An artist's concept of Pioneer 10. Photo: NASA

In a possibly last-ditch attempt to try to contact Pioneer 10, ground controllers successfully received the deep-space probe's faint radio signal Saturday, washing away fears that the craft had been forever lost.

The signal was received at a tracking station in Madrid, Spain, at 1727:30 GMT (1:27:30 p.m. EDT).

Pioneer 10 was last heard from in real time last July, while the last radio signal of any kind came in August. Officials were able to obtain five one-way communications sessions and three more opportunities of the two-way variety in early March.

Saturday's contact came during a two-way session, where controllers uplink commands to the spacecraft to lock on with a stable downlink signal. Engineers concluded that Pioneer 10 is currently in a mode where it will only communicate with ground controllers if a message is sent to the probe.

Pioneer's chief flight controller Ric Campo summed up the problem: "In order [for Pioneer 10] to talk to us, we need to talk to it."

This means that for controllers to communicate with Pioneer 10 in the future, they must reserve extra capabilities with the Deep Space Network (DSN). The DSN also supports more important missions such as Galileo, Cassini and Ulysses, making future communications more difficult to obtain and limiting the tracking capabilities for Pioneer 10.

The DSN is a collection of large dish-like antennas with installations in Goldstone, California, Canberra, Australia, and Madrid, Spain.

Currently, Pioneer 10 is 7.29 billion miles from Earth and its round-trip light time is 21 hours, 45 minutes. Its speed relative to the Sun is 27,380 miles per hour.

Pioneer 10 was officially retired March 31, 1997. The craft now serves as a training tool for ground controllers and its faint signal provides a radio beacon used by DSN and other facilities to confirm station tracking and receiver performance.