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Hayabusa2/LIDAR Laser Link Experiment Helps Calibrate Spacecraft


 We successfully conducted a laser link experiment between the laser altimeter (known as LIDAR: light detection and ranging) aboard the Hayabusa2 asteroid probe spacecraft and the Earth in December 2015. The important points of this experiment as are follows:

 

1. We confirmed that the LIDAR detected laser pulses from the ground-based laser station at Mt. Stromlo Observatory in Canberra, Australia, by scanning the spacecraft attitude (pointing) to change the LIDAR field-of-view direction. By using this result, we precisely determined the field-of-view direction with respect to the spacecraft frame.

 

2. The LIDAR is equipped with an observation mode in which it can time the interval between two pulses. As the interval between when the pulses were launched from the ground can be determined precisely, we also successfully demonstrated the technology to calibrate the clock frequency of the LIDAR by using the time interval of these pulses. In this experiment, the frequency offset was confirmed to be an appropriate value for the crystal oscillator which is used in the LIDAR.

 

3. The longest distance to the spacecraft during the successful laser link was 6.6 million km. Hayabusa2 became only 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.

 

 

Conceptual diagram of the experiment (not to scale): Ground-based laser stations shot lasers toward the spacecraft. Detection of laser pulses from the Australian station by the LIDAR was confirmed in December 2015. (The Tokyo NICT station also fired laser pulses, but cloudy weather interfered.) The longest distance was approximately 6.6 million km, or 22 light seconds. This distance was the farthest ever for a Japanese mission, and the third-most distant experiment for any nation.

[Top Inset]: LIDAR field-of-view determination data. The reader can verify that the alignment is offset slightly from the spacecraft frame.

[Left Inset]: Received laser intensity on the final day of the experiment.

Credit: NAOJ/NICT/JAXA/CIT/SERC/NEC, Noda et al.

 

 

Asteroids, small rocky bodies measuring meters to kilometers in size, are fascinating in their own right, but astronomers believe they may also hold important clues to how the planets and the Solar System formed. This makes them an important topic in astronomy. Because they are small, it is very difficult to observe them precisely with ground-based telescopes, so researchers use probe missions to get close to an asteroid, and in some cases even attempt to bring back samples for analysis in laboratories on Earth.

 

Hayabusa2 is Japan’s second asteroid sample return mission. The first Hayabusa mission successfully returned to Earth on June 13, 2010 after visiting the asteroid Itokawa. The improved Hayabusa2 was launched on December 3, 2014 and is scheduled to visit the asteroid 162173 Ryugu during June-July of 2018.

 

Among Hayabusa2’s instruments the LIDAR is particularly important. Not only will the LIDAR play a role in determining the surface features and size of the asteroid, its altitude readings are critical for helping Hayabusa2 touch down to collect samples.

 

Although the LIDAR was aligned and calibrated in the laboratory, it is possible that the vibrations during launch and the harsh environment of outer space might have effects on it. Therefore inflight calibration is important to achieve maximum accuracy.

 

This paper is regarded as an engineering paper rather than a scientific one. However, confirming the instrument performance is very important for the upcoming scientific observations. Part of that confirmation work was accomplished by this experiment, for example Point 1 mentioned above. It is also important that the LIDAR detected real laser pulses for the first time since the spacecraft was launched. Like laser range finders used on Earth, normally the 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.

   

The LIDAR will not be turned back on until the arrival of the spacecraft at the target asteroid, 162713 Ryugu, in the summer of 2018. After arrival, it is expected that full scientific observation will be done and many science results will be reported. Please stay tuned!

 

These results were published online in the journal “Earth, Planets, and Space” on January 3, 2017 as “Laser Link Experiment with the Hayabusa2 Laser Altimeter for In-flight Alignment Measurement” by Noda et al. This paper is open access, meaning that anyone can read it. The PDF file is available at the following link:

http://earth-planets-space.springeropen.com/articles/10.1186/s40623-016-0589-8

(text by Hirotomo Noda)