Researchers successfully established a laser link with the LIDAR instrument on the Japanese asteroid probe spacecraft Hayabusa2 after its Earth swing-by in the winter of 2015. This is the third time in the world that such a link was established with a spacecraft beyond the Earth-Moon distance. The researchers used the laser link to determine the alignment of the LIDAR receiving telescope and calibrate its internal clock. These engineering improvements will allow theLIDAR to take better scientific data once Hayabusa2 reaches its destination, the asteroid 162173 Ryugu.
Like laser range finders used on Earth, normally Hayabusa2’s laser altimeter (known as Light Detection And Ranging, or LIDAR ) emits a short laser pulse and then waits for reflected laser light to come back to it. But in this experiment we instead had the LIDAR watch for laser pulses fired from Earth. We confirmed that the LIDAR detected laser pulses from the ground-based laser station at Mt. Stromlo Observatory in Canberra, Australia. We checked the alignment of the LIDAR receiving telescope by having the spacecraft change its attitude (pointing) little by little and seeing at what angle it could best detect the laser light coming from Earth. In this way, we were able to precisely determine the direction of the LIDAR field-of-view relative to the body of the spacecraft. We found that the field-of-view was in fact tilted by roughly 3.5 milli-radian (~0.2°) with respect to the spacecraft frame.
In addition, the LIDAR is equipped with an observation mode in which it can time the interval between two pulses. By comparing the interval recorded by Hayabusa2 with the actual known interval between when the pulses were sent, we also succeeded in calibrating the onboard clock frequency.
The longest distance to the spacecraft during the successful laser link was 6.6 million km. Hayabusa2 became the third example of a long-distance laser link experiment farther than the Earth-Moon distance (ca. 380 thousand km), following the MESSENGER mission to Mercury and the Mars Global Surveyor of the United States.
These results were published online on January 3, 2017 in the Earth, Planets, and Space as “Laser Link Experiment with the Hayabusa2 Laser Altimeter for In-flight Alignment Measurement” by Noda et al.
More detailed information is provided on the following site:
NAOJ/RISE project website: Hayabusa2/LIDAR Laser Link Experiment Helps Calibrate Spacecraft
Jan. 10, 2017
Hirotomo Noda, Hayabusa2 LIDAR team