Planetarium

"I saw recently that the Voyager Probe encountered the Heliosphere. How long can it keep traveling before it runs out of fuel?"

 

The Voyager I and II probes were deployed in the late 70's to study and photograph the worlds in the outer solar system. They completed their planetary sojourns in the 80's and are continuing through the solar system's boundaries, defined, by some, as the heliosphere: the region where the solar wind is repelled by the interstellar material.

Assuming they don't encounter anything "solid," the Voyager probes will continue to travel indefinitely. In fact, the Voyager probes could move through intergalactic space for billions of years, barring any unforeseen encounter. Your question is predicated on the notion that all travel requires a constant fuel injection. (At least that it what I think it meant. My apologies if it didn't.) On our planet, motorists need to keep pressing down on the accelerator because of friction between the wheels and road and air resistance: both factors act on the cars to impede their motion, hence the necessity of constant acceleration.

The Voyager probes, moving through space, are not so impeded. The Voyager I probe, for instance, moves at 11 miles per second through the void because there is no frictional forces to retard its motion. This is all based on Newton's law, "A particle in constant motion will continue in that motion unless acted upon by an external force." Frictional forces act on Earth's moving objects. Now, the Voyager probe will encounter interstellar dust and gas, though in the rarefied region of interstellar space. Interstellar gas density is about 1 atom per cubic centimeter. However, the probe's mass is 753 kilograms: far more massive than any of these atoms. Cumulatively, over millions of years, the probe will slow down measurably as a consequence of repeated collisions, but will continue to travel inexorably through the galaxy. Of course, there is a possibility, albeit remote, that it will venture close enough to a star system or a renegade planet (those unattached to star systems) to be gravitationally captured. We know that in about 40,000 years, the Voyager I probe will pass to within about 1.5 light years of Giese 445: not close enough to be ensnared, of course. Might the Voyager's be stopped someday? We cannot know, but they will have to be captured gravitationally. They won't stop on their own accord.