On Friday, a spacecraft the size of an SUV will slingshot around Earth’s South Pole, altering its trajectory through space. The probe is NASA’s OSIRIS-REx, and its upcoming maneuver around our planet is known as a gravity assist — a way to harness Earth’s gravity to alter its orbit. The move is critical, since it will put OSIRIS-REx on course to meet up with an asteroid in the fall of 2018.
OSIRIS-REx launched last year with a relatively straightforward purpose: grab a sample of rocks from an asteroid and bring them back to Earth. If all goes well, the vehicle should retrieve the largest sample ever collected from an asteroid, and give scientists the chance to study the space rock components in more detail than ever before. But first, the probe has to reach its target — a nearby asteroid named Bennu.
Now it’s time for OSIRIS-REx to match Bennu’s orbit in space
NASA picked Bennu partly because the asteroid’s orbit is similar to Earth’s orbit, and that makes it an easier target to reach. But their paths aren't the exact same: Bennu's orbit is tilted by about six degrees compared to Earth's. In the past year, OSIRIS-REx has been orbiting in the same plane as Earth, traveling slightly ahead of our planet. And now it’s time for OSIRIS-REx to match Bennu’s orbit in space.
There are two main options to change a spacecraft’s trajectory: one is to use the vehicle’s onboard engines to propel the spacecraft in a certain direction. The problem with this option is that it uses up the spacecraft’s finite amount of fuel. And OSIRIS-REx would have needed a lot of fuel to alter its course to reach Bennu in time — more than the vehicle is carrying.
“This was the only option to reach Bennu, launching in 2016.”
So instead, the probe’s navigators opted to use the second option — a gravity assist. “This was the only option to reach Bennu, launching in 2016,” Michael Moreau, a flight engineer at NASA’s Goddard Space Flight Center, tells The Verge. This maneuver has been used on many previous space missions, to increase or decrease a spacecraft’s speed and course. It’s essentially an exchange of energy, similar to when a roller coaster speeds up while going down a hill. When OSIRIS-REx swings by Earth, it will steal a little bit of our planet’s momentum in order to change its orbit. Earth is so massive that the maneuver won’t really affect our planet. But OSIRIS-REx will change its speed and course by more than 8,400 miles per hour. That’s nearly twice the amount the spacecraft would get if it used up all its fuel.
This doesn’t mean OSIRIS-REx is speeding up or slowing down, though. The assist will mostly go toward changing the direction of the spacecraft’s orbit. When the spacecraft flies over the South Pole on Friday, OSIRIS-REx will shift its orbit by 6.2 degrees, putting it in the same plane as Bennu. “We’re targeting the spacecraft to fly by the Earth at a very specific point and time,” says Moreau.
OSIRIS-REx will approach the Earth at a speed of 19,000 miles per hour, flying over Australia first. It will then make its closest approach to Earth at 12:52PM ET, coming within 11,000 miles of Antarctica. Around that time, the vehicle will lose contact with NASA since it will be out of range with the space agency’s closest tracking stations. The blackout should last just 50 minutes, though, and NASA expects to regain communications around 1:40PM ET. The vehicle is also supposed to come into areas dominated by satellites, but NASA says it has taken steps to make sure no collisions happen during the assist.
After Friday’s maneuver, OSIRIS-REx will cruise through space for another year, reaching Bennu in October. At that point, the vehicle is supposed to fly around the asteroid for two years, surveying the rock’s surface, before actually grabbing the coveted sample and returning to Earth. The gravity assist is the first step to getting there, and it’ll allow the mission team to meet up with Bennu exactly when they needed to, while saving on fuel.
Bennu is one of the smallest objects NASA has ever orbited
That’s important, since OSIRIS-REx is going to need that fuel in the future. Bennu is roughly 1,600 feet wide, around the height of the Willis Tower in Chicago, making it one of the smallest objects NASA has ever orbited. That poses a big navigation challenge for the mission team, requiring much more frequent maneuvers to keep the spacecraft where it needs to be. During its cruise, the team has only had to adjust the spacecraft’s orbit every few months or so to stay on track. But once OSIRIS-REx reaches Bennu, there are times when the team will need to adjust the spacecraft’s direction every two days.
But first, Friday has to be a success. “The velocity change of the gravity assist is larger than of the other maneuvers we’re doing,” says Moreau. “It’s a big effect.”