Images: Serendipitous Discovery of Asteroid 128177 (2003 RA11)
What is an asteroid? An asteroid is a small chunk of rock orbiting our sun. The largest asteroid is nearly 1000 km across, although most are smaller, with many less than 1 km across. The majority spend their lives orbiting the sun in a diffuse belt between the orbits of Mars and Jupiter. They are considered to be remnants from the epoch of planet formation.
What does an asteroid look like? To a passing spacecraft, an asteroid appears to be a giant, pitted rock. However, from the distant platform of Earth, tiny asteroids look very much like any faint star. That is, they look like a tiny spot of light. How, then, does one distinguish between a star and an asteroid? An asteroid is much closer to us than a star. Therefore, one can see it appear to drift with respect to the background stars as the Earth passes by. But how does one discover an asteroid, since it seems you must already know where it is and follow it for several hours? The answer: by accident. At least, this is how we discovered this asteroid.
The story: In an attempt to measure the rotational period of a known asteroid, we took a series of 30 five minute exposures. It turns out that in the field of view of these images we saw not only the asteroid whose rotation we were trying to measure, but three other unexpected asteroids as well! We deduced they were all asteroids since they all looked like little stars that slowly moved across the star field as the night progressed. We checked their positions against those of known asteroids and found that one of them was not on the list. We had discovered a new asteroid!
The movie follows the new asteroid over a period of three hours (a faint, stationary spot near the middle of each frame). Stars drift up and to the left at a rate of 32 arcseconds per hour, in response to the Earth's motion past the asteroid. This is like in a home movie of a dog running around the back yard. The camera follows the dog, which appears stationary in the middle of each frame while the grass drifts by. The movie consists of ten frames created from the 36 images we took, so each frame is actually the average of three to four images. This helps to show the incredibly faint asteroid more clearly.
The still image shows a larger portion of the field of our observations. In this image we averaged together all of the movie frames, still holding the position of the new asteroid fixed. The motion of the stars up and to the left over three hours make them long trails in this image. The new asteroid (circled in red) shows as a pointlike image. There are also two short trails moving up and to the right. These are two other asteroids, moving more like the new asteroid than the stars, but differently enough to trail somewhat (also circled). The asteroid in the upper left corner is named Yokotatakao (6649), while the one in the lower right is van den Heuvel (3091). The latter was the target of the rotation study. That one is able to discover a new, exceedingly faint solar system object in a single field of view is a testimony to the power of an excellent telescope on a rare Michigan night of superb transparency.
Processing: The discovery images were taken on September 5, 2003 from 12:30 am to 4:00 am. The images were all five minute exposures taken with a red filter. After reducing the data by dark subtraction and flat fielding the images, astrometric positions and brightnesses of each of the asteroids in the field were determined. An even longer series of images were made the following night to confirm the existence of the new asteroid.
Positions for both nights were submitted to the Minor Planet Center, the institution responsible for keeping track of all the small bodies in the solar system. They confirmed that the object was indeed not previously observed and assigned a provisional designation of 2003 RA11 (based on the date of discovery). After additional observations over later oppositions, the orbit was sufficiently refined for it to receive its permanent numerical designation, 128177 (on March 20, 2006). Only at this point is the discoverer allowed to suggest a permanent name to the International Astronomical Union. [See our asteroid recovery page for an details on tracking Asteroid 128177.] The observations, data reduction, and image processing were all done by Calvin student Andrew Vanden Heuvel.
Orientation and scale: North is up and East is to the left. The angular dimensions of the movie is 4.4 by 4.1 arcminutes, and of the still is 10 by 10.6 arcminutes.The apparently bright star (GSC 5232:1066) approached by Asteroid 128177 in the movie has a visual magnitude of 15.5, some ten thousand times fainter than the faintest star visible to the unaided eye. Asteroid 128177 had a magnitude of about 19.5, forty times fainter again than the "bright" star.
On the night of its discovery, Asteroid 128177 was in the constellation of Aquarius, just 10 degrees north of the planet Mars, which dominated the night sky, shining eight hundred million times more brightly than the asteroid.
The celestial coordinates of the star GSC 5232:1066 are 22h20m25.2s, -6deg44'9'' (epoch 2000).