The Origins, Spectral Interpretation, Resource Identification, Security, Regolith Explorer, or in short, the OSIRIS-Rex, which was launched last year September from NASA’s Cape Canaveral launch site will slingshot the earth’s south pole, altering its trajectory through space.
The OSIRIS-REx probe, at the moment, is in orbit around the Earth, and to reach its intended destination, i.e., Asteroid Bennu, it needs to exit the Earth's gravitational field. This exit can be attained using thrusters or by a slingshot.
The slingshot manoeuvre doesn't require extra fuel to help it escape as it uses the Earth's gravitational force to alter course. This procedure, in scientific parlance, is termed as a “gravity assist”.
The OSIRIS-REx just doesn't have the required amount of fuel that would be required to use thrusters instead of a gravity assist.
This kind of a maneuver has been the go-to solution for NASA quite a few times earlier too in older space missions.
The Science Behind “Gravity Assist”
The gravity assist maneuver is basically just exchange of energy; similar to how a roller coaster picks up speed while going downhill to expend it while going uphill (gain momentum). When the OSIRIS-REx does its flyby at the South Pole this Friday, it will steal a bit of the Earth's momentum (energy) to change its orbit.
This also gives rise to the question - “Will Earth's orbit be affected?”. Well, the answer is yes, it will be affected. But with the Earth's size being so huge, the difference will be next to negligible.
Using the gravity assist maneuver, the OSIRIS-REx is going to alter its own speed by 8400 miles per hour. It doesn't mean that the OSIRIS-REx is going to gain or loose speed; the maneuver is to shift its orbit by 6.2 degrees to come in line with that of the asteroid Bennu.
OSIRIS-REx will be traveling at a speed of 19,000 miles per hour. At 0052 hrs ET it will be the closest to the Earth, flying over the Antarctic Continent with the closest it being at 11,000 miles. While over Antarctica, OSIRIS-REx will lose contact with NASA for around fifty minutes since it will be out of range of the closest tracking stations. OSIRIS-REx will come back online with NASA at 0140 hrs ET.
OSIRIS-REx’s mission is to travel through space and then land on an asteroid named Bennu. It then is supposed to collect rock samples from Bennu.
The reason that NASA chose the asteroid Bennu and not any other asteroid or celestial body is because of its orbit is very similar to that of the Earth’s, thereby making it an earlier target to reach.
To quell fears of a crash sometime in the future, Bennu’s orbit is tilted by six degrees to that of the Earth. Over the past year, OSIRIS-REx has been orbiting the Earth, gaining the required momentum to propel itself to match Bennu's orbit and complete its mission.