All but lost in the recent news of the Stardust sample return and New Horizons launch, the Messenger spacecraft continues on its journey to Mercury. And significant science has already occurred, particularly the laser link set up across a record 24 million kilometers (15 million miles) between the spacecraft and Earth. Laser communications with spacecraft are still in their infancy, but this test showed the potential of moving past microwaves into far more effective laser channels.
The beauty of a laser signal is that it spreads much more slowly than conventional radio signals, a huge factor given the need to return significant data at maximum speed. Consider this: the Mars Pathfinder mission returned a radio signal that spread to hundreds of times the diameter of the Earth by the time it reached us. The 23-watt signal of the distant Voyagers broadcasts a beam now a thousand times the Earth’s diameter. These numbers play havoc with signal strength. Voyager’s signal to Earth during the Neptune encounter was twenty billion times less than the power needed to operate a digital wristwatch.
Now ponder that a probe circling Centauri A or B would return a radio signal with 1/81.000.000 the energy that Voyager returned from Neptune. No wonder we have to look to laser methods as we contemplate missions into nearby interstellar space. At that, the Messenger signals were primarily designed to test the spacecraft’s laser altimeter, but they offered a fine proof of concept. From 24 million kilometers, the laser pulses had expanded to roughly 1900 kilometers. That tighter signal will one day allow higher data rates, though it also demands refinements in the precision of spacecraft tracking.
A key paper for those interested in the application of lasers to interstellar communications is James Lesh, C. J. Ruggier, and R. J. Cesarone, “Space Communications Technologies for Interstellar Missions,” Journal of the British Interplanetary Society 49 (1996): 7–14. Also be aware of Alex Harwit, Martin Harwit, and Joss Bland-Hawthorn, “Laser Telemetry from Space,” Science 297, no. 5581 (July 26, 2002): 523, where the theoretical case for lasers is developed.
The Messenger tests are analyzed in David E. Smith, Maria T. Zuber et al., “Two-Way Laser Link over Interplanetary Distance,” Science Vol. 311 No. 5757 (January 2006), p. 53. The abstract is here.