After almost 27 months of space travel, NASA's OSIRIS-REx probe will begin to operate around the asteroid Bennu this Monday, December 3. Until March 2021, the spacecraft will inspect and map Bennu, navigate near the asteroid and finally touch the surface for five seconds to obtain a sample. It will be then when you begin your journey back to Earth with your precious cargo.
After today's meeting, the ship will make a total of five passes over the north pole, the equator and the south pole at a distance of 7 kilometers. The main scientific objectives of this Preliminary Phase are to estimate the mass of Bennu, refine the asteroid's state of rotation model and generate a global model at a resolution of 75 centimeters.
Already in Orbital Phase A, the spacecraft will be placed in a gravity orbit around Bennu for the first time. There are no scientific requirements, as this phase is designed to provide the mission team with the experience of navigating close to a small body. The spacecraft will surround Bennu at a distance between 1.4 and 2 kilometers, and each orbit will last approximately 50 hours. This phase marks the closest a spacecraft has orbited around a small body.
During this phase, the navigation equipment it will go from star-based navigation to navigation based on reference points. The use of reference points, such as rocks and craters on the surface of Bennu, to determine the position of OSIRIS-REx will allow the navigation team to maneuver the spacecraft very accurately, which will be critical during the next phases of the mission.
The in-depth study of Bennu begins during the Detailed Study. OSIRIS-REx will make multiple passes around Bennu to produce the wide range of viewing angles needed to fully observe the asteroid. The ship will also use its OTES spectrometer to map the chemical composition of the entire Bennu surface. The images obtained during this phase will have a high enough resolution to produce digital terrain maps and global image mosaics for the proposed sample sites. The Bennu site will be examined in bulk and the sections will be classified as "safe" or "unsafe", with the results displayed on a hazard map.
In a second phase of the Detailed Study, the ship will make the necessary scientific observations to help the team concentrate on the best location in Bennu to collect a sample of regolith (loose surface material). To obtain this data, the ship will execute a series of turns between the north and south poles of Bennu while making observations from seven different stations on the equator. These data will be studied to understand the geology of Bennu. The spacecraft will also search for dust and gas.
The wide range of data products developed during this phase will be analyzed and combined to produce several maps. In the end, the team will have the necessary information to select up to 12 candidate sample sites. In addition, the team will map the global properties of the asteroid, achieving an important scientific objective of the mission.
At the end of the Detailed Study, the spacecraft will enter a close orbit (with a radius of 1 km) around Bennu and start the Orbital B phase, beating the record of the orbital phase A. New data collected will be used to evaluate the Potential sample sites for three key elements: safety, sampling capacity and scientific value.
During the reconnaissance phase, the ship will make a series of low-altitude reconnaissance observations of the two candidates from the final sample site. These observations, obtained from 225 meters above the surface, will show objects on the ground that are as small as 2 centimeters. The context images of the sites will also be taken during the upper passes at a height of 525 meters. Both sites will be studied in their entirety so that the team can begin planning immediately for sample collection at the backup site if necessary.
Because the collection of samples is a critical event, the mission has planned at least two trials before the final execution. In the first test, OSIRIS-REx will practice leaving its orbit, maneuvering to a predefined control point located 125 meters above the sample site, and then returning to orbit. The second test will take the spacecraft from the orbit to a coincidence point, where it will move over the sampling location before returning to orbit.
During each trial, the spacecraft will collect and analyze tracking data, LIDAR ranges, and OCAMS and TAGCAMS images so that the team can verify the performance of the flight system before the actual sample collection maneuver.
When the time is right, OSIRIS-REx will use the TAGSAM instrument to collect a sample of Bennu regolith. TAGSAM is an articulated arm in the spacecraft with a round head sample at the end. During the Touch-and-Go (TAG) maneuver, the sampler's head will extend towards Bennu, and the impulse of the slow and descending trajectory of the ship will push it against the surface of the asteroid for about five seconds, enough to get a sample. In contact, the nitrogen gas will blow over the surface to take the dust and small stones, which will then be captured in the head of TAGSAM.
After the spacecraft is activated to get away from Bennu, the mission team will measure the amount of sample collected by turning the spacecraft with the extended TAGSAM arm. Then they will compare the change in the inertia of the spacecraft with a previous turn of empty TAGSAM to ensure that enough sample has been collected. The ship has three nitrogen gas canisters on board, which allows three sampling attempts. Once it is determined that sample collection is successful, the TAGSAM head will be placed in the sample return capsule to return to Earth. After the successful stowage, the ship will be placed in a safe remote position from Bennu, where it will remain until the start of its return trip to Earth in March 2021.