Originally posted on the Nature news blog

The Hubble Space Telescope has begun searching for an icy world in the outer Solar System that NASA’s New Horizons probe can visit after it flies past Pluto in July 2015.

The awarding of Hubble observing time, announced today, could greatly increase the chances of the mission’s success. The spacecraft was meant to fly first past Pluto and then past another object in the cluster of icy bodies known as the Kuiper belt. But mission scientists have been unable to identify a suitable Kuiper belt object (KBO) using big telescopes on the ground. They needed the space-based vision of Hubble.

“Hubble is coming to the rescue of New Horizons, and we’re very excited about it,” says Alan Stern, principal investigator for the mission and a planetary scientist at the Southwest Research Institute in Boulder, Colorado.

Launched in 2006, New Horizons is currently about nine-tenths of the way to Pluto. Mission planners woke it up yesterday for an extended diagnostic assessment.

NASA has yet to approve funding to fly past a second target after Pluto, but without a candidate KBO the question was moot. The problem has been how to pick out KBOs, which are far away and thus very faint, from the crowded background field of Milky Way stars against which the New Horizons probe is travelling. Mission scientists began their ground-based search in earnest in 2011, but they’ve been stymied by bad weather at observing sites, and the fact, discovered only recently, that that there are actually fewer faint KBOs than one might expect given the number of bright ones. They have discovered more than 50 faint KBOs, but none are in the right region of space for New Horizons to make a close-up visit.

New Horizons has been given an initial allotment of 40 Earth orbits of Hubble observing time, or about 40 hours. If it finds two faint KBOs during that period, then a second observing period of 156 orbits will kick in. The first set of observations is meant to show whether there are enough faint KBOs, statistically speaking, for it to be worth Hubble’s time to continue with the full search. The group that allocates telescope time agreed “that going forward with at least the pilot was a good use of Hubble time,” says Matt Mountain, director of the Space Telescope Science Institute in Baltimore, Maryland.

First observations began over the weekend and the data are already being processed. Time is of the essence because the team must identify at least one KBO target by roughly the end of August to be able to follow its trajectory for at least a year and accurately plan a visit. The probe has limited fuel; mission scientists intend to fire the probe’s thrusters soon after the Pluto flyby next summer in order to accurately set it on a course to visit a KBO.

Ideally, Stern says, the Hubble search will turn up several candidate KBOs that are in the right place for a New Horizons visit. The team will continue to use ground-based telescopes to hunt throughout the year, but they calculate that having Hubble time raises their chances of finding a suitable KBO target from less than 40% to more than 90%.

“This is going to make all the difference,” he says.

This article is reproduced with permission from the Nature news blog. The article was first published on June 16, 2014.