Hubble's Search for Kuiper Belt Objects
A Kuiper Belt object (KBO) that is potentially reachable by NASA's Pluto-bound New Horizons probe is visible in multiple exposures taken with the Hubble Space Telescope. Hubble tracked the KBO (labeled PT1) moving against the crowded background field of stars in the summer constellation Sagittarius. The object is no bigger than 19 to 28 miles across, and it is a deep-freeze relic of what the outer solar system was like 4.6 billion years ago, during the period when the Sun formed. The image at right shows the KBO at an estimated distance of approximately 4 billion miles from Earth. As the KBO orbits the Sun, its position noticeably shifts between exposures taken approximately 10 minutes apart. Following an initial proof of concept of the Hubble pilot observing program in June, the New Horizons team was awarded telescope time by the Space Telescope Science Institute for a wider survey in July. When the search was completed in early September, the team identified this KBO as "definitely reachable" by the New Horizons spacecraft.