On Sept. 12, OSIRIS-REx pointed its medium-range science camera, MapCam, toward asteroid Bennu 621,000 miles (one million kilometers) in the distance. Slewing gently side to side and up and down as it captured 64 images, the spacecraft scanned the area around the asteroid in a carefully choreographed pattern. The day before, it had collected a similar mosaic of images with its long-range science camera, PolyCam.
Over the next few days, scientists on the ground pored over the images, looking for any signs of dust in the vicinity of the asteroid, which could present a hazard to the spacecraft as it approaches. Ultimately, they determined that the coast is clear – for now.
But OSIRIS-REx will look for natural satellites (small moons) and conduct another search for dust plumes when the spacecraft is closer to Bennu.
While comets, with their characteristic tails and comas, are known for releasing plumes of volatile materials like ice, gas, and dust, this behavior has also been observed in some asteroids. If dust had been visible in the images collected in mid-September, it would have suggested that Bennu had comet-like plume activity in the recent past, probably in the weeks or months before OSIRIS-REx conducted its first dust search.
Although OSIRIS-REx is designed to withstand the rigors of spaceflight and the occasional collision with stray particles, flying through a dust plume would pose a risk to the spacecraft’s instruments and solar panels. If the mission team had identified plume activity in the images, they had contingency plans to execute a braking maneuver, placing the spacecraft at a safe distance so that the dust activity could be studied further.
The existence of dust plumes would suggest that Bennu has active deposits of ice or other volatiles. Finding frozen water on the asteroid would be an exciting result for mission scientists who are in part studying Bennu to understand whether asteroids could have been the delivery mechanism for the water and organic materials needed to seed life on Earth billions of years ago. Plumes would also have implications for where OSIRIS-REx could safely collect a sample of material from Bennu’s surface in 2020 – and what types of material would likely be in that sample.
“We probably wouldn’t want to sample too near a vent for safety reasons,” says Carl Hergenrother, the OSIRIS-REx Astronomy Working Group Lead, who helped plan the hazard searches. “But it would be interesting since plumes mean that there could be subsurface volatile material nearby.”
OSIRIS-REx’s second dust plume search, scheduled for two days in Spring 2019 when the spacecraft will be about 3.1 miles (five kilometers) from Bennu, will look for active dust plumes coming off Bennu’s surface. For those observations, the spacecraft will be positioned between the Sun and the asteroid (at a high phase angle) so that Bennu is backlit and any dust plumes are more visible. Some of the 13 mosaics that the spacecraft captures will include offset images of the asteroid so that any jets coming from the surface are easier to see against the dark backdrop of space.
Dust isn’t the only potential hazard that OSIRIS-REx is looking out for. Later this fall, the spacecraft will use PolyCam and MapCam to search for natural satellites – any chunk of rock orbiting Bennu that is larger than 10 centimeters and bright enough to be seen (which requires an albedo of at least 0.03). While most asteroids exert a weak gravitational pull due to their relatively small sizes (Bennu has a diameter of roughly 500 meters), they are capable of holding small moons in orbit around themselves. In fact, asteroid 243 Ida, the second asteroid ever visited by a spacecraft, surprised scientists when images from the Galileo mission revealed it had a small moon, now called Dactyl.
To look for moons, two of OSIRIS-REx’s cameras will again capture a series of carefully planned mosaics covering the area around Bennu. First, PolyCam will map the asteroid’s entire Hill Sphere (the area where a satellite could theoretically exist), looking for objects that are one meter or larger. Then, as the spacecraft gets closer, MapCam will conduct a search pattern for smaller satellites (down to 10 centimeters), which could only exist in a stable orbit closer to Bennu.
Similar to a dust plume discovery, if OSIRIS-REx were to detect a natural satellite orbiting Bennu, it would trigger a contingency plan. The spacecraft would conduct a braking burn and stop its approach to the asteroid about 40 or 50 kilometers out. The team would then take a few weeks to closely map the moon’s orbit around Bennu and decide whether any changes need to be made to the mission plan for the spacecraft to safely avoid the satellite. Later on, the team would study the moon in more depth, collecting images and other data about its color, reflectivity, shape, size, and other features.
Also similar to a dust plume detection, a moon would be an interesting scientific discovery. “If we did find a satellite, mapping its orbit would allow us to refine the mass of Bennu before going into orbit around the asteroid or even doing close approaches,” says Hergenrother. “It would also tell us more about Bennu’s history.”
While potential hazards like dust and natural satellites present navigation, safety and other challenges, they are part of the inherent adventure of exploring a never-before-visited world. Although Bennu has been thoroughly studied from Earth, the asteroid may have many surprises in store for the mission team. Careful planning and thorough observation strategies will ensure that these surprises are transformed from potential hazards into new scientific knowledge.