OLA: turn a light-travel time into a landscape

Canada contributed the OSIRIS-REx Laser Altimeter, or OLA, to NASA’s asteroid sample-return mission. The instrument measured Bennu’s surface with laser pulses to help construct a three-dimensional model. Mapping supported the choice of a sampling location by describing slopes and other surface features. This was a contribution to the wider mission, distinct from the mechanism that actually collected material.

The OSIRIS-REx launch photograph shows the mission leaving Earth on 8 September 2016, with its spacecraft enclosed inside the launch vehicle. It cannot show OLA’s optical components or Bennu’s terrain. Those questions require different evidence. Matching an image to the stage of a mission helps keep a dramatic launch scene from standing in for measurements that were made later at the asteroid.

A laser pulse travels from the instrument to a surface and then back to the receiver. In a simplified example, suppose the round-trip time is ten microseconds. Using 300 metres per microsecond for light speed, the total traveled distance is 3,000 metres. The one-way range is half of that: 1,500 metres. Forgetting the return journey would double the inferred distance.

That single range is not yet a complete map. The team also needs the direction of the measurement and the spacecraft’s position and orientation. Repeated observations from different viewing conditions can then be placed in a common reference frame. Our calculation neglects detailed timing calibration — open definition and geometry corrections; it demonstrates why a time measurement and a position model must work together.

A reflected pulse makes a round trip

An invented landing-site comparison makes the decision clearer. Site A has scientifically interesting material but lies beside a steep slope and large obstacles. Site B has less dramatic terrain and still addresses the central scientific question. Selecting a site requires comparing access, hazards and scientific value. The largest number of laser measurements does not automatically make one site the best choice.

OLA’s measurements helped the actual team evaluate candidate sites before the Nightingale collection in October 2020. Canada’s instrument contribution also secured a share of the material returned to Earth in 2023. Laboratory analysis and surface mapping answer related questions at very different scales. A sample’s meaning depends partly on knowing where it came from, so the mission’s mapping record remains valuable after the return capsule has landed.