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The New Horizons spacecraft approaches Pluto in this artist's conception.

The New Horizons spacecraft approaches Pluto in this artist's conception.


Hubble telescope to look for follow-on target for Pluto-bound probe

New Horizons, NASA’s mission to the outer solar system, has been given a large chunk of time on the Hubble Space Telescope to assist an increasingly desperate search for an icy object the spacecraft can study after it hurtles past Pluto in July 2015, NASA headquarters announced today.

The mission team has so far been unable to find a suitable Kuiper belt object (KBO) for follow-up study, but needs to as soon as possible so that it can plan orbital adjustments with its limited supply of fuel.

Beginning this week, the mission team will get 40 orbits of Hubble time from the discretionary budget of the director of the Space Telescope Science Institute (STScI) in Baltimore, Maryland, which operates the telescope. It takes the telescope 97 minutes to orbit Earth, but because Earth blocks intended targets for much of this time, each orbit is worth about an hour of observation.

Researchers will use the time to search a quarter of the space within the realm of the spacecraft’s thrusters. A Time Allocation Committee (TAC) of 18 astronomers gave the New Horizons team an additional 120 orbits to search the remaining space if the initial search turns up a reasonable number of KBO candidates. And if a prime candidate is found, the team will get 30 more orbits of time for precision studies to nail down the KBO orbit. “This program was strongly supported by the TAC, whether they were cosmologists, solar system or exoplanet people,” says Neill Reid, the head of the science mission office at STScI. “This is the one chance to go visit there.”

One major problem has been that the bright, star-filled center of the Milky Way is directly behind the search area, making it difficult for faint KBOs to stand out. The mission team has already been granted dozens of nights of time on some of the largest ground-based telescopes on Earth, to no avail. Reid says the team should have improved chances with Hubble because of its sharper vision, and also because the background sky as seen from space is a bit darker. But success is by no means a given—KBOs have turned out to be far less numerous than mission scientists thought when New Horizons launched in 2006. “It depends on whether the solar system is going to cooperate or not,” Reid says. “They made a good case that they had done as much as they could from the ground.”

With fierce competition for Hubble time, some observers worried that the TAC might dismiss the large request from the New Horizons team. But the dilemma even got the attention of the Senate subcommittee in charge of science appropriations, chaired by Senator Barbara Mikulski (D–MD), whose state includes both the STScI and the Johns Hopkins University Applied Physics Laboratory, which operates the New Horizons mission. The lawmakers backed an expanded KBO survey in a report accompanying the NASA spending bill that Mikulski’s committee approved earlier this month. “The Committee strongly supports surveying the accessible region of space that the New Horizons spacecraft will be able to transit,” the panel wrote, “in order to determine if potential targets of opportunity exist that the spacecraft can explore.”