A NASA spacecraft, Osiris-Rex, is speeding toward Earth after a year looping around the sun. On Friday afternoon, it will miss the planet by about 11,000 miles, zooming underneath our blue orb at 19,000 miles per hour, passing over Australia and Antarctica.
The near miss is deliberate.
The Earth’s gravity will fling the spacecraft upward by about six degrees so that its trajectory will match the tilt of the orbit of its destination: a small near-Earth asteroid named Bennu.
“We’re essentially stealing a bit of the Earth’s momentum as we go by,” said Michael Moreau, who leads Osiris-Rex’s navigation team at NASA’s Goddard Space Flight Center in Greenbelt, Md.
As a consequence, Earth’s tilt will change ever so slightly, too small to be worth calculating. “It would be a very small number,” Dr. Moreau said.
Osiris-Rex — a shortening of Origins, Spectral Interpretation, Resource Identification, and Security, Regolith Explorer — was launched last year and circled the sun, returning for Friday’s flyby. It is to arrive at Bennu in about a year. The asteroid periodically crosses Earth’s orbit, and there’s even a 1-in-2,700chance that it could hit Earth between 2175 and 2196.
Scientists believe that Bennu, a dark asteroid about 500 yards in diameter, is full of carbon-rich molecules dating back to the birth of the solar system 4.5 billion years ago. Those molecules might have been the ingredients that led to life on Earth. Osiris-Rex will attempt to collect a few pounds of rock and dirt from Bennu by gently bouncing off the surface like a pogo stick and collecting material that it disturbs with a burst of nitrogen gas. It will bring the samples back to Earth in 2023 for closer study.
For the flyby, there is no chance that Osiris-Rex, about the size of an S.U.V., will veer off course and slam into Earth. Spacecraft navigators have become adept at using precise flybys as slingshots to steer spacecraft through the solar system. NASA’s New Horizons spacecraft, for example, added nearly 9,000 miles per hour to its speed with a Jupiter flyby in 2007, shortening its travel time to Pluto (It still took another eight years to get there).
Dr. Moreau said Osiris-Rex will pass within a kilometer of the targeted spot above Earth. The timing is precise too, within a few tenths of a second. It will make its closest approach to Earth at 12:52 p.m. Eastern time on Friday.
Dr. Moreau said his team will face larger navigational challenges once Osiris-Rex gets to Bennu in 2018. “It’s the smallest object that has ever been orbited by a spacecraft,” he said. “And that’s exciting.”
The European Space Agency’s Rosetta spacecraft spent a couple of years exploring Comet 67P/Churyumov–Gerasimenko, a comet about 2.5 miles wide. Bennu is about one-eighth that diameter, and Osiris-Rex will come within a kilometer of the center of Bennu, Dr. Moreau said.
“We are much closer to the object than Rosetta was,” he said. “It means a lot of the errors in your estimation of the trajectory and navigation are less forgiving.”
With gravity near the asteroid so slight, the navigators need to keep track of even very minute forces, including heat from the spacecraft radiators and the momentum imparted by particles of light hitting the solar panels.
For Friday, the Osiris-Rex team is encouraging amateurs to photograph the passing spacecraft and share the images on the mission website.
The Desert Fireball Network, a project based at Curtin University in Perth, Australia, will use high-end digital single lens reflex cameras to photograph Osiris-Rex from different angles. Usually, the project tracks meteors burning up in the Earth’s atmosphere.
This time, the different angles will allow scientists to reconstruct the three-dimensional path that Osiris-Rex took as it swung by.
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