For the past year, NASA's OSIRIS-REx satellite has been on a solar orbit in our galaxy, readying itself for its ultimate goal of heading out to the asteroid Bennu, surveying it, and grabbing a sample from its surface. On Friday, Sept. 22, it will finally head off to its target with a little help from Earth's gravity.
OSIRIS-REx has certainly come a long way since New Atlas first got a sneak peak at the satellite last year. Almost 600 million mi (965 million km) be exact. That's about how far it has traveled since its launch atop an Atlas V rocket on Sept. 8 2016. During that time, NASA has fired the satellite's thrusters to make small course corrections and activated its MapCam imager to search its surroundings for Earth-Trojan asteroids, chunks of rock that share stable positions in Earth's orbit around the sun (it didn't find any).
But now it's time for the Origins-Spectral Interpretation Resource Identification Security REgolith Explorer to move closer to its real goal – a rendezvous with the asteroid Bennu in 2018. To do that, it's going get a propulsive boost from our planet through what's known as an Earth gravity assist in which it briefly dips into Earth's orbital energy and then uses that energy to slingshot off into space with enough velocity to get it to Bennu. That's why NASA engineers have positioned the satellite to pass within 11,000 miles (17,000 kilometers) of Earth's surface at 12:52 pm EDT on Friday.
"The Earth Gravity Assist is a clever way to move the spacecraft onto Bennu's orbital plane using Earth's own gravity instead of expending fuel," said Dante Lauretta, OSIRIS-REx principal investigator at the University of Arizona, Tucson.
Gravity assists have been used for years by NASA to get spacecraft where they need to go in our galaxy without wasting fuel. The first instance of the maneuver happened when the Voyager satellites borrowed a bit of Jupiter's gravity to slingshot further on their missions, allowing Voyager II to head toward Neptune and Uranus and flinging Voyager I toward interstellar space, which it reached in 2012.
OSIRIS-REx will make its closest approach to Earth south of Cape Horn, Chile over Antarctica while traveling at about 19,000 mph (30,500 km/h). At that point NASA will temporarily lose communications with the craft.
"For about an hour, NASA will be out of contact with the spacecraft as it passes over Antarctica," said Mike Moreau, the flight dynamics system lead at Goddard. "OSIRIS-REx uses the Deep Space Network to communicate with Earth, and the spacecraft will be too low relative to the southern horizon to be in view with either the Deep Space tracking station at Canberra, Australia, or Goldstone, California."
Also, during the gravity assist maneuver, NASA engineers will have to take precautions not to hit other satellites orbiting the Earth. Should the mission look like it's heading in that direction, the team can execute a small maneuver to steer the satellite to safety. The team can also execute another maneuver after the gravity assist to make sure the satellite stays on track to keep its Bennu rendezvous.
"The asteroid's small size and low gravity makes OSIRIS-REx the most challenging mission that I have worked on," said Peter Antreasian, the navigation team chief from KinetX Aerospace. "At roughly 500 m (1640 ft) in diameter, Bennu will be the smallest object that NASA has orbited."
In late 2018 the satellite will be slowed so that it can dock with the asteroid to begin its survey and sampling mission. It will lift off from the asteroid sometime after March 2021 and return to Earth in 2023.
The following video illustrates the gravity assist maneuver. You can also download a poster of the historic event from NASA.