One of NASA's Mars rovers has sent pictures relayed by the European Space Agency's Mars Express orbiter for the first time, demonstrating that the orbiter could serve as a communications link if needed.

The link-up was part of a set of interplanetary networking demonstrations paving the way for future Mars missions to rely on these networking capabilities. The American and European agencies planned them as part of continuing efforts to cooperate in space exploration.

On Aug. 4, as Mars Express flew over NASA's Mars Exploration Rover Opportunity, the orbiter received data previously collected and stored by the rover. The data, including 15 images from the rover's nine cameras, were subsequently forwarded to the European Space Operations Centre, Darmstadt, Germany, and immediately relayed to the rover team based at NASA's Jet Propulsion Laboratory, Pasadena, Calif.

Mars Express also is demonstrating two other networking modes with Opportunity and the twin rover, Spirit, between Aug. 3 and Aug. 13.

Two NASA orbiters, Mars Odyssey and Mars Global Surveyor, have relayed most data the rovers have produced since they landed in January. In addition, Mars Express demonstrated communication compatibility with the rovers in February, but at a low rate that did not convey much data. The Aug. 4 session, receiving 42.6 megabits from Opportunity in about 6 minutes, set a new mark for international networking at another planet.

The ability for a European mission and a NASA mission to communicate with each other at Mars required years of groundwork on Earth, said JPL engineer Peter Shames. Both Mars Express and the rovers use a communications protocol called Proximity-1. The Consultative Committee for Space Data Systems, an international partnership for standardizing techniques used for handling space data, developed Proximity-1.

Mars Express was 1,400 kilometers (870 miles) above Mars' surface during the Aug. 4 session with Opportunity. In that session, the goal was reliable transfer of lots of data. The orbiter was about four times higher when it listened to Spirit on Aug. 3 and Aug. 6. Those sessions demonstrated a mode useful for catching fewer bits of a weaker signal during critical events, such as a landing or an approach to another craft for a rendezvous in orbit. The final session of the series, scheduled for Aug. 13 with Opportunity, was to demonstrate a mode for gaining navigational information from the Doppler shift in the radio signal.

"Establishing a reliable communication network around Mars or other planets is crucial for future exploration missions," said Con McCarthy of the European Space Agency's Mars Express project. "This will allow ESA and NASA to more accurately track spacecraft during their approach, atmospheric entry and even descent, as well as to increase the coverage and the amount of data that can be brought back to Earth."

"We're delighted how well this has been working, and thankful to have Mars Express in orbit," said JPL's Richard Horttor, project manager for NASA's role in Mars Express.

JPL engineer Gary Noreen of the Mars Network Office said, "The capabilities that our international teamwork is advancing this month could be important in future exploration of Mars."

JPL's Dr. Mark Adler, Mars Exploration Rover mission manager, said, "A lot of the people who have worked on the Mars Exploration Rovers will be working on future Mars missions or have already switched to working on future missions, so we are very glad to see these capabilities demonstrated."

JPL, a division of the California Institute of Technology in Pasadena, manages the Mars Exploration Rover Project for NASA's Science Mission Directorate, Washington.