As the $700 million New Horizons probe approaches its July 2015 encounter with Pluto, scientists back on Earth are worried that a priceless chance to study a Kuiper Belt Object (KBO) beyond it may be slipping away.

Even with the most capable ground-based telescopes, a New Horizons search team has failed to find a KBO that New Horizons can reach as it hurtles toward interstellar space following its Pluto flyby. The search continues, but with time running short the project is seeking time on the Hubble Space Telescope to improve the odds that a feasible target can be found.

The New Horizons project needs to know where that target will be when the spacecraft passes through, so it needs some lead time to calculate the object’s orbit around the Sun. Otherwise, a once-in-a-lifetime opportunity to study one of the mysterious bodies at the edge of the Solar System may be squandered.

“The scientific bounty of a spacecraft encounter with a primitive KBO is realizable in our lifetimes, but only with New Horizons and only if a suitable target can be found while there is still time to reach it,” wrote two NASA scientific advisory groups in an April 30 joint statement. “No other mission currently in flight, in build, or in design will reach the Kuiper belt. Time is of the essence for New Horizons.”

The New Horizon’s team has asked for an initial 40 orbits – about 2.5 days – of time on the Hubble to peer “deeper” into the region beyond Pluto for promising candidates. Bill McKinnon, a professor of Earth and Planetary Science at Washington University, said the ability of the space telescope to detect objects that are much fainter than can be seen from the ground, combined with a narrowing of the region that needs to be searched, suggests that a target can be found during a “trial run” this summer. If no target appears, the search will provide a statistical justification for applying more Hubble time to the problem.

“Forty orbits may be enough to get our man,” said McKinnon, a member of the New Horizons science team and a former chairman of the NASA Outer Planets Assessment Group that joined the Small Bodies Assessment Group in calling for the Hubble observation. “We may actually find the one we need, so we can declare victory and go home.”

The ideal target will be a KBO on the order of 25 km across, McKinnon said, on a trajectory that will put it where New Horizons can reach it with its remaining fuel after the Pluto flyby next year. The spacecraft will use its instruments to characterize the composition, reflectivity and density of the object; map its surface features, and look for smaller moons in orbit around it. Given the great distance and the small size of the objects, scientists “really have no idea” what KBOs are like, although they are pretty sure they have been there since before the Solar System took the form we know today.

“Pluto is a planetary outlier in the Kuiper belt,” the two advisory groups stated. “It is entirely unlike the far more numerous, small, primitive planetesimals (KBOs) left over from the era of planet formation that populate the classical Kuiper belt, and especially its dynamically ‘cold’ core that New Horizons will soon pass through. This population is also the most accessible example of a debris disk, like those detected around numerous other stars.”

New Horizons was launched in 2006 on an Atlas V, and picked up a gravity assist from Jupiter 13 months later that sent it on the long transit to Pluto.