in my eyepiece, unfortunately a bank of clouds rolled in at 4:30 and by 5:00am not even bright Venus could be seen in the sky. Such is astronomy in the Northeast. So I hope my fellow IOTA enthusiasts who were spread out across Massachusetts and Connecticut had clearer skies.
There are hundreds of occultations each year, so I will try again soon.
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