Using their new lunar laser link, NASA and MIT have succeeded in beaming data from the Moon at 622 megabits per second. This impressive feat is an important step in moving away from radio communications, which have reached the limit in the face of the new data demands for work in space. In shedding the need for large antennas, communications systems can be designed around the spacecraft, rather than having to design spacecraft that can accommodate their communications.
The Lunar Laser Communications Demonstration (LLCD) relies on the Lunar Atmosphere and Dust Environment Explorer (LADEE) spacecraft for sending data between Earth and the Moon. The 622 Mbps is a maximum speed for the downlink from lunar orbit, which is achievable only when the Moon is high in the sky on Earth. When the Moon is low, atmospheric effects like scattering and absorption reduce the rate to half that. The uplink, from a ground station at White Sands, New Mexico, was a mere 20 Mbps.
The longer term plan for the LLCD is to use communications satellites to bounce transmissions between ground stations at 1.25 gigabits per second. That number happens to be pretty close to the 1.26-second delay that a light transmission between Earth and Moon would incur. Communication over long distances imposes a few constraints on what kinds of protocols might be used for an interplanetary internet. So-called delay-tolerant networking, or DTN, has emerged as an approach to handle the problems that typical routing protocols face when end-to-end paths can not be instantaneously established.
Routing protocols therefore must take a “store and forward” approach, where data is incrementally moved and stored through network nodes. The difficulty in the initial setup of space networks is obviously the that nodes are sparse. The nodes that do exist will therefore require substantial storage capacity and bandwidth, along with flexibility and creativity in scheduling. One store and forward method of routing is the known as the bundle protocol. These protocols can easily cope with intermittent connectivity issues, or periods of reduced transmission, like when the Moon is low in sky.
The directed nature of laser communications has built-in security that more diffuse RF transmissions cannot match. Many forward thinkers, including Steven Hawking, have noted that security is an issue that extends past our terrestrial environment, to any as-yet-unknown recipients from beyond. Predicting whether a remote intelligence is friendly or hostile is as difficult as it is for us to predict own true nature in the face of meeting one.
On the other hand, there are the extremely optimistic luminaries like Craig Venter, sequencer of the first human genome (his own). Venter notes in his new book, “Life at the speed of Light,” that right now his sequence is streaming out from our planet as a radio transmission with an expanding front that will eventually reach the far reaches of the galaxy. Furthermore, he envisions the most important data that the interplanetary network might send are commands for robots capable of sequencing and synthesizing, to print new life — and beam back the sequences of any they might find.