The two Voyager spacecraft do actually have some propulsion on board – though you’re right that these thrusters are not where the majority of their forward motion is coming from. That outward speed comes from the combination of a high speed launch away from Earth, followed by a big gravitational slingshot past Jupiter. The thrusters which are on board are mostly for controlling the direction the spacecraft are pointing – they’d be very slow to increase the speed of the craft.
That said, it’s not like the spacecraft need much help speeding up – both Voyager 1 and Voyager 2 are moving at such a high speed that they will escape our solar system entirely. Voyager 1 is moving away from our solar system so fast that it could make it from the Sun to the Earth – a 93 million mile trip – in 3 months and a week.
Both spacecraft are slowing down, but this is because they’re still escaping the gravitational pull of our Sun. It takes a lot of energy to escape the solar system entirely, and these spacecraft are continually losing some of their speed to the pull of the Sun, which – even at the distance of these two craft – is still significant. So by the time the Voyagers make it through the heliosheath (the space between how far our sun’s solar wind reaches undisturbed and the end of the regime of our solar wind in any respect), they will be going slower relative to the sun than they currently are.
Given how fast these crafts are going, unless they smash into something (which is an option), the density of the interstellar medium should not pose a significant barrier. However, they are truly exploring uncharted territory. We have estimates of what the interstellar area should be like, from watching those areas of the sky, but we’ve never sent anything out there to measure it. With any luck, we’ll be able to learn a lot more about the edges of our solar system in the next few years. We’ll need a little luck, though, since we don’t know how thick the heliosheath is, and the Voyagers will eventually run out of power.
Both Voyager spacecraft run on the heat generated by radioactive decay, since the sun is way too faint in the outer reaches of the solar system to provide any useful amount of power. But by nature of a radioactive source, the amount of heat you generate declines over time, and so instruments on board the spacecraft must gradually be shut down to avoid pulling more energy than the craft has to offer. We may have to switch off the gyroscopic operations in the next year or two, depending on how well the power is holding up, but otherwise the spacecraft should continue to operate until at least 2020, keeping us posted here on Earth with the news from the very edges of the solar system.
At some point these craft will go silent – either through a total lack of power, or the inability to accurately find our faint star to direct its signal. After that, we must bid our dutiful craft a farewell and let them meander the galaxy. As NASA rather poetically put it, “The Voyagers are destined—perhaps eternally—to wander the Milky Way.”
Sign up for the mailing list for updates & news straight to your inbox!