Targeting Crisis Averted for New Horizons

NASA scientists have found three potential Kuiper belt objects in the nick of time, saving the Pluto-bound probe from missing out on half of its mission.

In July 2015, the New Horizons space probe will whiz past Pluto just 10,000 kilometers from its surface. This long-awaited flyby will provide unprecedented views of the dwarf planet, its moons, and the environment around it.

But scrutinizing the Pluto system is only half of New Horizons’s mission.

Soon afterward, the spacecraft will fire thrusters to set itself on a course to fly past a Kuiper belt object, or KBO. Because planning for that redirection must begin soon, project managers have worked hard to find a KBO worthy of such a visit. Yet, despite 3 years of searching, by mid-2014 a suitable candidate had not been found.

But now, the science team can breathe easier. NASA’s Hubble Space Telescope has uncovered three KBOs viable for a quick visit. “The story of the discovery is one of persistence, ingenuity, uncertainty, and a bit of a gamble,” says team member Keith Noll (NASA Goddard Space Flight Center).

Hubble Saves the Day

In a perfect world, project scientists would have identified a suitable KBO long before the space probe’s launch in 2007. But they postponed their main search until 2011 for multiple reasons.

By waiting longer, all possible KBOs began converging on the narrow cone of space that New Horizons will be able to reach after its Pluto encounter. With less sky to search, detecting suitable KBOs should — in principle — be easier, even if done at a later date.

Pluto has been crossing the constellation Sagittarius, home of our star- and dust-filled galactic center. The crowded field makes finding any intrinsically faint object extremely challenging. Fortunately, Pluto is moving in front of dark clouds, allowing the search to continue without so much background confusion.

The team members were also hoping that, by waiting, they could utilize bigger telescopes and much better instruments than were available 10 years ago. And in fact, they’ve used the 10-meter Keck Telescopes and 8.2-meter Subaru Telescope in Hawaii, the 8.1-meter Gemini telescopes in Hawaii and Chile, and the 6.5-meter Magellan Telescope in Chile.

But, despite using some of the best ground-based telescopes in the world, the search turned up only 50 new KBOs, none of which were close enough for New Horizons to reach. So the team turned to Hubble and was awarded roughly two weeks of observing time — a large chunk on the most sought-after telescope. And it paid off. Hubble quickly picked up three potential targets.

“The Hubble Space Telescope has rescued the exploration of the Kuiper Belt,” says principle investigator Alan Stern (Southwest Research Institute).

But Stern also argues that having dedicated project scientists was equally important. “This team did about two years of work in four months,.” he explains. “It was night and day, seven days a week. They wrote crazy amounts of code. They did crazy amounts of rapid-fired data reduction, analysis, and follow-up.”

Uncharted Territory

The Kuiper Belt is a vast ring of primordial debris encircling our solar system. Its objects are so intrinsically faint, small, and distant that they’re nearly impossible to study from Earth. And yet they contain crucial clues about the formation of the solar system. Because KBOs have not been heated by the Sun, their ices are therefore a sample of what the early solar system was like 4.6 billion years ago.

The first target, designated “Potential Target 1,” was discovered on June 27th — just 11 days after the search team was awarded time on Hubble. Since then, the object has been imaged four times. These follow-up images allowed the team to constrain its orbit and size (roughly 30 to 45 km across) and provided assurance that New Horizons will reach it in January 2019.

Even better, PT1 is reachable with only two-thirds of the spacecraft’s remaining fuel supply, says Stern. So if the project scientists need to make slight adjustments to the spacecraft’s orbit, once PT1 comes into view, they can.

But PT1 is incredibly faint, roughly 200 million times fainter than the naked eye can see. So although PT1 is the most likely of the Hubble-discovered objects to be targeted, it’s possible that follow-up observations might make PT2 or PT3 more desirable. They’re both slightly brighter and therefore bigger than PT1, which will make targeting them easier.

Their larger surfaces also have a huge advantage. “By looking at their surfaces we can learn about how battered they have been by collisions over the years — how violent or quiet things have been in the outer region of our solar system,” says team member Susan Benecchi (Planetary Science Institute).

But the project scientists still don’t know what to expect. “What we’ve found in planetary science is that when we send a spacecraft for the first time to a new place, we’re often wrong,” says Stern. “For example, no one expected river valleys on Mars, volcanoes in the outer solar system, and oceans on the inside of worlds.”

A typical KBO is about 10 times larger than 67P/Churyumov–Gerasimenko (Rosetta’s comet), and about 100 times smaller than Pluto. So they serve as great stepping-stones from smaller cometary objects to larger dwarf planets.

“We’re either going to confirm conventional wisdom, which would be important, or we’re going to find a big surprise,” says Stern. “Either way it’s a data point — and one we can’t get unless we go there.”

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Learn more about the Secrets of the Kuiper Belt in the February 2014 issue of Sky & Telescope.