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After four weeks of navigating in the vicinity of asteroid Bennu, NASA’s OSIRIS-REx spacecraft fired its thrusters for eight seconds Monday to slip into orbit around the carbon-rich object, making Bennu the smallest planetary body ever orbited by a spacecraft.

OSIRIS-REx arrived at Bennu on Dec. 3, ending a journey from Earth that lasted more than two years and spanned 1.2 billion miles (2 billion kilometers). Since then, the robotic spacecraft has surveyed the 1,600-foot-wide (492-meter) asteroid through a series of flybys as close as 4.4 miles (7 kilometers) over Bennu’s north pole, south pole and equator to measure the asteroid’s gravitational tug on OSIRIS-REx, which helped scientists determine the object’s mass.

The mass estimate helped navigators refine the parameters of OSIRIS-REx’s maneuver to enter orbit around Bennu. The craft’s thrusters ignited for 8 seconds at 2:43:55 p.m. EST (1943:55 GMT) Monday to slightly adjust OSIRIS-REx’s velocity, nudging it just enough for Bennu’s tenuous gravity to capture the probe into orbit.

“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.”

Orbit insertion burn design has been finalized: @OSIRISREx will fire its thrusters to enter orbit about Bennu at 19:43:55 UTC (14:43:55 EST) tomorrow, December 31st! The engines will burn for 8 seconds. pic.twitter.com/gSI24og2Ix — KinetX SNAFD (@KinetXSNAFD) December 31, 2018

Launched in September 2016, NASA’s $1 billion Origins, Spectral Interpretation, Resource Identification, Security, Regolith Explorer was built to travel to Bennu and collect samples for return to Earth.

When the solar system formed more than 4.5 billion years ago, chunks of rock and ice collided as they circled the sun like the balls on a billiard table, eventually building up planets. The leftovers became asteroids and comets, and scientists believe Bennu still harbors the basic carbon-bearing organic molecules that were present in the early solar solar system, the stuff that may have helped seed life.

Bennu is in an orbit around the sun that crosses Earth’s orbit, making it a potentially hazardous asteroid that could eventually pose an impact threat with our planet.

The spacecraft carries three cameras — one for long-range viewing, a color camera for mapping, and another imager to take pictures as OSIRIS-REx collects samples from the asteroid’s surface. The rest of OSIRIS-REx’s suite of science instruments includes a thermal emission spectrometer to detect heat coming from the asteroid, a visible infrared spectrometer to locate minerals and organic materials, a laser altimeter provided by the Canadian Space Agency to create topographic maps, and a student-built X-ray spectrometer to identify individual chemical elements present on the asteroid.

Besides revealing Bennu with high-resolution images, OSIRIS-REx has already made discoveries at the asteroid.

Data gathered by thermal emission and visible infrared spectrometer instruments — OTES and OVIRS — indicates clay minerals on the asteroid’s surface contain hydroxyl molecules with oxygen and hydrogen molecules bonded together. This finding suggests Bennu’s surface was once in contact with water, likely when the asteroid was part of a much larger parent body that was smashed to bits in a collision in the chaotic early solar system.

“The presence of hydrated minerals across the asteroid confirms that Bennu, a remnant from early in the formation of the solar system, is an excellent specimen for the OSIRIS-REx mission to study the composition of primitive volatiles and organics,” said Amy Simon, OVIRS deputy instrument scientist at NASA’s Goddard Space Flight Center in Greenbelt, Maryland. “When samples of this material are returned by the mission to Earth in 2023, scientists will receive a treasure trove of new information about the history and evolution of our solar system.”

“Our initial data show that the team picked the right asteroid as the target of the OSIRIS-REx mission. We have not discovered any insurmountable issues at Bennu so far,” said Lauretta said. “The spacecraft is healthy and the science instruments are working better than required. It is time now for our adventure to begin.”

The first high-resolution images of Bennu taken by OSIRIS-REx revealed a miniature world containing a few relatively smooth regions without many large rocks, interspersed with boulder-filled regions that might be treacherous for the spacecraft’s sample collection touch-and-go maneuver.

One prominent feature of Bennu’s landscape is a large boulder protruding from the surface near the south pole. While ground-based radar images suggested the boulder to be at least 33 feet, or 10 meters, in height, OSIRIS-REx imagery indicates is closer to 164 feet, or 50 meters, tall with a width of approximately 180 feet, or 55 meters, according to NASA.

On approach to Bennu, ground controllers at Lockheed Martin in Denver — where OSIRIS-REx was built — unlatched the probe’s robotic arm from its launch restraint for the first time. Over several days, the ground team commanded the arm to bend its joints and jettison a launch cover over the sample collection mechanism, which will release compressed air during a touch-and-go maneuver to force gravel and surface material into an on-board chamber for the journey back to Earth.

Due to the asteroid’s weak gravity, OSIRIS-REx was predicted to move around Bennu at a speed of just one-tenth of a mile per hour, or about 5 centimeters per second, once in orbit. The spacecraft’s first orbital phase will last until mid-February, when OSIRIS-REx will again make a series of flybys of Bennu.

During the mission’s first orbital phase, OSIRIS-REx is orbiting the asteroid at a range of 0.9 miles (1.4 km) to 1.24 miles (2.0 km) from the center of Bennu, setting another record for the closest distance any spacecraft has orbited to a planetary body.

Orbital speeds are determined by the gravitational pull of the parent planet or star. For comparison, objects in low Earth orbit have to travel at around 17,500 mph (7.8 kilometers per second) to avoid falling back into the atmosphere.

“It’s Bennu’s size and small mass that make the navigation challenges on this mission unprecedented, really,” said Michael Moreau, OSIRIS-REx flight dynamics system manager at NASA’s Goddard Space Flight Center, in a conference call with reporters in August.

Comet 67P/Churyumov-Gerasimenko, which was orbited by the European Space Agency’s Rosetta spacecraft from 2014 through 2016, is between five and 10 times larger than Bennu, depending on how you measure. Asteroid Ryugu, where Japan’s Hayabusa 2 spacecraft is currently exploring, is nearly twice the size of Bennu, and Hayabusa 2 will not enter orbit around it.

“Entering orbit around Bennu is an amazing accomplishment that our team has been planning for years,” Lauretta said.

OSIRIS-REx will orbit and explore Bennu for more than a year, allowing scientists and mission planners to examine the asteroid and determine a safe location for the craft’s touch-and-go descent in July 2020 to snag samples from the asteroid. The spacecraft will depart Bennu and head back to Earth, releasing its sample carrier for re-entry and landing in Utah in September 2023.

Scientists will take the samples to an ultra-clean facility at NASA’s Johnson Space Center in Houston for detailed analysis.

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Follow Stephen Clark on Twitter: @StephenClark1.