Physics Photo of the Week

November 11, 2011

Asteroid 2005 YU55
Last Tuesday evening (November 8, 2011) we had the rare opportunity to view an asteroid as it zipped past the Earth at about 3/4 the distance to the Moon.  Astronomy students Brandon Knutson, Katie Anderson, and Josh Jenkinson assisted in capturing a series of telescopic images of this asteroid.  Although the asteroid was bright as asteroids go, it still required a telescope to observe.  The above photograph is a sum of about 40 5-second exposures as the asteroid swept past the field of view of the telescope camera in a period just under 5 minutes.  According to NASA the asteroid is about 800 meters (1/2 mile) in diameter.  The Moon was very bright that night as well, making the background sky rather bright.  The narrow "squiggly" lines throughout the image are scattered "hot pixels" in the camera.  In the
excitement we forgot to turn on the camera cooling mechanism, which would have virtually eliminated the hot pixels.  The "movement" of the hot pixels is an artifact.  The telescope is clock-driven to keep up with the stars, but the guiding is not perfect.  The stars move slightly relative to the hot pixels.  The stack of images was aligned with the stars, thus causing the hot pixels to appear to move.

The animated pigure at the right shows a succession of alternate 5-second exposures played back at about 20 times the real-time speed.  It was amazing to sit at the telescope and computer monitor watching the star-like object  move to a new position with every 5 second image.  Of course the images of the asteroid are elongated because of its continual movement during the 5 second exposures.

NASA keeps close watch on asteroids and other objects that come anywhere near to Earth's orbit.  The orbit of 2005 YU55 is so well-known that there is no chance of any devastating collision with Earth within the next 100 years

Special thanks are also extended to physics assistant Mayuri Patel for repairing the electrical circuit that supplied power to the telescope earlier that day.

Physics Photo of the Week is published weekly during the academic year on Fridays by the Warren Wilson College Physics Department.  These photos feature interesting phenomena in the world around us.  Students, faculty, and others are invited to submit digital (or film) photographs for publication and explanation.  Atmospheric phenomena are especially welcome.  Please send any photos to 

All photos and discussions are copyright by Donald Collins or by the person credited for the photo and/or discussion.  These photos and discussions may be used for private individual use or educational use.  Any commercial use without written permission of the photoprovider is forbidden.

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