Earth lives in the middle of a shooting gallery. We are surrounded by thousands of asteroids that whiz close to our planet. While we don't know of an imminent threat to humanity as of now, history shows that eventually, something big will hit us. Just ask the dinosaurs.
On September 8, if all goes according to plan, an SUV-sized spacecraft will blast off on its mission to a potentially hazardous nearby asteroid called Bennu. One important goal of OSIRIS-REx (Origins, Spectral Interpretation, Resource Identification, Security, Regolith Explorer) is to help scientists better predict the path of these small, rocky bodies as they careen through the solar system.
The NASA-led mission, which includes participation from the Canadian Space Agency, will stay at Bennu for two years, mapping its shape and size, and scooping a sample from the surface to return to Earth in 2023. For asteroid defense purposes, scientists will watch how solar heating affects the asteroid's movements through a phenomenon called the Yarkovsky effect. "The asteroid absorbs the heat and it re-radiates that heat, which gives a subtle push to its orbit," said Lindley Johnson, NASA's planetary defense officer, in an interview with Motherboard. "Over time, especially for smaller asteroids, it changes the orbit subtly."
Scientists have tracked Bennu's Yarkovsky effect since 1999, when the Arecibo Observatory in Puerto Rico took measurements using radio wavelengths. Those measurements were repeated in 2005 and 2011, said mission principal investigator Dante Lauretta at a televised NASA press conference Aug. 17.
NASA TV briefing previews launch of asteroid-bound spacecraft. Video: NASA/YouTube
OSIRIS-REx will gather data near Bennu between 2018 and 2021, giving investigators two decades of measurements, Lauretta added. The spacecraft's instruments will measure the physical properties and the rotation of the asteroid up-close for the first time, providing an unprecedentedly precise picture of the Yarkovsky effect—and providing us with more information on how asteroids move through space, which will be critical in determining which ones are the potential Earth-killers.
"We're going to see if theory matches observation," Lauretta said. "If it doesn't," he added, "and sometimes nature surprises you, then we're going to have to go back and sharpen our pencils and refine our theory until we get to the right answer."
Dancing up close
NASA has several other asteroid missions planned in coming years. The proposed Asteroid Impact & Deflection Assessment mission (with the European Space Agency) would leave Earth in 2020 to hit an asteroid's moon and watch how much it deflects. This will be practice for using a "kinetic impactor" to move a hazardous asteroid out of the way of Earth, one tactic that scientists have considered implementing if and when this happens.
NASA also hopes to run a human mission to an asteroid sometime in the 2020s, which would involve a robot carving a huge boulder from an asteroid and moving it closer to Earth for astronauts to visit and study. What is needed between now and then, therefore, is lots of practice working near an asteroid.
"Close-proximity operations to a small body is more like docking to a [second] spacecraft than orbiting a planet," Johnson said.
"Since it's a small, low-gravity environment, the amount of thrust used for attitude control and adjusting your orbit has bigger effects. If you use too much power, you knock yourself out of orbit quickly."
"We want to understand these asteroids, [including] what they're made up with and how they evolve"
While a few missions have previously visited comets and asteroids, only a handful have attempted landings. The first-ever comet landing took place in November 2014, when the Philae lander left the European Rosetta spacecraft and touched down on Comet 67P/Churyumov–Gerasimenko. It was an astonishing feat, given no good maps of the comet even existed before Rosetta arrived, just three months before. (OSIRIS-REx won't be landing on its targeted asteroid, exactly, but rather lowering itself down close to the surface to retrieve a sample.)
Read More: Goodbye Philae
"You get there, and you don't know the object yet," Andrea Accomazzo, the flight director for the Rosetta mission, recalled in a Motherboard interview.
He said that the same problem will be true of any spacecraft (manned or not) visiting an asteroid, because telescopes near Earth are not powerful enough to see these small objects. By contrast, spacecraft preparing to land on Mars already have many high-definition images from other robots who mapped the planet's surface.
OSIRIS-REx's precious sample of Bennu will be carried back to Earth for analysis by scientists, to learn more about what stuff asteroids are made of. The spacecraft will skim a kilogram or so of material off the surface. It's a small amount, but it will be the biggest haul yet from another body in our solar system (excluding the human Apollo moon missions).
Lowering the spacecraft safely towards the asteroid will require careful study of optical images, as well as Canadian laser altimeter measurements of its surface. The instrument responsible for this (called OLA, or OSIRIS-REx Laser Altimeter) will be able to see rocks as small as 5 centimeters across, even in areas of shadow on the asteroid's surface.
"We want to have a region that is as flat as possible," Catherine Johnson, co-investigator on the OSIRIS-REx mission and the deputy instrument scientist for the altimeter, told Motherboard. "You don't want it tilted. You want a place that is basically very smooth, very low slope, no big boulders."
Philae's fate demonstrates just how tricky the maneuvering can be: after touchdown, it failed to properly grip the comet's surface, and landed in a dark spot where its solar-powered batteries couldn't charge. The Philae lander performed only 64 hours of experiments. Controllers heard from it sporadically afterwards, and then it died for good.
As for OSIRIS-REx, the OLA instrument's heritage comes from two lidars (lasers that work like radars) that flew on previous missions: the 2008 Phoenix mission to Mars' north pole, and a test US Air Force Research Laboratory satellite built by Canada's MacDonald, Dettwiler and Associates (or MDA, which also developed the iconic Canadarm and Dextre robotic arm).
Mike Daly, the instrument scientist, created the concept while he was still working at MDA, which he eventually left for Toronto's York University. Besides scanning the surface in detail and learning the asteroid's size and shape, the lidar will take the topography of the entire asteroid, he told Motherboard.
"From a science perspective we want to understand these asteroids, [including] what they're made up with and how they evolve," he said, adding that one thing the lidar can chart is the flow of asteroid regolith or material across the surface. "That topography feeds back into the Yarkovsky effect and will predict the asteroid's path in the future."
Correction: The OSIRIS-REx mission will not be landing on the asteroid, as was implied in a previous version of this story, but rather lowering itself close to the surface to retrieve a sample. The story has been updated to reflect that.