In a sign of cosmic communications to come, last week mission controllers sent signals to a Mars-orbiting European spacecraft, which relayed the instructions to NASA's Spirit rover on the surface, and a signal was returned to Earth back along the same path.
It was the initial transmission across what could be called the first-generation Interplanetary International Internet.
"We have an international interplanetary communications network established at Mars," said Jennifer Trosper, Spirit mission manager at NASA's Jet Propulsion Laboratory.
NASA has used its own orbiters to communicate with surface probes at Mars before, but the Feb. 6 "pioneering demonstration," as officials called it, was the first to involve multiple nations.
The Mars Express orbiter, which relayed the signals, is a project of the 15-nation European Space Agency. It reached Mars late last month and has been returning photographs of the planet from above and recently added to the case for ancient rivers on Mars.
"This is the first time we have had an in-orbit communication between ESA and NASA spacecraft, and also the first working international communications network around another planet," said Rudolf Schmidt, ESA's project manager for Mars Express.
The planned demonstration is part of an ongoing effort between the two agencies to cooperate, including using joint communication assets, according to a NASA statement released today.
Today's solar system network is crude and limited to what some envision. Other plans are underway for eventually creating a true Interplanetary Internet that is more like the one you're using to read this story.
The IPN, as it's been dubbed, might one day serve as a backbone connecting hubs at various planets to spaceships. If built -- NASA is among the institutions researching the idea -- it could overcome a drawback to the current system: Spacecraft can only communicate with Earth via line-of-sight. That means a craft on the far side of Mars can't send signals to Earth.
The IPN would behave more like e-mail. Data could be stored at any of various hubs around the solar system and transmitted to their destination via the best path at the moment.
The capability will be more important in the future when more spacecraft, with more complex tasks, reach the red planet, perhaps including human missions.
The terrestrial portion of the test communication last week was routed from JPL in Pasadena, Calif, to ESA's European Space Operations Center in Darmstadt, Germany. Spirit returned basic telemetry data in the trial effort.
"This is excellent news," said JPL's Richard Horttor, project manager for NASA's role in Mars Express. "The communication sessions between Mars Express and Spirit were pristine. Not a single bit of data was missing or added, and there were no duplications."
Spirit and its twin rover, Opportunity, normally get communications support from NASA's Mars Odyssey and Mars Global Surveyor orbiters. Each rover can also communicate directly with Earth. All these signals originating on or near Mars are received by NASA's Deep Space Network of three radio receivers, in California, Spain and Australia.
Radio signals take several minutes, travelling at the speed of light, to traverse the void between the two planets. Most of the communications are made in what engineers call the X-band, high-frequency waves that are much higher than those received by an FM radio.