Asteroid Operations for the OSIRIS-REx mission began in August 2018 – when the spacecraft captured its first image of Bennu from a distance of about 1.2 million miles (two million km) – and continue until March 2021 – when the spacecraft begins its return trip to Earth. The spacecraft’s official Arrival at Bennu occurred on Dec. 3, 2018, when OSIRIS-REx transitioned from flying toward Bennu to operating around Bennu. While at the asteroid, OSIRIS-REx will survey and map Bennu, navigate in close proximity to the asteroid, and ultimately touch the surface for five seconds to gather a sample of the asteroid.
Asteroid Operations are divided into nine phases, which are each specifically designed to allow the mission team to build its knowledge of the asteroid, learn how to safely navigate the spacecraft in microgravity, and identify the best sample site.
Approach Phase began on August 17, 2018, when the spacecraft was still about 1.2 million miles (two million km) away from Bennu, and it continued until the spacecraft arrived at the asteroid on December 3, 2018. The primary goals of Approach were to visually locate Bennu for the first time, survey the surrounding area for potential hazards, and collect enough imagery of Bennu for scientists to generate a detailed shape model of the asteroid, assign a coordinate system, and understand its spin state.
Preliminary Survey Phase began with the spacecraft’s arrival at Bennu on December 3, 2018, and marked the first time that the OSIRIS-REx spacecraft operated around the asteroid. The spacecraft made a total of five passes over the north pole, equator, and south pole at a range of 4.3 miles (7 km). The primary science goals of Preliminary Survey were to estimate Bennu’s mass, refine the asteroid’s spin state model, and generate a global shape model at a resolution of 75-cm.
In Orbital A Phase, the spacecraft was placed into a gravitationally-bound orbit around Bennu for the first time on Dec. 31, 2018. There were no science requirements for Orbital A, as this phase was designed to provide the mission team with experience navigating in close proximity to a small body. The spacecraft circled Bennu at a distance between 0.99 and 1.3 miles (1.6 and 2.1 km) and travelled around 0.11 mph (5 cm/sec), with each orbit lasting about 61.4 hours. This phase marked the closest that a spacecraft has ever orbited around a small body.
During this phase, the navigation team transitioned from star-based navigation to landmark-based navigation. Using landmarks – such as boulders and craters on Bennu’s surface – to determine the position of OSIRIS-REx allows the navigation team to maneuver the spacecraft very precisely, which will be critical during upcoming mission phases.
Detailed Survey: Baseball Diamond
The in-depth study of Bennu began in earnest during Detailed Survey: Baseball Diamond Phase, which kicked off on Feb. 28, 2019. OSIRIS-REx is making multiple passes around Bennu to produce the wide range of viewing angles necessary to fully observe the asteroid. The spacecraft will also use its OTES spectrometer to map the chemical composition of Bennu’s entire surface. Images obtained during this phase will be of high enough resolution to produce digital terrain maps and global image mosaics for proposed sample sites. Bennu’s terrain will be surveyed in bulk and sections will be classified as either “safe” or “unsafe,” with the results visualized on a hazard map.
The phase’s name comes from the early stage of mission design when the stations the spacecraft would traverse were arranged in the shape of a baseball diamond. Although the mission design has since evolved, the original name for the phase remains.
Detailed Survey: Equatorial Stations
During Detailed Survey: Equatorial Stations Phase, the spacecraft will make scientific observations needed to help the team home in on the best location on Bennu to collect a sample of regolith (loose surface material). To obtain this data, the spacecraft will execute a series of slews between Bennu’s north and south poles while taking observations from seven different stations above the equator. These data will be studied to understand the geology of Bennu. The spacecraft will also conduct searches for dust and gas plumes.
The wide range of data products developed during this phase will be analyzed and combined to produce the Integrated Global Science Value Map, the Global Safety Map and the Global Sampleability Map. At the end of Detailed Survey: Equatorial Stations, the team will have the information needed to select up to 12 candidate sample sites. In addition, the team will map the global properties of the asteroid, accomplishing a major science objective of the mission.
At the end of Detailed Survey, the spacecraft will enter a close orbit (with a radius of .6 miles (1 km)) around Bennu and begin Orbital B Phase. This phase breaks the record OSIRIS-REx set in Orbital A for the closest that a spacecraft has ever orbited around a small body. The primary science activities for this phase are global mapping of Bennu, the development of shape modeling based on OLA data, and the execution of a Radio Science experiment. These data are used to evaluate potential sample sites for three key elements: safety, sampleability and science value. Orbital B concludes with the team narrowing in on a primary and a back-up sample site.
During Recon Phase, the spacecraft will make a series of low-altitude reconnaissance observations of the two final sample site candidates. These observations, obtained from 738 ft (225 m) above the surface, will show objects on the ground that are as small as .8″ (2 cm). Context images of the sites will also be taken during higher passes at an elevation of 1,722 ft (525 m). Both sites will be fully studied so that the team can immediately begin planning sample collection at the back-up site if it becomes necessary.
Because sample collection is a critical event, the mission has planned for at least two rehearsals prior to final execution. In the first rehearsal OSIRIS-REx will practice leaving its orbit, maneuvering to a pre-defined Checkpoint located 410 ft (125 m) above the sample site, and then returning to orbit. The second rehearsal will take the spacecraft from orbit to a Matchpoint, where it will hover over the sampling location before a return to orbit. During each rehearsal, the spacecraft will collect and analyze tracking data, LIDAR ranges, and OCAMS and TAGCAMS imagery so that the team can verify the flight system’s performance before the actual sample collection maneuver.
When it is time, OSIRIS-REx will use the TAGSAM (Touch-and-Go-Sample-Acquisition-Mechanism) instrument to collect a sample of regolith from Bennu. TAGSAM is an articulated arm on the spacecraft with a round sampler head at the end. During the Touch-and-Go maneuver (TAG), the sampler head will be extended toward Bennu, and the momentum of the spacecraft’s slow, downward trajectory will push it against the asteroid’s surface for about five seconds—just long enough to obtain a sample. At contact, nitrogen gas will blow onto the surface to roil up dust and small pebbles, which will then be captured in the TAGSAM head.
After the spacecraft fires its thruster to back-away from Bennu, the mission team will measure the amount of sample collected by spinning the spacecraft with the TAGSAM arm extended. They will then compare the change in the spacecraft’s inertia with a previous, empty-TAGSAM spin to ensure that enough sample was collected. The spacecraft has three nitrogen gas canisters on board, allowing for three sampling attempts. Once it is determined that sample collection is successful, the TAGSAM head will be placed in the Sample Return Capsule for return to the Earth. After successful stowage, the spacecraft will be put in a slow drift away from Bennu to a safe distance, where it will stay until its departure in March 2021 for the Return Cruise Phase back to Earth.