鈥淏ennu or bust鈥� is one of the slogans of the OSIRIS-REx mission, and 鈥渂ust鈥� is officially off the table. On 3 December, the NASA spacecraft will arrive at its destination, the asteroid Bennu, after a two billion kilometre journey that took about two years.
鈥淭his is a milestone for our spacecraft team, because now we鈥檙e not just approaching Bennu, closing in on it, we鈥檙e at a part of the mission where we need to think about how we鈥檙e going to manoeuvre around the object,鈥� says Daniella DellaGiustina, an OSIRIS-REx team member at the University of Arizona in Tucson.
Bennu has an average diameter of less than 500 metres, which will make it the smallest object ever orbited by a spacecraft. The first few circles around it will help the spacecraft navigators figure out how to fly near it, DellaGiustina says. A big part of that is making precise models of its shape and any variations in gravity around it that might be caused by boulders or denser bits inside the asteroid.
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Because Bennu is so small, it has extremely weak gravity, which makes orbiting difficult. Even the force from sunlight hitting the spacecraft could push it out of a stable orbit. Because of that, OSIRIS-REx will circle the asteroid in what is called a terminator orbit, remaining always on the edge of night and day so that the sunlight hitting it is always the same and as weak as possible without plunging the craft into total darkness.
Orbiting Bennu for an extended period of time is particularly useful because it is one of a few asteroids whose orbits take them dangerously close to Earth. Their small sizes make their trajectories difficult to predict, so buddying up to a near-Earth object for a while might help us predict whether any might eventually end up on collision courses with the planet.
As OSIRIS-REx circles its target, the team on Earth will be keeping close watch for a safe area for it to briefly touch down and grab a sample of dust. Sometime in mid-2020, it will drop down towards the surface, extend its robot arm, and tap the asteroid with a small sampler. 鈥淲e鈥檙e not quite landing 鈥� we鈥檙e pogo-sticking off the asteroid with our sampling mechanism,鈥� says DellaGiustina.
After that, it will be time for the mission to live up to its other slogan: 鈥淭o Bennu and back.鈥� In March 2021, OSIRIS-REx will leave Bennu and blast back towards home with its precious cargo. It will arrive at Earth in 2023.
But the mission hardly ends then 鈥� once the spacecraft falls back to Earth, scientists will begin the analysis of the space dust. 鈥淏ennu is a relatively primitive object, which means that its history might contain clues to the history of the solar system,鈥� says DellaGiustina. 鈥淪tudying its material in particular should be able to help us answer some questions about the origins of organic material on Earth.鈥�
