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Voyager 2 started its journey some 37 years back so all of the gases must have been exhausted, but it's still moving at a speed of 15km/s. I can say that this might be due to no opposing force to slow down the speed. But if some space debris gets in its way, or any of the extra-planetary objects get in its way or comes too close, then how will that change its path?

I think the only the way to change the direction is by gases; are there any gases still present?

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"space debris comes on its way .. then how it changes its path?" By colliding with it. The people who built them had greatest fear about things like Neptune's rings, but decided it was an acceptable risk (of not hitting anything worth worrying about). There was no effort made to detect or avoid potential collisions. – Andrew Thompson Aug 26 '14 at 10:52
Of course there will be damage if it collides, and also a change of momentum/vector. – Andrew Thompson Aug 26 '14 at 11:56
@SpringLearner - There is almost nothing in space for Voyager to collide with. Should a collision occur with even a tiny fragment it would likely be catastrophic for the spacecraft, so this has likely not happened. Given the speed at which Voyager is moving avoiding a collision would be virtually impossible, the hazard would become visible fractions of a second before impact. – superluminary Aug 26 '14 at 14:55
It is perhaps worth noting that the mass-budget for a mission like this is tighter than ... (well, I had in mind a crude idiom my father-in-law uses, but it's not appropriate for polite company) ... a very tight thing indeed. If there had been a kilogram to spare it wouldn't have been used for a collision warning system (which you couldn't have designed into a kilogram anyway), it would have been used for another useful instrument. – dmckee Aug 28 '14 at 1:12
Suppose the Voyager hit something the size of a 50 caliber bullet. (A 50 cal bullet can rip big holes through cars.) Voyager's velocity is over 15 times the muzzle velocity of a 50 cal. Since energy is proportional to velocity squared, that collision would obliterate the vehicle. Even something the size of a grain of sand would do considerable damage. Another thing to remember: It's very, very, very dark that far from the Sun. Even if the vehicle's sensors were powered, the vehicle couldn't see the hazard about to hit it. – David Hammen Aug 28 '14 at 4:48
up vote 22 down vote accepted

But if some space debris comes on its way or any of the extra planetary objects comes on its way then how it changes its path?

Not only won't Voyager 2 change it's path, it can't change it's path.

Suppose you drop your cellphone from the top of a tall building. Your cellphone is going to fall and hit the ground, hard. Your cellphone has no sensors to detect the hazard, no effectors to avoid the hazard, and no hazard avoidance software to develop a plan that uses those non-existent effectors to avoid the hazard.

Now suppose one of those Voyager spacecraft is on a collision trajectory with some Oort cloud object. Just as your cellphone is going to hit the ground, hard, so will that Voyager spacecraft hit that object. The Voyager spacecraft have no way to detect that they are on a collision course because the cameras that took all of those beautiful images were powered off a long time ago. Even if they spacecraft could see the imminent collision, the spacecraft have so little fuel left in them that nothing could be done to avoid that collision. Finally, even if the spacecraft still had powered sensors to detect the collision and powered effectors to avoid the collision, it still couldn't avoid that collision (without help from the Earth) because the computers on those spacecraft and the software on those computers are extremely primitive.

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Not really. They are instead on an end-of-life mission. The probability of a collision in the next several thousand years is near zero. The probability that they will run out of the tiny bit of fuel they have left in them (this fuel is used for attitude control only) during the next twenty years or so is 100%. The probability that they will run out of whatever little power is left in the RTGs in the next ten years or so is 100%. The vehicles will die long before they collide with something. – David Hammen Aug 26 '14 at 12:48
Nitpick: accelerometers (which can detect falling) are quite common in cell phones now. – Andrew Medico Aug 26 '14 at 20:12
@AndrewMedico Voyager, and the cell phone at terminal velocity would read 0 on an accelerator as they are no longer accelerating. – asawyer Aug 26 '14 at 20:31
@Ben -- It reads 1g, oriented upward. Accelerometers don't sense gravity. They sense everything but gravity, and in the case of a cellphone sitting on your desk, the cellphone detects the upward normal force exerted by the desk on the cellphone. – David Hammen Aug 27 '14 at 1:05
@thegrinner Nah. The phone detects the transition from not-falling to falling, but it doesn't detect the imminent collision. Mind you, the engineer may have built the assumption fall=collision in as they do for laptop hard-drive (which park their heads in the event of any large acceleration), but that is an educated guess not a collision warning system (which is hard). – dmckee Aug 28 '14 at 1:17

Space is almost completely empty. The voyager probes are exceedingly unlikely to collide with anything (as demonstrated here: What is the possibility of Voyager 1/2 colliding with matter (Asteroids or planetoids) present in space?) larger than a mote of dust. If they did collide with anything at 15 km/s, then it slowing them down would be the least of their worries.

They do have some hydrazine fuel left, which they use to control their orientation (attitude).

Hydrazine, by the way, is not a gas, but a liquid. The probe thrusters catalytically split the hydrazine to produce gases for thrust.

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Thanks for the answer,+1.isnt there any possibility to collide with orphaned rocks,meteors,comets or anything? – SpringLearner Aug 26 '14 at 11:32
@SpringLearner "isnt there any possibility to collide with orphaned rocks,meteors,comets or anything?" What is the possibility of Voyager 1/2 colliding with matter (Asteroids or planetoids) present in space? – Michael Kjörling Aug 26 '14 at 11:41
@MichaelKjörling Thanks for the link,colliding with the dust wont slow down too much but it will surely damager the vehicle,wont it? – SpringLearner Aug 26 '14 at 12:21
SpringLearner - On Stack Exchange comments aren't for asking more questions,they are mostly for clarifying posts. SE works a bit differently to online forums. If you have new questions, please ask them as questions. – Rory Alsop Aug 26 '14 at 12:52

When Voyager was a shiny new probe just heading out back in 1977 (or Pioneer, Apollo, or really anything else we've sent beyond low earth orbit) it didn't have the ability to avoid hazards. It's trajectory was more-or-less fixed the moment the Titan booster cut it loose.

The fuel it carried was for course corrections - gentle nudges to go past Jupiter at just the right angle to get to Saturn in a few years. Yes, we could consider Jupiter to be a hazard it avoided but moon-size and larger hazards are very predictable - the Titan profile considered all of that.

True hazards, the ones that pop up without planning, come in "dust speck" and "rocks and larger" categories. We have nothing, even today, to detect dust specks coming toward us at spacecraft speeds and rocks would need to be pretty big to notice 24+ hours before collision. You need that much time to collect sensor data, return it to earth, analyze it, predict a collision, program a maneuver to avoid the collision, send it to the spacecraft and fire the thrusters to nudge it out of the way. You can't do this just-in-time without pretty big fuel reserves, but if you see it far enough out you only need 1 Newton in the right direction to avoid it.

Every spacecraft gets hit by dust. If it takes a big particle (say, 1mm) in something critical that component is dead. 1 gram at LEO speeds produces 22,000 joules on impact - this is more than a .50BMG round point-blank, and that round can split a truck engine in half. Rises to 144,000 Joules at Voyager's speed. That's a 30mm cannon round.

if it gets hit by an impossible-to-see-coming apple-sized object, it's space junk.

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good explanations +1 – SpringLearner Aug 27 '14 at 8:37
And of course, if it does get hit, we won't have any idea exactly what happened. Most likely, it'd just stop transmitting all of a sudden. – Michael Kjörling Aug 27 '14 at 9:31

We should also consider two things: First, the computer on the Voyager probes is an ancient, CPU-less system with a bunch of kilobytes of core memory (actually sheet memory, a type of core memory) and the processing speed of a 8-bit home computer. It would be unable to react autonomously. Second, the distance to earth is a bunch of light-hours, meaning that even if the cameras were powered, we would see any object hours after the probe saw it. A command to avoid the object would then take the same amount of hours to reach the probe. Furthermore, as the baud rate at these distances is very low, the probe can deliver images with a framerate of maybe an image per hour or less. So any object colliding with it would have to be big enough to be seen at least some hours before or the probe wouldn't have time to transfer the image. Won't happen though, there's almost nothing out there.

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There is nothing to collide to.

The space is really empty. How far the objects are is very well demostrated at this site:

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This answer does not address the question which is "Can it"; not "is there anything out there" – James Jenkins Aug 27 '14 at 11:26
Besides; yes, there is something to collide with. Lots of somethings. Theoretically trillions of them, but at the very, very least thousands of thousands of them (they call those millions). So how likely said collision might be is a statistical calculation with a margin of error, involving lots and lots of relatively small, and potentially some really big, fast-moving objects out in the Kuiper belt and Oort cloud that we can't even see individually until the odd one falls in really close and fast, like comets. But every time a meteor collides with the earth, well, you know what I mean. – Craig Aug 28 '14 at 7:21

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