KENNEDY SPACE CENTER, FL — A spacecraft built to rendezvous with an asteroid, collect a sample, then return it to Earth is sitting in NASA’s Payload Hazardous Servicing Facility (PHSF) as it undergoes final inspection before its terrestrial departure September 8.
Once launched, OSIRIS-REx (Origins Spectral Interpretation Resource Identification Security Regolith Explorer) will begin a two year journey to Bennu, the lucky asteroid chosen from over 7,000 near-Earth asteroids, to study its composition, orbital dynamics, and assess the asteroid’s potential for impacting Earth.
Bennu, named after the Egyptian mythological bird, is roughly 500 meters in diameter, and was chosen because of its size, chemical makeup, and orbital proximity to our planet. Bennu and asteroids like it are extremely carbon-rich and have not changed since their creation during the solar system’s infancy nearly 4 billion years ago. It’s even possible that an asteroid such as this impacted Earth, and seeded our planet with the origins of life.
OSIRIS-REx’s primary mission goal is to return a sample from Bennu known as regolith - or, more simply put, space dirt. After rendezvousing with the asteroid in August of 2018, OSIRIS-REx will spend another two years mapping, analyzing, and surveying Bennu’s thermal, chemical, and geological properties, in part to determine an optimal contact point for sample collection.
The probe won’t actually move into position to make contact with Bennu until July 2020. When it does, the Touch-and-Go Sample Acquisition Mechanism (TAGSAM) will press against the asteroid’s surface for a total of five seconds. Releasing a burst of nitrogen gas, TAGSAM will blow loose rocks and soil into a sample container, and then stow it in OSRIS-REx’s Sample Return Capsule for the journey back to Earth.
OSIRIS-REx NASA HQ Program Scientist Jeffrey Grossman outlined the importance of both the return sample and the overall mission itself during a press briefing:
“ORISI-REx is going to help us understand how the solar system works, and the nature of the bodies in the solar system. Its going to help us understand how the solar system formed. Its going to inform our understanding for the potential for life in the solar system; on Earth and elsewhere. But it also advances our more practical goals of understanding the resources of the near-Earth solar system, as well as the hazards; characterizing those, and better understanding the processes that are involved with those [hazards].”
Those hazards are another reason NASA chose Bennu. Every six years, Bennu’s orbit brings the asteroid to within 0.002 AU (Astronomical Units - the average distance between Earth and the Sun). That’s very close. For comparison, the Moon is only 0.002454 AU from Earth, on average.
Though it may not be within our lifetime, a 2135 fly-by of the asteroid will place it between the Earth and the Moon. OSIRIS-REx Principal Investigator Dante Lauretta pointed out during a press conference at NASA’s PHSF, “that 2135 fly-by is going to tweak Bennu’s orbit, potentially putting it on course for the Earth later that century.” And while the asteroid may not be on the list of globally-catastrophic sized space rocks, it certainly wouldn’t simply burn up in the atmosphere. An impact from an asteroid of Bennu’s size could easily devastate a metropolitan area for miles.
The “Security” title in the OSIRIS-REx acronym holds the focus to this aspect of the probe’s mission. Bennu’s orbit has been observed in close detail since 1999, and while scientists have a theory for the forces that can influence the asteroid’s orbit, data from OSIRIS-REx will allow the refinement of current models and measurements used to track the orbits of such planetary bodies.
Known as the Yarkovsky effect, an asteroid absorbs solar radiation during its natural rotation relative to the Sun, then expels that heat like a radiator as thermal radiation. This expulsion creates a minuscule, but impactful force of thrust as the asteroid spins that can alter its orbit over time. Lauretta explains,
“The Yarkovsky effect turns out to be a fundamental process that’s effecting these small asteroids across our solar system…The asteroids get their energy from the Sun. A lot of that energy, especially on a dark asteroid like Bennu, is absorbed, causing the asteroid’s surface to heat up. It has to release that energy back out into space, and when that happens, it acts like a thruster and changes the trajectory of the asteroid. So if you want to predict where an object like Bennu is going to be in the future, you have to account for this phenomena. And we’re going to provide the best ever scientific investigation of this fascinating concept.”
One might wonder, if the small amount of thrust caused by the Yarkovsky effect is enough to potentially put Bennu’s trajectory on a collision course with Earth, how will it be effected by a spacecraft bumping into it and releasing a burst of pressurized gas? Thankfully, Lauretta was quite reassuring.
For the immediate future, the risk of an asteroid impacting the Earth seems minimal. The data project-scientists seem to be most looking forward to will arrive with OSIRIS-REx’s return sample on September 24, 2023, which they expect to be able to study for decades to come as they search for valuable insights into the origin of life in our solar system and on Earth.