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Hubble finds fresh targets for NASA's New Horizons mission to Pluto

Paul Sutherland, Feature writer
Oct 17, 2014, 15:00 UTC

Sen—The Hubble Space Telescope has successfully discovered three remote, icy objects in the outer Solar System that NASA’s New Horizons spacecraft could potentially visit after it shoots past Pluto in July next year.

New Horizons was the fastest probe ever sent into space when it launched in 2006, to help it reach Pluto in reasonable time. That means it will have only a short time near Pluto to scan that ex-planet before it is carried far away again.

Mission specialists realised there was an opportunity for the probe to perform more science by studying one or more of a class of body in the Solar System called Kuiper Belt Objects (KBOs). It has long been believed that a vast swarm of these surrounds the Sun at a distance from it of about 6.5 billion km (4 billion miles). The trouble was that only a few tens of them have so far been found.

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Two Kuiper Belt Objects detected by the Hubble Space Telescope are shown, in a combined sequence of exposures, moving faintly against the background of stars in Sagittarius. Image credit: NASA, ESA, SwRI, JHU/APL, New Horizons KBO Search Team

The decision was taken to use some of the powerful Hubble telescope’s valuable time to scour the region of sky through which New Horizons will travel in the hope of finding more of the elusive KBOs.

It sounded like looking for a needle in a haystack. But the dedicated search by members of the New Horizons team, using the telescope jointly operated by NASA and ESA, was successful. The three icy KBOs that Hubble detected are around ten times larger than typical comets but less than two per cent the size of Pluto.

Discovered in the constellation of Sagittarius, two of the KBOs are about 55 km (34 miles) wide and the other perhaps 25 km (15 miles) across. They lie around 1.6 billion km (one billion miles) beyond Pluto, which means it will take the spacecraft another three to four years to reach them after reaching its primary target.

The team began looking for candidate KBOs in 2011, using some of the biggest telescopes based on the Earth. They managed to discover several dozen, but none was close enough for New Horizons to reach it using its available fuel.

New Horizons science team member John Spencer, of the Southwest Research Institute, in Boulder, Colorado, said: “We started to get worried that we could not find anything suitable, even with Hubble, but in the end the space telescope came to the rescue. There was a huge sigh of relief when we found suitable KBOs. We are ‘over the moon’ about this detection.”

Hubble began its intensive search in July after a trial run in June showed its potential by spotting two KBOs. The objects differ from asteroids because they have not been warmed by the Sun and so remain in a pristine state, like samples of the primordial Solar System kept in a deep freeze. They are also thought to be the material that came together to build dwarf planets like Pluto.

Following Hubble’s success, the mission team will now apply to NASA in late 2016 for an extended mission to visit one of the newly-found KBOs.

New Horizons’ Principal Investigator Alan Stern, also of SwRI, commented: “This has been a very challenging search and it’s great that in the end Hubble could accomplish a detection—one NASA mission helping another.”