Searching for lines in the picture
Astronomers have stumbled upon an unusual asteroid-hunting ground by accident: in the Hubble Telescope archives they have found a plethora of images with glowing lines. These are the tracks of small asteroids that passed through the frame during long exposures. According to the latest calculations by astronomers from the Jet Propulsion Laboratory (JPL), the asteroid 1997 XF11 will pass the earth in October 2028 at a much greater distance than scientists initially thought. After evaluating images from the Palomar Observatory in 1990, Donald K. Yeomans and Paul W. Chodas concluded that there would be a good 960,000 kilometers between Earth and the asteroid. In their opinion, a collision can be ruled out.
At the time the images now being consulted were taken, 1997 XF11 had not yet been discovered. The actual target of the long-exposure photos were more distant celestial objects. The asteroid just happened to be passing by without paying any attention. Recently, astronomers have increasingly used archival images to map the profusion of minor planets in our solar system. The Hubble Space Telescope offers you particularly beautiful and productive sources.
The hunt by JPL's Robin Evans and Karl Stapelfeldt has resulted in a sizeable catch of around 1000 small asteroids. Their preliminary analysis suggests that a total of 300,000 minor planets - essentially boulders one to three kilometers in diameter - orbit between Mars and Jupiter. So far, the orbits of 8,319 asteroids have been measured in this main belt of planetoids, about the same number of asteroids have been sighted, but their detection has not yet been confirmed.
Over a three-year period, the two astronomers and their collaborators combed through more than 28,000 images from the Wide Field and Planetary Camera 2 (WFPC2) in search of broad, intricate streaks of light - clear signs of asteroids in the telescope. Most of the ones they found are too faint to be observed using search programs currently based on Earth. The Hubble Telescope captures these images purely by chance: nearby asteroids are bound to drift through the field of view at some point, while other higher-priority targets are observed.
"The stock images are fairly evenly distributed across the sky, so the number and location of asteroids found reflects the actual conditions in the sky," Evans said. "As expected, we see a concentration of asteroids in the direction of the ecliptic, and we see small asteroids because they are the most numerous. These small main belt asteroids have the best chance of being steered onto a course toward Earth by their larger neighbors."
An accurate count of asteroids is an important part of calculating how many of these small bodies might pose a threat to Earth. The new information from the Hubble archives could help scientists better assess the risk that the asteroids pose to Earth.
Hubble images capture an asteroid as a long line created by the asteroid moving through the camera's field of view. The curvature of the lines results from the movement of the telescope around the earth. It allowed scientists to determine the distances and sizes of the asteroids.
According to Evans and Stapelfeldt, the image archive data also limits the maximum number of small comets that could pass very close to Earth. In 1997, Louis A. Frank of the University of Iowa, using data from the Polar Spacecraft, reported finding evidence that about a dozen small comets per minute are impacting the Earth's upper atmosphere. Evans and Stapelfeldt believe these small comets should be bright enough to leave thousands of detectable tracks in the Hubble images - but they weren't.
