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NASA’s OSIRIS-REx spacecraft moved within about 131 meters, or 40 meters, of the Bennu asteroid this week in the final $ 1 billion mission practice run ahead of a touchdown and descent to the asteroid in October to collect samples for return to Earth.
During the sampling test on Tuesday, the engineers who monitored the spacecraft maneuvers from Earth confirmed that the navigation algorithms, computer programs for autonomous operations, scientific instruments and multiple mechanical systems all functioned as expected.
On Tuesday, the spacecraft OSIRIS-REx fired intruders to leave a “safe house orbit” approximately 0.6 miles (1 kilometer) from the asteroid Bennu, then landed at a point about 410 meters (125 meters) above the asteroid surface. Spaceships then fired at cars to perform a so-called “checkpoint” burnout to begin a free fall toward the asteroid.
Eight minutes later, OSIRIS-REx struck the control planes again for a “match point” burn to match the aircraft ship’s motion with the asteroid rotation. This allows the spacecraft to make its final landing on the surface above the location of the target samples.
The spacecraft continued to descend in the direction of the asteroid for three minutes as the match point burned, reaching a point 131 meters from Bennu before shooting the strikers back for a back maneuver.
The position that OSIRIS-REx reached during Tuesday’s test marked the closest the spacecraft has come to the asteroid Bennu, following a similar sampling drill in April that practiced mission downhill maneuvers up to a point of 213 meters, or 65 meters, from the asteroid.
After launching from Earth in 2016 and reaching its asteroid target in 2018, Origins, NASA Interpretation, Resource Identification, Security, Regolith Explorer, or OSIRIS-REx, NASA has mapped Bennu with cameras, mineral-olfactory spectrometers and a Canadian built laser to measure its roughness.
Formed as a revolving peak, Bennu measures about 1,614 meters (492 meters) wide. At the time of Tuesday’s test, the asteroid was located about 179 million miles (288 million kilometers) from Earth. At that distance, it takes 16 minutes for commands from ground teams to reach the OSIRIS-REx spacecraft.
“This time delay prevented direct command of ground-based flight activities during the test,” NASA said in a statement. “As a result, the spacecraft performed the entire test sequence autonomously.”
Ground controllers further linked the landing sequence to spacecraft ahead of time, and OSIRIS-REx flew the landing profile on autopilot, as it would in the current sampling effort.
OSIRIS-REx has been the search sites for the sample collection, but the scientists found that Bennu is more rugged than expected.
The lunar space sampling test targeted a location called Nightingale, the main sampling site located within a 460-meter crater in the northern hemisphere of Bennu. The area is surrounded by sharp rocks and terrain, and that forced engineers to develop updated navigation skills to allow the spacecraft to steer itself into a safe and touching landing.
OSIRIS-REx uses a feature called natural feature tracking to take a series of images with a navigation camera to autonomously identify rocks, craters and other marks on the asteroid’s surface, giving data on relative position and velocity . The spacecraft computer compares the figures with a hazard map loaded on the computer before landing. If OSIRIS-REx discovers that it is approaching a dangerous area, it may order an abortion and return away from Bennu.
Natural trace algorithms were successfully tested during Tuesday’s test. The spacecraft’s cameras captured higher-resolution images of the sampling location at Nightingale when OSIRIS-REx was flying directly to the site, allowing ground crews to update and refine the ability to track natural features before the actual effort of sampling in October.
“Many important systems were exercised during this test – from communications, ship propellers, and most importantly, the nature trail tracking system and hazard mapping,” said Dante Lauretta, OSIRIS-REx lead investigator at the University of Arizona. Tucson. “Now that we have completed this milestone, we are confident of completing the procedures for the TAG (Touch-And-Go) event. This test confirmed that the crew and all of the ship’s systems are ready to collect a sample in October.”
OSIRIS-REx also extended its sample collection arm, known as TAGSAM, during Tuesday’s test. The spacecraft also moved its two wings of the solar array in a “Y-wing” configuration to position them safely away from the asteroid surface, just as they would be positioned during the touch landing and go.
The TAGSAM wing later retreated to a settled position after OSIRIS-REx began flying away from Bennu at the end of the test on Tuesday.
During the actual mission examination, a device mounted on the bottom of the TAGSAM wing of the spacecraft will contact the asteroid surface for a few seconds and the nitrogen gas compressed by the fire. The gas cartridge will break the rock at the sampling site and the spacecraft will capture some of the materials in a room to return to Earth.
Scientists hope to collect at least 2.1 ounces (60 grams) of pristine samples from the Bennu surface, and return the material to Earth in September 2023 for analysis in sophisticated laboratories. Scientists will examine specimens to look for signs of organic matter and other chemicals critical to the origin of life.
Built and operated by Lockheed Martin, the OSIRIS-REx spacecraft launched from Cape Canaava in September 2016. A limited number of personnel passed Tuesday’s checkpoint test from a Lockheed Martin control center in Colorado, plus locations at the Center Goddard Space Flight to Maryland and the University of Arizona.
Other team members participated remotely due to the coronavirus pandemic, NASA said.
Earlier this year, mission managers decided to give teams more time to prepare for the OSIRIS-REx sampling effort after officers adapted to new operations schemes to minimize risks from COVID- 19.
This decision delayed the second OSIRIS-REx sampling trial from 23 June to 11 August and again postponed the current execution examination from 25 August to 20 October.
If the first attempt at sampling turns out to be in vain in October, scientists are hampered by the possibility of additional evidence before the ship leaves for Bennu next year to begin its return voyage to Earth.
Follow Stephen Clark on Twitter: @ StephenClark1.