High art recently met high tech as NASA’s Lunar Reconnaissance Orbiter (LRO) received an image of the Mona Lisa via laser. Traveling about 240,000 miles (386,000 km), the image was sent to the probe in lunar orbit using a laser beamed from NASA’s Next Generation Satellite Laser Ranging (NGSLR) station at NASA's Goddard Space Flight Center in Greenbelt, Maryland as a demonstration of lasers as a deep-space communications tool.
Painted by Leonardo da Vinci between 1503 and 1519, the Mona Lisa is one of the most famous images in the world. The enigmatic portrait has been reproduced and parodied countless times over the centuries, so it was a logical choice for the demonstration. It was received by the Lunar Orbiter Laser Altimeter (LOLA) aboard the LRO, which was launched June 18, 2009. The LOLA uses lasers to map the topography of the Moon as the LRO travels in a polar orbit at an altitude of 50 kilometers (31 miles) above the lunar surface.
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The LOLA routinely receives laser signals from Earth as NASA tracks its position, and it’s the only satellite outside of Earth orbit to be tracked this way. For the demonstration, the image of the Mona Lisa was broken into an array of 152 pixels by 200 pixels. These were converted into shades of gray by assigning each a number from zero to 4,095. The laser was fired in pulses with the length of each pulse corresponding to one of the 4,096 numbers, at a rate of 300 bits per second. These were received by the LOLA and reconstructed based on the timing of the pulses.
Once completed, the image was then re-transmitted to Earth by radio telemetry. Turbulence in the Earth’s atmosphere degraded some of the information, resulting in a spotty version of the famous painting, but the team applied Reed-Solomon coding, which is an error-correction algorithm commonly used in CDs and DVDs, to correct it.
Image as sent through the Earth's atmosphere and after applying the Reed-Solomon error correction (Image: Xiaoli Sun, NASA Goddard)
"This is the first time anyone has achieved one-way laser communication at planetary distances," said LOLA's principal investigator, David Smith of the Massachusetts Institute of Technology. "In the near future, this type of simple laser communication might serve as a backup for the radio communication that satellites use. In the more distant future, it may allow communication at higher data rates than present radio links can provide."
This demonstration is only the first step in a more ambitious program. According to the LRO deputy project scientist, Richard Vondrak, "This pathfinding achievement sets the stage for the Lunar Laser Communications Demonstration (LLCD), a high data rate laser-communication demonstrations that will be a central feature of NASA's next moon mission, the Lunar Atmosphere and Dust Environment Explorer (LADEE)."
After LLCD, NASA plans the Laser Communications Relay Demonstration (LCRD), which will be NASA's first long-duration optical communications mission with the aim of developing the system for deep space communications.
The video below outlines the laser transmission operation.