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News Release 4 of 48

November 11, 2004 09:00 AM (EST)

News Release Number: STScI-2004-31

Hubble Tracks Asteroid's Sky Trek

A Hubble Heritage Release

November 11, 2004: While analyzing NASA Hubble Space Telescope images of the Sagittarius dwarf irregular galaxy (SagDIG), an international team of astronomers led by Simone Marchi, Yazan Momany, and Luigi Bedin discovered 13 sucessive faint trails left by a tiny asteroid. The trails are seen as a series of reddish arcs on the right in this August 2003 Advanced Camera for Surveys image.

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Q & A: Understanding the Discovery

  1. 1. Is the asteroid in our solar system, our galaxy, or in the Sagittarius dwarf irregular galaxy (SagDIG)?

  2. The asteroid is located within our own solar system, most probably between the orbits of Mars and Jupiter. Other objects visible in the photograph include several bright foreground stars, which are located within our Milky Way galaxy, generally a few hundred to a few thousand light-years away. The majority of the numerous faint, bluish stars are members of SagDIG, lying about 3.5 million light-years away from Earth. Also visible are a number of background galaxies (the small, reddish/brown extended objects with spiral arms and halos). These lie at distances of many millions of light-years. There is thus a vast range of distances among the objects visible in this photo, ranging from about 169 million miles for the asteroid, up to many quadrillions of miles for the faint, small galaxies.

  3. 2. Why are the asteroid trails curved?

  4. As the Hubble Space Telescope orbits around the Earth, and the Earth moves around the Sun, the asteroid will appear to move with respect to the vastly more distant background stars, due to an effect called parallax. It is somewhat similar to the effect you see from a moving car, in which trees by the side of the road appear to be moving much more rapidly than background objects at much larger distances. If the Hubble exposure were a continuous one, the asteroid trail would appear like a continuous wavy line. However, the exposure with Hubble's camera was actually broken up into more than a dozen separate exposures. After each exposure, the camera's shutter was closed while the image was transferred from the electronic detector into the camera's computer memory; this accounts for the many interruptions in the asteroid's trail.

  5. 3. Is it rare for Hubble to catch an asteroid during an exposure?

  6. It is rare for Hubble to photograph asteroids by chance, given the small field of view of its cameras, but it does happen occasionally. The odds are highest when Hubble is looking in the direction of the asteroid belt. The vast majority of asteroids orbiting the sun are located in a region called the asteroid belt. This belt lies between the orbits of Mars and Jupiter. As the planets and asteroids move around the sun, they are confined to an area of the sky that is comparable to the equator of the sun projected onto the sky. The constellation Sagittarius lies in this area, called the ecliptic. Thus, when one looks at galaxies or other objects within constellations along the ecliptic, there is a higher chance of also imaging the path of an asteroid.

  7. 4. How do dwarf irregular galaxies differ from other galaxies?

  8. As their name implies, dwarf irregular galaxies differ from their spiral and elliptical cousins because of their much smaller physical size and lack of definite structure. There are many more dwarf irregular galaxies than there are large spirals and elliptical. It is thought that most large galaxies, like our Milky Way, have been formed by large numbers of dwarf galaxies that have coalesced together over many billions of years.

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Credit: NASA, ESA, and Y. Momany (University of Padua)