In a breakthrough new experiment, scientists working with NASA's Lunar Reconnaissance Orbiter (LRO) have used a high-power laser to transmit an electronic copy of da Vinci's famous painting. This represents the first laser communication with a spacecraft not in orbit about the Earth.
The LRO already makes unique use of laser systems. It's the first interplanetary mission to be tracked by a large laser mounted at the Goddard Space Flight Center. For other missions, both tracking and communication are handled by radio dishes mounted on the spacecraft and pointed back at the Earth. LRO is the first to be tracked by laser, and the upcoming Lunar Atmosphere and Dust Environment Explorer (LADEE) will be the first to contain an experimental laser communication system.
Communicating with a laser beam works by timing the pulses sent from Earth. The time between each pair of pulses indicates the value of a bit of data, in this case the brightness of a pixel of the Mona Lisa. The computer onboard the LRO then converts this information back into the picture, which is transmitted to Earth via the radio dish. Unfortunately, even on a calm, clear day, the atmosphere interferes with the precise timing and transmission of these pulses. To correct for this, the computer applies algorithms similar to those which correct for smudges and scratches on DVDs.
Right now, laser communication is slower and less reliable than traditional radio signals, but engineers are hopeful that this will change. In the future, laser systems could transmit back the vast quantities of data collected by space probes more efficiently than is possible today. In the interim, laser systems might serve as a valuable backup communication method in the case of trouble with the main antenna.