o reach a given phone number from outside the United States, callers must use an international dialing code, such as dialing 44 to reach the United Kingdom. In a similar move, certain NASA Web sites may soon be appending a ".earth" suffix to their Internet addresses in order to exchange mail and other data with computers in interplanetary space.
The Transmission Control Protocol/Internet Protocol is familiar as the underlying "language" of the Internet. (You're using it right now, to read this article.) TCP/IP was invented in 1973 to address the problems of the computer networks of that day, which were modeled on telephone networks meant for continuous, two-way voice communication. When computers began squirting small packets of binary numbers at each other at wildly varying rates, a new method was needed to permit numerous signals to share the same "wire"--actually a continuously optimized pathway from computer to computer to computer.
This design, first implemented in a U.S. experiment called ARPANET, provided a crucial feature: decentralization. With no "master" and no Achilles' heel, Internet protocols permit automatic rerouting of signals when a hardware link goes offline or becomes congested. This robustness has permitted TCP/IP to conquer virtually every corner of the Earth, and it has even seen some limited use in outer space for communication to and between satellites. But the protocol's inventors had no way to envision extraterrestrial applications, and so made a number of unfortunate assumptions:
- That the paths between network nodes are fixed
- That all signal delays are caused by congestion
- That communication between senders, receivers, way stations and support systems (e.g., the Domain Name Servers that let computers know how to find each other) is interactive, with delays of a few seconds or less.
For geosynchronous satellites, which hold a fixed position 22,000 miles (or about 0.1 light-seconds) above Earth's equator, these assumptions remain essentially valid, although radio noise and the speed of light begin to become problematic. But for more complex orbits, and particularly for deep-space probes and for constellations of spacecraft whose relative positions are always changing, the assumptions break down altogether.
The art of sharing
Why does this matter? Traditionally, to receive instructions and send back data, both manned and unmanned spacecraft have relied on two-way point-to-point radio--essentially the old telephone model of communications. This is fine when you have a single spacecraft to worry about, and a single ground-based transceiver (or transceiver network) to talk to it, and a dedicated radio frequency that no one else is using. But even deep space is filling up these days with all manner of yammering, bandwidth-hogging spacecraft, resulting in ever-greater inefficiencies and resource-scheduling headaches. As in the days of ARPANET, it's time to re-learn the art of sharing.
Nowhere is this more apparent than in NASA's ambitious Mars exploration program. Every two years, a new launch window opens up and a few more orbiters and landers are hurled at the Red Planet, with the goal of mapping the planet and its resources, and eventually setting up a complete robotic base equipped with "virtual scientists" and even fuel and air refineries for the human astronauts we hope to send. But data transmission rates are already abysmally slow--NASA's beloved Pathfinder mission had about 0.5 percent of the speed of a typical desktop modem--and the problem will only get worse as our probes get smaller, cheaper, and more numerous.
Mars Online
So beginning in 2003, America's Mars orbiters will be designed to serve as general-purpose communication relays, running a network scheme known as Space Communication Protocol, or SCP, which will pick up transmissions from other orbiters and from equipment on the Martian surface and rebroadcast them to designated sites on Earth, complete with the security and error-correction codes we Internet users take for granted.
Since Domain Name Servers for planet Earth are in short supply in outer space, Martian computers would have their own small "address books" servicing a ".mars" domain, and would transmit messages in a manner similar to e-mail, with the receiving address, say, a particular computer at the Jet Propulsion Laboratory, decoded by a terrestrial "gateway" such as NASA's Deep Space Network. So the TO: address for a particular data packet might read: colossus.jpl.nasa.gov@dsn.earth.
Pretty cool, eh?
The SCP, still very much in its infancy, already includes a File Protocol (SCS-FP), a Transport Protocol (SCS-TP), a Security Protocol (SCS-SP), and a Networking Protocol (SCS-NP), which are the cosmic equivalents of the FTP, TCP, ESP, and IP that form the Internet we know and love. But these new protocols have been designed with an eye toward slow, flaky, variable-geometry radio links operating over very large distances, where interactivity takes minutes or hours, assuming it's possible at all.
Interestingly, though, SCP's design committees have relied on a new networking assumption: that all the computers in the solar system can be synchronized to a single universal clock. Since this is true only if their relative velocities are very small fractions of the speed of light, it isn't hard to envision a future in which fusion-powered interplanetary clippers and antimatter-driven starships push an evolving Solarnet well past its breaking point.
So someday, we may see gateways in the ".sol" domain accepting relativistically dilated GalacTP transmissions from very distant, very speedy network nodes like enterprise.starfleet.mil@ufp. Won't that be a kick in the pants!
Wil McCarthy is a rocket guidance engineer, robot designer, science fiction
author and occasional aquanaut. He has contributed to three interplanetary
spacecraft, five communication and weather satellites, a line of
landmine-clearing robots, and some other "really cool stuff" he can't tell
us about. His short fiction has graced the pages of Analog,
Asimov's, SF Age and other major markets, and his
novel-length works include Aggressor Six, the New York Times Notable
Bloom, and upcoming The Collapsium.
