American Scientist

In the July-August 2014 issue, we spoke with Dante Lauretta, a planetary scientist at the University of Arizona and principal investigator of the OSIRIS-REx mission to asteroid Bennu, which launched in 2016. Here, Lauretta provides a follow-up to the mission since it launched.

The OSIRIS-REx spacecraft is currently surveying near-Earth asteroid Bennu, with the primary objective of selecting a site on the surface to collect a sample. We are also studying Bennu’s physical, chemical, and geological properties to provide context for the sample that we collect, assess Bennu’s resource potential, refine estimates of its impact probability with Earth, and “ground-truth” the extensive astronomical data set collected before the spacecraft encounter.

It has been a long journey. After five years of development, the spacecraft launched on September 8, 2016. The outbound trajectory included a one-year loop around the Sun that culminated in an Earth gravity assist (EGA) to boost the spacecraft to its destination. During this transit, the spacecraft approached an area of space called the Sun-Earth L4 Lagrangian point, which we thought might harbor an as-yet-undetected population of “Earth-Trojan asteroids,” or objects trapped in this stable region that co-orbits the Sun with Earth. Although we found no Earth-Trojan asteroids, the survey yielded many main-belt asteroid detections, demonstrating our camera’s sensitivity. The EGA rotated the spacecraft trajectory six degrees into Bennu’s orbit plane, after which the spacecraft spent another year traveling to the asteroid. We took advantage of the Earth-Moon flyby to calibrate our science instruments.

The spacecraft began observing Bennu in August 2018, when it was sufficiently close to the asteroid for it to be just bright enough for detection. As the spacecraft approached Bennu, we discovered that it is spinning faster over time, most likely because of what’s called the YORP effect—a windmill-like phenomenon related to radiation pressure that can cause the asteroid rotation rate to increase or decrease over time. We found that the rotation period is getting shorter by about 1 second per century—a seemingly slight effect, but one that could eventually spin the asteroid apart.

We also acquired an early assessment of the minerals on the surface from our spectrometers. A ubiquitous spectral absorption band in the near-infrared (at a wavelength of 2.7 micrometers) indicates the presence of hydrated silicates. These water-bearing minerals are precisely the kind of material we were hoping to find, because they lend support to the idea that volatiles such as water could have been delivered to Earth by primitive asteroids.

Based on measurements of the asteroid that were previously taken by the Spitzer Space Telescope, we expected Bennu’s surface to be covered in small, sand-sized particles. But the early high-resolution images from the spacecraft’s cameras revealed the surface to be much rougher, with many boulders ranging from meters to tens of meters in diameter, the biggest of which is about nine stories tall and nearly as wide as the wingspan of a 747. This surprise means that we need to improve the spacecraft guidance system to target much smaller areas for sample collection than originally planned.

The data acquired during the approach to and initial survey of Bennu also allowed us to produce a stunning 75-centimeter-resolution shape model, a mission-critical product for both science and navigation. This model shows that Bennu is shaped like a spinning top, as we predicted. The spacecraft flew in hyperbolic trajectories over the north and south poles and the equator, taking radio science measurements that determined Bennu’s mass. This value, together with the volume from the shape model, showed us that Bennu has the low density (high pore space) characteristic of a “rubble pile” asteroid—a cluster of constituent pieces, rather than a solid body, probably formed from the remnants of other asteroids broken up by collisions. As much of 60 percent of Bennu’s interior may be empty space.

With the asteroid’s mass, shape, and rotation state constrained, the navigation team had all the information needed to place the spacecraft into orbit on New Year’s Eve of 2018. This feat was recognized with two Guinness World Records: the smallest celestial body ever orbited (just about 500 meters in diameter, a little bigger than the Empire State Building), and the closest orbit ever achieved (at 1.6 to 2.1 kilometers).

This “Orbital A” phase provided the biggest surprise of the mission. Back when the spacecraft was still on its approach to Bennu, we searched for dust plumes and natural satellites in the vicinity of the asteroid, and none were detected. And yet, in navigational images taken a week after entering orbit, the team spotted what appeared to be particles ejecting from the surface of Bennu. We increased our observation cadence and soon detected several more particle ejection events. Although many of the particles were launched clear into space, we tracked some particles that orbited Bennu before returning to the asteroid’s surface. Luckily, the particles do not pose a safety risk to the spacecraft. The team is working hard to understand this amazing phenomenon, which has never been seen before.

The characterization of Bennu began in earnest this spring with the “Detailed Survey” mission phase, wherein the spacecraft flies over Bennu repeatedly to characterize the asteroid from different angles. Images that we acquire in this phase will be used to produce digital terrain maps of any potential sample site at an extraordinary scale of 5 centimeters per pixel. The images are also keeping us busy mapping the geological features on Bennu’s surface, including craters, boulders, grooves, faults, and regolith. Data that we collect in this phase will further reveal the temperature and composition of Bennu’s surface in detail, feeding into the selection of a sampling site that is both safe and scientifically valuable.

By December, we will have zeroed in on a site for sampling in mid-2020. The goal is to return at least 60 grams of material to Earth. When the mission returns to Earth in 2023, it will complete our seven-year journey to bring pieces of the early Solar System home for further study.