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... JPL reported Wednesday that the maneuver, completed in March, restarted Voyager 1's primary roll thrusters, which are used to keep the spacecraft aligned with a tracking star. That guide star helps keep its high-gain antenna aimed at Earth, now over 15.6 billion miles (25 billion kilometers) away, and far beyond the reach of any telescope.

Those primary roll thrusters stopped working in 2004 after a pair of internal heaters lost power. Voyager engineers long believed they were broken and unfixable. The backup roll thrusters in use are now at risk due to residue buildup in their fuel lines, which could cause failure as early as this fall.

Without roll thrusters, Voyager 1 would lose its ability to stay properly oriented and eventually drift out of contact. To make matters worse, the only antenna on Earth with enough power to send commands to the Voyager probes -- the 230 foot (70 meter) wide DSS-43 dish in Australia -- is undergoing an upgrade shutdown until next February, with only a couple of brief operational windows in August and December.

While other dishes around the world can still receive data from the Voyagers, those windows are the only opportunities to send commands to the spacecraft, which remain the most distant human-made objects in existence. ...