It is not everyday that we get to hear about missions to asteroids and that’s the reason behind recent excitement in the world of astronomy as US space agency NASA has confirmed that its OSIRIS-REx asteroid sample return mission is a go and slated for launch next month.
One key factor that will play a major role in the success of the mission is the ability to determine from where the sample will be extracted. To do this the spacecraft and mission scientists on Earth would need to know the topography of the asteroid.
This task will be carried out by a robotic cartographer dubbed OSIRIS-REx Laser Altimeter, or OLA. The instrument has been developed by the Canadian Space Agency (CSA) and according to the space agency, OLA will create three-dimensional global topographic maps of Bennu and local maps of candidate sample sites.
Scientists at CSA explain that OLA’s light detection and ranging (LIDAR) has been entrusted with an important task. LIDAR is similar to radar, but uses light instead of radio waves to measure distance. OLA will emit infrared laser pulses toward the surface of Bennu as the spacecraft moves around the asteroid. The laser pulses reflect back from the surface to a detector. The team will measure the time difference between outgoing and incoming pulses to calculate the distance between the spacecraft and Bennu.
LIDAR has been used on prior spacecraft, including the Mars Global Surveyor and the Lunar Reconnaissance Orbiter. Those laser altimeters are fixed to the spacecraft, meaning that the laser pulse will only travel in the direction that the spacecraft is pointing. This can limit the coverage and spatial resolution of their topographic maps. So, while they have generated a vast amount of data, fixed LIDAR are not ideal for missions where the data must be gathered quickly.
“OLA is the first scanning LIDAR to fly on a planetary mission,” said Beau Bierhaus, an OLA team member at Lockheed Martin. “Because the LIDAR can articulate independently of the spacecraft, the LIDAR provides improved operational flexibility, and more importantly, much greater spatial coverage and resolution.”
OLA is expected to thoroughly map Bennu with about 6 billion measurements of the asteroid’s surface, which measures about one-third of a mile (one-half kilometer) in diameter. In comparison, the laser altimeter on the Lunar Reconnaissance Orbiter has received more than 6.8 billion measurements of the surface of the moon, which has a diameter of about 2,159 miles (3,500 kilometers).
The fundamental data of the asteroid’s shape and topography that OLA will provide are essential for several key phases during the mission.
The science team will use the high-resolution topographic data, in conjunction with camera images and on-board navigation algorithms, to navigate around the asteroid and guide the spacecraft to the selected sample site.
“We’re measuring topography down to one centimeter,” said Olivier Barnouin, the Altimetry Working Group lead at Johns Hopkins University Applied Physics Laboratory in Laurel, Maryland. “We’re looking at an asteroid at a scale that no other mission has before. We don’t want to be off in some unknown area during sample acquisition.”
The three-dimensional maps will also give geologic context to the returned asteroid sample. Just as geologists on Earth document where they collect their samples in the field on topographic maps, OLA will allow the science team to take their measurements and observations of the collected sample and apply them to their broader understanding of Bennu.
OLA will also allow the science team to study how regolith, or loose surface material, behaves in a microgravity environment. Scientists have done similar studies on the moon and Mars, but unlike Bennu, these bodies have relatively high gravity.
“What happens on asteroids is that you take that gravity dial and turn it way down,” Bierhaus said. “The dynamics of how regolith moves on the surface of the asteroid are foreign to us. OLA data will give us a greater understanding of how granular material behaves in space.”
This understanding is especially important for future asteroid missions. Scientists will need to know how regolith behaves in micro-gravity environments if we want to send astronauts to an asteroid someday to collect samples.