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How a Satellite Just Used Earth Like a Slingshot

On its way to study an asteroid for clues about the origins of life, OSIRIS-REx got a little boost from Earth’s gravity

An illustration envisioning how the satellite OSIRIS-REx will collect rocks from the asteroid Bennu (NASA/GSFC)
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Friday, the satellite OSIRIS-REx swung around Earth on a slingshot journey toward a nearby asteroid waiting to be studied.

OSIRIS-REx was launched by NASA a year ago this month to travel to the asteroid Bennu, a 1,600-feet-wide chunk of rock with an orbit that approaches fairly close to Earth. Over the course of the next year, it will travel to Bennu and poke it a few times using a stream of gas to disturb the dust on the asteroid's surface, reports Kenneth Chang for the New York Times. Then it will collect some of that dust and bring it back to Earth by the year 2023.

What makes that dust from an ordinary-looking asteroid worth all of this effort? Its age.

Some scientists theorize that not only water, but the organic compounds that birthed Earth's earliest life were brought to our planet from outer space by meteorites called "carbonaceous chondrites." It's believed that asteroids like Bennu that were formed during the birth of the solar system some 4.5 billion years ago are the source of these meteorites that could have seeded life on Earth.

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ORISIS-REx streaks across the sky. (Taken by Mike Read via NASA)

NASA chose Bennu for this mission because it follows an orbit only slightly wider than Earth's, meaning it's fairly easy to reach from here, reports Loren Grush for The Verge. However, some effort is still required, and that's where the slingshot maneuver performed today comes in. Fuel is a precious resource on a spacecraft, and navigating toward a very small asteroid will require a lot of adjustments with thrusters along its journey. Thus, to save some fuel in getting OSIRIS-REx going on its trip, scientists decided to nab a boost from Earth's gravity.

By using the momentum gained from orbiting the Earth, the satellite's handlers launched it at roughly 19,000 miles per hour toward Bennu without the need of its thrusters, noted Marcia Dunn for the Associated Press. The orbit also helped tilt the satellite's direction by about 6 degrees to put it on the correct plane to intercept the asteroid, reports Leah Crane for New Scientist.

Gravity boosts like these are fairly common tools used for saving satellite fuel—the Voyager satellites took advantage of a particular alignment of the outer planets of the solar system to get boosts from all four of the gas giants. Closer to home, the Juno satellite sent to Jupiter got an 8,800 mile per hour speed boost by vaulting around the Earth, reports Mike Wall for Space.com.

Juno came within just 347 miles of the Earth's surface on its slingshot, but OSIRIS-REx kept its distance, approaching Earth at roughly 11,000 miles above Antarctica at its closest this afternoon. If you want to see the satellite streak by, the ORISIS-Rex team is on it collecting images online from people lucky enough to catch a glimpse.

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