A daring mission to sample a distant asteroid is starting to get very interesting, with NASA's OSIRIS-REx probe setting off some New Year's fireworks of its own in an eight-second burn to enter orbit around Bennu. This maneuver on the final day of 2018 saw the asteroid become the smallest object ever to be orbited by a spacecraft, with mission scientists now gearing up for a series of close flybys to help them pinpoint a sampling site on the surface.

NASA's OSIRIS-REx mission has gone rather smoothly since setting off on an Atlas V rocket in September 2016. It then spent a year in orbit around the Sun before leveraging the Earth's gravity to slingshot into space for its faraway encounter.

In August 2018 it then started to close in on the asteroid, and in December fired its thrusters and entered a parallel orbit at a distance of around 11.8 mi (19 km). Though its science-gathering efforts had barely begun, the probe found evidence that the asteroid was rich in water molecules just a week later.

And things are now set to ramp up even further. Not only did the New Year's Eve maneuver make Bennu the smallest object ever orbited by a spacecraft, it also put OSIRIS-REx in the tightest orbit around a celestial object of study in history. The probe is now circling the asteroid from a distance of around a mile (1.6 km) from the surface, where it will gather even sharper images of its surface and learn more about its mass and gravity.

The reason OSIRIS-REx needs to orbit Bennu so tightly is largely due to its incredibly weak gravitational field, which is around five millionths as strong as Earth's and barely strong enough to hold a vehicle in a stable orbit.

"The gravity of Bennu is so small, forces like solar radiation and thermal pressure from Bennu's surface become much more relevant and can push the spacecraft around in its orbit much more than if it were orbiting around Earth or Mars, where gravity is by far the most dominant force," Dan Wibben, OSIRIS-REx Maneuver and Trajectory Design Lead.

While the team was well prepared for these conditions and used complex simulations to safely navigate the spacecraft safely into orbit, the weak gravitational pull will probably mean they'll need to make adjustments to keep it there. When or if other forces, such as the thermal pressure of solar radiation mentioned above, push the probe away from its orbit, the team will make small burns called "trim" maneuvers to correct its course.

All going well, the team will have OSIRIS-REx carry out a series of close flybys over several months beginning in February. These closer encounters will allow the team to gather high-resolution images of every square inch of the asteroid's surface.

This, along with the other data it collects, will guide the team in selecting a sampling site. This sampling phase of the mission is set to take place in 2020, and will see the spacecraft blast the surface of the asteroid with nitrogen gas to stir up some material for retrieval with its robotic arm. These are to be returned to Earth in 2023, at which point scientists will begin studying their chemical composition for clues as to the origins of the solar system, potential mining resources and organic compounds such as sugars and amino acids.

"The team continued our long string of successes by executing the orbit-insertion maneuver perfectly," said Dante Lauretta, OSIRIS-REx principal investigator at the University of Arizona, Tucson. "With the navigation campaign coming to an end, we are looking forward to the scientific mapping and sample site selection phase of the mission."

Source: NASA

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