Here’s an interesting observation from Joss Bland-Hawthorn, who is head of instrument science at the Anglo-Australian Observatory in Sydney: “Astronomers are losing vast amounts of data from recent satellite missions to Mars. We collect a hundred times more than we can transmit back.”
The comment appears in the current issue of New Scientist, in an article by Maggie McKee called “Mars Laser Will Beam Super-fast Data.” And the problem identified is one that will plague us more and more the farther we get from Earth. Radio signals are inherently less efficient than lasers, and not only because shorter wavelengths can carry more information in the same unit of time. A laser signal transmitted from a Mars orbiter, says New Scientist, will only spread to a width of a few hundred kilometers by the time it reaches the Earth. A radio signal, by contrast, diffuses rapidly with distance.
How rapidly? Well, JPL’s James Lesh told me in a telephone interview last year that the Mars Pathfinder mission returned a signal that had spread to hundreds of times the diameter of the Earth by the time it arrived. And because the data rate depends on the power of the signal, getting good information out of the ether can be quite a challenge. For an Alpha Centauri probe, the numbers become staggering: a radio signal from such a probe would reach Earth with 81 million times weaker a signal than the one the Voyager probe sent back from Neptune.
The good news is that the Mars Telecommunications Orbiter, to be launched in 2009, will include a laser communications package along with its more traditional radio equipment. The five-meter Hale Telescope in California and a still to be constructed array of smaller telescopes will be used to detect the Mars signals. Lasers are the future when it comes to communicating with the outer Solar System, not to mention the nearest star.
An excellent reference here is Lesh’s “Space Communications Technologies for Interstellar Missions,” Journal of the British Interplanetary Society 49 (1996): 7-14, which was written with C. J. Ruggier, and R. J. Cesarone.
And a final thought: why isn’t a bibliography of so essential a journal as JBIS available online? One of my great hopes for the Interstellar Flight Foundation is that it may be able to reintroduce an interstellar bibliography of the kind that Robert Forward and Eugene Mallove once edited.
