Artwork showing OSIRIS-REx flying past Earth above Antarctica and South America. Credit: NASA Goddard Space Flight Center/University of Arizona
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Artwork showing OSIRIS-REx flying past Earth above Antarctica and South America. Credit: NASA Goddard Space Flight Center/University of Arizona

OSIRIS-REx gets flung by the Earth Friday!

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Mar 25, 2021, 8:00 PM EDT (Updated)

On the morning of Friday, September 22, 2017, the Earth will experience a close encounter with a spaceborne object. But never fear! We’re perfectly safe. That’s because the space traveler is the NASA probe OSIRIS-REx, and it will pass more than 17,000 km above the Earth’s surface.

The flyby is designed so that the spacecraft will steal a little bit of the Earth’s orbital energy, using it to fling itself up, changing its own orbital plane to match that of its target, the asteroid Bennu. OSIRIS-REx will pass closest over Earth’s south pole, and the Earth’s gravity will naturally bend the probe’s path up, up, and away.

This is the third event in the mission’s life in space, counting launch as the first. It launched a bit over a year ago and was placed into an orbit similar to that of Earth around the Sun. In January 2017 it performed a “deep space maneuver,” firing its engine enough to change its velocity by about 1600 kilometers per hour, putting it on the correct course for the flyby.

If you want the details of this flyby, then (as always) you should check in with my friend Emily Lakdawalla at The Planetary Society, who has the info.

The spacecraft has already been spotted by Earthbound telescopes; the Large Binocular Telescope in Arizona saw it on September 2:

Animation showing the movement of OSIRIS-REx on September 2, 2017, when it was still 12 million km away. Credit: Large Binocular Telescope Observatory

I know, it doesn’t look like much, but c’mon: It was 12 million kilometers away and at a magnitude of 25. The faintest star you can see with your naked eye is 40 million times brighter! So this is actually pretty good.

If I’ve done the math right, it’ll be roughly magnitude 11 or so when it passes Earth on Friday. That’s still faint, though within reach of a good telescope. The mission web page has advice and links for trying to see it. Given how far south it’ll be, that means it’s easiest from southern locations; in Australia the Desert Fireball Network will use the flyby to test out their cameras. They’ll observe OSIRIS-REx from different locations and use that to get its 3D trajectory in space. They use the same technique to track material like meteors burning up in Earth’s atmosphere.

I mentioned three events in the mission’s space life so far, but the fourth event is the big one: arrival. Approach starts in August 2018, when OSIRIS-REx is about 2 million km from Bennu. It’ll begin a series of engine burns to slow its approach relative to the asteroid until it goes into orbit. Starting on October it’ll begin surveying Bennu, and will continue to do so for a year.

The orbit of Bennu (blue) is similar to Earth's. This shows their relative positions on the day of the OSIRIS-REx flyby. Credit: NASA / JPL

The orbit of Bennu (blue) is similar to Earth's. This shows their relative positions on the day of the OSIRIS-REx flyby. Credit: NASA / JPL

Bennu is a pretty interesting asteroid (if it weren’t, then duh, we wouldn’t be sending a spacecraft to it). It was discovered only in 1991, and is on an orbit similar to Earth’s, though slightly bigger, more elliptical, and tilted to ours by about 6°. That’s a substantial inclination, taking a lot of energy to match, which is why the spacecraft is using Earth to whip it around. Bennu only approaches Earth about once every six years (its orbital period is about 1.2 years, so it takes a while for it and the Earth to sync up).

Bennu itself is about 500 meters across, a decent-sized chunk of rock (though it will be the smallest object NASA will have ever had a spacecraft orbit, an interesting statistic). It’s what’s called a B-type asteroid, meaning it’s rich in carbon as well as what are called volatiles: materials with low boiling points. Even though it’s small, it may have water inside it, trapped in materials like clays.

It’s shaped roughly like a top or a walnut, slightly wider than it is high. It rotates once every 4 hours or so. Its overall shape was determined from both radar mapping as well as how it changes brightness with time (for example, a very long object can get much brighter when it’s broadside to you, and fainter when it’s end-on). Interestingly, its mass is low; given its size it’s barely denser than water! It’s likely to be a rubble pile, a collection of loosely bound rocks held together by gravity and other forces. That can happen as an asteroid suffers low-speed impacts over billions of years, shattering it in place. Lots of voids form between the rubble, accounting for the low density.

Other than that, it’s thought that Bennu hasn’t undergone much change since it formed 4.5 billion years ago. It’s hoped to be a time capsule dating back to the formation of the solar system itself!

NASA made this spiffy short video explaining more about Bennu, OSIRIS-REx, and the mission itself:

Oh, one more thing for now: OSIRIS-REx is loaded with instruments to examine the asteroid, including cameras, LIDAR, and a spectrometer. But it also has another package: a sample return capsule (SRC). While at Bennu, it will collect a sample of surface material, squirrel it away inside the capsule, then send it back to Earth! This has been done by a mission before (the Stardust mission to a comet), so it’s tested tech.

Scientists want to collect at least 60 grams of material, though they might get more. The mechanism to collect the sample will puff nitrogen gas onto the asteroid surface and then collect the material that floats off. They have enough gas to try this three times, so it seems likely they’ll get what they need.

Then the SRC will be sent on its way back to our planet, arriving as a fireball in the sky and then falling to Earth in July 2020. It’ll be collected and the samples brought to labs where this pristine asteroid material can be studied in much greater detail than is possible with a spacecraft.

But that’s all still far in the future. First things first! Let’s get the flyby done, and then we can start looking ahead to seeing Bennu up close and personal next year.