In February 2017, the OSIRIS-REx spacecraft will undertake a search for “Earth-Trojan” asteroids while on its outbound journey to the asteroid Bennu. Earth-Trojans are asteroids that share an orbit with Earth while remaining near a stable point 60 degrees in front of or behind the planet.

The University of Arizona-led OSIRIS-REx mission has been given a side job during its seven-year cruise to study and sample an asteroid named Bennu.

The NASA spacecraft’s cameras will be looking for “Trojan” asteroids that follow the Earth’s orbit and could be hiding in the sun’s glare, undetectable from Earth.

OSIRIS-REx is in the fourth month of a seven-year mission to return a pristine sample of an asteroid to Earth. Its principal science mission doesn’t begin until it gets close to Bennu in 2018, but NASA and project scientists decided to take advantage of its position in space to search for Trojans.

Between Feb. 9-20, the spacecraft will turn on its camera suite, built by scientists at the UA’s Lunar and Planetary Laboratory, to scan the space ahead of Earth’s orbit for asteroids.

There are points, fore and aft of the Earth in its orbit, where the pull of Earth and the sun are in equilibrium. Chunks of rock trapped there or left there from formation of the solar system would orbit along Earth’s path. OSIRIS-REx will be passing right by one of those points.

Those objects would be difficult to image from telescopes on Earth, because they are potentially visible only near the horizon at twilight when telescopes have to look through a lot of atmosphere.

They would be small, only partially lit by the sun and about 93 million miles away, said Carl Hergenrother, the OSIRIS-REx scientist who is coordinating the search for them.

OSIRIS-REx, on an orbit near the Earth right now, will be as close as 5 to 6 million miles to the theoretical “cloud” of asteroids, looking away from the sun and unbothered by atmospheric scattering, said Hergenrother.

Astronomers and planetary scientists have discovered Trojans orbiting with six planets in the solar system. More than 6,000 are known to orbit with Jupiter.

Only one Earth Trojan has been detected — a 300-meter-diameter rock discovered by NASA’s WISE space telescope in 2010.

Hergenrother said the search for Trojans has both scientific and operational purposes.

The science could be exciting, he said. “There could very well be a population of objects that formed when Earth was forming.”

Hergenrother said the search is also an important test of MapCam, the wide-field camera on the spacecraft.

“It’s a dress rehearsal for observations we’ll be making at the asteroid when we’re looking for satellites around Bennu,” he said.

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