On September 21st, 2017 we learned about

OSIRIS-REx spacecraft buzzes the Earth for a course-correcting boost to Bennu

After a year in space, the OSIRIS-REx spacecraft is returning to Earth for a very brief, friendly shove. To properly align with the asteroid Bennu, which is traveling a very similar orbital path to our planet, OSIRIS-REx needs to adjust its heading by around 6°. Rather than burn a bunch of fuel to push itself in the right direction, the spacecraft will come up behind our planet and borrow some energy in the form of a tug from the Earth’s gravity. This should line it up properly for the next phase of its mission, plus give us a chance to practice tracking small objects moving very quickly in our planet’s direction.

The idea of using a planet’s gravitational pull to adjust a spacecraft’s course isn’t new. Many other probes we’ve sent into space, including Cassini, Juno and both Voyagers, have used what’s often referred to as a gravitational assist maneuver to both adjust their heading and speed up or slow down to be better oriented to reach their destinations. As the craft approaches the planet, it essentially starts to fall towards that larger body. However, thanks to careful alignment and high speeds, spacecraft can get a pull without being completely pulled out of the sky, careening onward past the planet without burning fuel. These moments can sometimes be a chance to take new measurements of the assisting planet, but often, as with OSIRIS-REx’s pass on Friday, instruments are turned off to avoid any chance of interference with the change in velocity.

Fly-by photo-op

That’s not to say that OSIRIS-REx is only coming by to steal a tiny bit of Earth’s orbital energy without giving us anything in return. Around four hours after OSIRIS-REx zips by us, it will test cameras and two spectrometers on both the Earth and the Moon. Even after it’s moved away from its closest proximity of 10,710 miles, the spacecraft will still be within range to make some measurements for around 10 days. So if all goes well, we should be getting around 1,000 postcards of our own planet and Moon from OSIRIS-REx’s brief visit.

Teams in Australia will also be taking pictures from the ground. OSIRIS-REx will be mostly buzzing by Antarctica, so only Australians will have a chance to see it at its lowest altitude. Even then, the small craft won’t be easy to spot, although that makes it a good target to practice tracking similarly-sized meteors. The Desert Fireball Network, with help from citizen scientists in Fireballs in the Sky, aim to build a network of cameras around the world to track small meteors in our cosmic neighborhood. By taking photos from a variety of locations, they should be able to plot the course of an object like OSIRIS-REx in 3D. The fact that we know OSIRIS-REx’s route already makes this a great chance to check if the system is working designed.

Once the photos and measurements come in from this flyby, OSIRIS-REx’s next big moment will be in about another year. In August 2018, the spacecraft will be in range to begin syncing itself closely to Bennu, preparing to capture a small piece of the asteroid. We won’t get our hands on those samples until 2023, but souvenirs are more exciting than postcards, so it should be worth the wait.

Source: OSIRIS-REx Earth flyby: What to Expect by Emily Lakdawalla, The Planetary Society

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