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 will 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 is surveying and mapping Bennu, navigating in close proximity to the asteroid, and will ultimately touch the surface for five seconds to gather a sample of the asteroid.
Asteroid Operations are divided into ten 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 made multiple passes around Bennu to produce the wide range of viewing angles necessary to fully observe the asteroid. The spacecraft also used its OTES spectrometer to map the chemical composition of Bennu’s entire surface. Images obtained during this phase were of high enough resolution to produce digital terrain maps and global image mosaics, which were then used to identify proposed sample sites. Bennu’s terrain was surveyed in bulk and sections were 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 made 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 executed a series of slews between Bennu’s north and south poles while taking observations from seven different stations above the equator. These data were studied to understand the geology of Bennu. The spacecraft also conducted searches for dust and gas plumes.
The wide range of data products developed during this phase were 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 had the information needed to select four candidate sample collection sites. In addition, the team mapped the global properties of the asteroid, accomplishing a major science objective of the mission.
At the end of Detailed Survey, the spacecraft entered a close orbit – with a radius of 0.6 miles (1 km) – around Bennu to begin Orbital B Phase. This phase broke 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 were the global mapping of Bennu, the development of shape modeling based on OLA data, and the execution of a Radio Science experiment. These data were used to evaluate potential sample collection sites for three key elements: safety, sampleability and science value. Orbital B concluded with the team narrowing in on a primary and a back-up sample site.
At the end of Orbital B, the spacecraft transitioned to the slightly higher Orbital C for additional particle ejection observations. During Orbital C, the spacecraft flew approximately 1.3 kilometers above the asteroid’s surface.
During Reconnaissance A phase, the OSIRIS-REx spacecraft performed four flyovers – one for each potential sample collection site. Site Sandpiper was observed first, followed by site Osprey and then site Kingfisher. Observations for this phase concluded with site Nightingale. The spacecraft performed these observations during October 2019 at a distance of approximately 0.6 miles (1 km) from asteroid Bennu’s surface.
This mission phase helped the team assess which two sites were best suited for sample collection by thoroughly examining the sampleability, topography, albedo, and color of each site. During Recon A, the spacecraft’s Natural Feature Tracking (NFT) autonomous navigation system began initial calibration. NFT will be used for navigating to the asteroid’s surface during the sample collection event.
During Orbital R phase, the OSIRIS-REx spacecraft flew a terminator orbit approximately 0.9 miles (1.4 km) above asteroid Bennu’s surface from November 2019 through early January 2020. This orbit is designated “Orbit R” because it is a sub-phase of the larger Reconnaissance phase.
During Reconnaissance B phase, the OSIRIS-REx spacecraft performed two flyovers – one over the primary sample collection site Nightingale, and another over the backup site Osprey. The spacecraft observed the two sample collection sites from an altitude of approximately 0.4 miles (625 m), which was closer than the previous Reconnaissance A flyovers (approximately 0.6 miles/1 km). When the spacecraft was not performing flyovers, it spent its time orbiting Bennu at a distance of 0.9 miles (1.4 km).
A primary goal of this mission phase was to collect the imagery required to complete the spacecraft’s Natural Feature Tracking image catalogue. The team used these images to create a “footprint” of each site’s topographical makeup. During the sampling event, this footprint will inform the spacecraft of the surface features particular to the sample collection site so that it can accurately target Bennu’s surface.
During Reconnaissance C phase, the OSIRIS-REx spacecraft performs two flyovers – one of the primary sample collection site Nightingale, and another of the backup site Osprey. The spacecraft will observe the two sample collection sites from an altitude of approximately 820 ft (250 m), which is closer than the previous Reconnaissance B flyovers (approximately 0.4 miles/625 m).
The purpose of this mission phase is to collect high-resolution images of the two sites. This imagery is crucial for identifying which regions within each site contain an abundance of fine-grained, sampleable material.
Because sample collection is a critical event, the mission planned two rehearsals prior to final execution. In April 2020, OSIRIS-REx performed its first rehearsal. The spacecraft practiced leaving its orbit, maneuvering to a pre-defined Checkpoint located 410 ft (125 m) above the sample site, and then returned to orbit. In August 2020, the second rehearsal will take the spacecraft from orbit through the Checkpoint burn to the Matchpoint burn, where it will approach – but not touch – the sampling location before returning to orbit. During each rehearsal, the spacecraft collects and analyzes tracking data 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.