NASA is about to achieve another first for their organization. In about three weeks time, on October 20th, the OSIRIS-REx spacecraft will descend to Bennu’s surface, briefly touch down, and collect a sample from the asteroid. The spacecraft will collect a minimum of 60 grams (2 oz.) of material for return to Earth.
When the spacecraft arrived at Bennu in December 2018, it turned out that the asteroid was much rockier than thought, and the surface was strewn with boulders. OSIRIS-REx took its time selecting an appropriate sampling site. Initially, there were four candidate sites. After a lot of examination and consideration of the sites, mission operators settled on the site named Nightingale: a 16m (52 ft.) clearing among the boulders.
Nightingale was chosen because it contains the largest amount of fine-grained material suitable for sampling. But it’s also surrounded by hazardous boulders that’ll make it challenging for the bus-sized spacecraft to maneuver around.
The spacecraft and the asteroid are 334 million km (207 million miles) from Earth. That distance means there’s an 18.5 minute delay in communications between mission control and the spacecraft. So OSIRIS-REx (Origins, Spectral Interpretation, Resource Identification, Security, Regolith Explorer) will execute a set of instructions and collect the sample autonomously.
As it finds its way down to Nightingale, OSIRIS-REx will rely on its Natural Feature Tracking navigation system. That system will compare real-time images of the site with images stored online to guide itself down between the boulders. There’s a fail-safe built into the system, and if the spacecraft decides to, it can abort the descent and try again.
Once the spacecraft makes its way to the surface safely, it’s time for the TAGSAM (Touch-And-Go Sample Acquisition Mechanism) system to take center stage.
TAGSAM features a sampling head on the end of an instrument arm 3.35 m (11 ft) long. OSIRIS-REx will extend that arm as it approaches the asteroid, and while OSIRIS-REx is slowly lowering itself over the sampling site, the momentum will carry the sampling head onto the asteroid’s surface. It’ll only be there for about five seconds, long enough for TAGSAM to release puffs of nitrogen to stir up particles. The particles will be caught by surface pads.
After collecting its sample, OSIRIS-REx will slowly back away from Bennu. Then it’ll perform two separate checks to see if the sampling was successful. First it’ll use its SamCam (Sampling Camera) to capture images of the sampling head to verify the presence of material. Then i’ll perform a spin maneuver that allows it to determine the mass of the collected sample. If there were problems, the spacecraft has enough nitrogen for two more sample attempts. If it can’t navigate safely to Nightingale, then it can attempt a sampling operation at back-up site Osprey.
If all goes well, OSIRIS-REx will depart from Bennu in 2021 and return to Earth in 2023. On September 24th, 2023 the spacecraft will send its Sample Return Capsule (SRC) down to Earth by parachute, to land at the Air Force’s Utah Test and Training Range. Then, the science can begin.
Bennu is a C-type (carbonaceous) asteroid in the near-Earth Apollo group of asteroids. C-type asteroids are the most common type, and make up about 75% of known asteroids. C-type asteroids are divided into sub-groups, and Bennu is a B-type within the C-type family.
Scientists think of Bennu as a sort of time capsule. It contains material from the primordial solar nebula, before any planets were fully-formed. Researchers are keen to examin material from that distant time, to learn more about the evolution of the Solar System, and the bodies in it, including Earth. They also want to know if asteroids like Bennu are the source of the some of the organic compounds that aided the rise of life on Earth.
This will be the first time that NASA has collected an asteroid sample and returned it to Earth. But it won’t be the first time that’s ever been done. JAXA (Japan Aerospace Exploration Agency) has already returned samples of asteroid Itokawa to Earth with its Hayabusa space probe. Those samples arrived at Earth in 2010, and have been studied in depth.
JAXA launched the second of its sampling missions, Hayabusa 2, in December 2014. It has succesfully collected a sample from asteroid Ryugu and is on its way back to Earth. It’ll deliver its sample to Earth in December 2020, then will continue on to another target, asteroid 1998 KY26. (It’ll only observe and study its next target, not sample it.)
We’re becoming accustomed to complex missions like this, and we kind of expect them to succeed. But take a step back and look at the OSIRIS-REx mission with fresh eyes:
A spacecraft the size of a bus will navigate its way down to the surface of a boulder-strewn, rotating asteroid over 300 million km away. It has no way to communicate with mission operators as it does so. It’ll use its autonomous Natural Feature Tracking (NFT) navigation system to avoid obstacles as it descends. When its extended TAGSAM (Touch-And-Go Sampling Acquisition Mechanism reaches the surface, it’ll release a puff of nitrogen to drive rock particles into the air, to be collected in the sampling head.
Then it’ll slowly back away from Bennu, autonomously again, to a safe distance. Next it’ll use its camera to check and see if the sampling head contains any material. Then it’ll perform a spin maneuver to determine the mass of the sample. Eventually, OSIRIS-REx will head back to Earth and release its sample, sending it down to a predetermined location on Earth for retrieval.
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