The Mercury laser altimeter to be launched by ESA has been installed on the Mercury Planetary Orbiter (MPO). Developed by the European team led by the University of Bern in Switzerland, the Colombo Laser Altimeter (BELA) is part of the Colombo Mercury mission and was launched in April 2018. This is the first implementation of a laser pointer gauge for interplanetary flight in Europe.
Coupling system, the instrument will measure the terrain of Mercury that will be detected on the Mercury planetary orbiter, and will be one of the two spacecraft in the planetary orbit, which is also part of Colombo’s mission. The Mercury Planetary Orbiter is manufactured by ESA, and the Mercury Magnetospheric Orbiter (MMO) is manufactured by Japan Aerospace Research and Development Agency (JAXA).
The purpose of the Colombo mission, Europe’s first Mercury probe program, is to provide information on the composition, geophysics, atmospheric information, Mercury magnetosphere and history of Mercury on the planet. The two spacecraft will travel to Mercury as part of a coupled system. They will reach Mercury in 2024, MMO will conduct research on the planet ’s magnetosphere through an orbit, which will be the closest point near Mercury ’s surface, up to a distance of 590 kilometers, and the Mercury planet orbiter will follow a more The near orbit is 480 kilometers, and the surface and internal structure of the planet are investigated.
BELA is one of eleven devices that Mercury ’s planetary orbiter will carry. The laser meter uses a direct detection method. This high-power green laser pointer is developed and designed by Cassidian Optroniks. This device will emit a 50 mJ pulse with a frequency of 10 Hz and a wavelength of 1064 nm. These will be reflected back from the surface of Mercury and received by the receiving telescope (RTL) after a delay of about 5 milliseconds. Then focus the image on the silicon avalanche photodiode-these are based on the photodiode of the Mars Universal Exploration Spacecraft and NASA’s Messenger spacecraft laser altimeter, which flew around Mars between 2011 and 2015. The signal from the photodiode is analyzed by an electronic module developed by the Swiss technology company RUAG to determine the time of flight (hence the range and altitude), the integrated pulse intensity and its width.
Mirror RTL is a double-sided reflective telescope, designed and developed by the company. The equipment is located inside the spacecraft, it needs to cope with the temperature change from minus 20 ° C to 45 ° C without deformation, and it needs to be lightweight. “We decided to build the telescope to provide thermal compensation entirely from beryllium,” said the Co-Principal Investigator of the Colombo Laser Altimeter. “The 20 cm diameter system weighs only 600 grams.” The optical surface is made of diamond-processed copper deposited on the element of beryllium.
Mercury is very close to the sun, and BELA will have to face intense sunlight and heat. RTL’s “baffle unit” can reflect 90% of the sunlight to protect the equipment. “If we have a traditional black baffle, not only will the temperature reach 450 ° C, but we inject more than 300 W of thermal energy into the spacecraft,” Thomas explained. “The outer ring (ceramic) still reaches 200 ° C, which is the worst case, but now 30W of thermal energy is dumped into the spacecraft.” Some small protection modules of the laser are also developed by the DLR of the Berlin Aerospace Center in Germany.
Feel the thermal energy, deal with the sunlight reflected by Mercury, all the optical systems, the light transmitted from the telescope to the photodiode and the red laser pointer are equipped with interference filters. The filter of the optical system isolates light at a laser wavelength of 1064 nanometers, and the filtered laser is light that prevents the system from reaching a dangerous level.