Astrobotic Engineers Radiation-Tolerant Flight Computing
Astrobotic’s first mission will land near a pit in the Moon’s Lacus Mortis region and demonstrate unprecedented landing precision. Astrobotic’s landing technology registers high-quality, real-time camera images taken during the lander’s descent with terrain maps derived from orbital imagery to achieve safe landing at a precise destination. This capability enables Astrobotic’s Griffin lander to fly directly over the pit, capturing close-up views of the floor and walls that cannot be seen from orbit or the ground, and then to land within 100m of the pit. Successful performance of the landing software relies on Griffin’s onboard flight computing. Beyond Earth’s magnetic fields, the lunar surface may experience solar particle events and galactic cosmic rays (GCR). Solar particle events are relatively rare, so the Griffin lander will encounter primarily GCR from the radiation background of space. The GCR spectrum consists mostly of protons (85%) and alpha particles (helium nuclei) (14%) at extremely high energies. These high-energy particles pose a risk to onboard flight electronics. All electronic components are susceptible to damage or malfunctions caused by ionizing radiation (i.e., particle radiation and high-energy electromagnetic radiation). While the effects of ionizing radiation are usually transient, they can cause soft errors (i.e., “bit flips”, where the binary state of a bit is changed by physical deposition of a particle’s charge onto the component). These bit flips can lead to destruction of the device by triggering other damage mechanisms. For this reason, space hardware has traditionally been radiation hardened, meaning that it has been made resistant to the damaging effects of ionizing radiation. Unfortunately, radiation-hardened components perform at speeds that are a fraction of their terrestrial counterparts. For Astrobotic’s landing technology to perform the task of guiding Griffin to a precise location and avoiding hazards to land safely, Griffin’s onboard computing must run at near-terrestrial speed. The most common method for radiation mitigation for space applications is simple material shielding around the sensitive electronics. Shielding is an effective technique to minimize the total radiation dose that the electronics will encounter during the mission. For example, 2.5 millimeters of aluminum shielding will attenuate large amounts of proton radiation and reduce the total radiation dose well below the damaging limit for most commercial-grade electronics. As an additional step to mitigate faults caused by radiation, error-correcting computer code can be implemented to check for faulty data. Without this backup check, computers might send the wrong output or cause system malfunctions. Even electronics on Earth can experience radiation effects, and many commercial grade chips use error-correcting code. Astrobotic was recently selected for the Google Lunar XPRIZE Milestone Prizes for Landing, Mobility, and Imaging. One Landing milestone is to develop a flight computer that can perform in the radiation environment of space. The computer will use terrestrial grade components for computing-intensive functions and slower radiation-hardened components to mitigate radiation-induced errors. Proper shielding reduces these errors to a minimum and ensures that the Griffin will land successfully.