Truly incredible that two probes built and launched 35 years ago are still functioning at all, let alone extending their mission and literally expanding our sphere of scientific understanding.

When I see NASA spacecraft doing such extraordinary things, long beyond their expected lifespan, I always feel a little guilty when a simple function I wrote can't even process a test file without crashing!

Surprisingly, according to the Wikipedia article on RTGs (Radioisotope Thermoelectric Generators, the power source for the Voyager probes), by 2001, the probes' RTGs were down to 67% of capacity, instead of the 83-and-change percent expected. [1]

So, while still impressive, they're unfortunately not working as well as they could be.

[1] https://en.wikipedia.org/wiki/Radioisotope_Thermoelectric_Ge...

>However, the bi-metallic thermocouples used to convert thermal energy into electrical energy degrade as well

Are there copies of these RTGs on Earth for analyst or it was defined by lack of any other explanation?

I think it is likely. I was under the impression that space programs try to keep identical subsystems on Earth so they can use them to debug in-mission issues. Hard to attach a JTAG debugger when your device under test is 34 lighthours away! =)

34 hours is the roundtrip time, the distance is 17 light hours.

Thank you, I didn't catch that.

They're working just as well as expected.

There are three factors that reduce the output of an RTG over time. The first, and most obvious, is the radioactive decay of the fuel source. (As an aside, potentially this can be more complicated than one might think because as the primary isotope decays it builds up other radioactive isotopes which also produce heat and have different half-lives, so you need to solve a series of differential equations to determine the heat output over time, but for Pu-238 it's actually fairly simple since the main decay product is a much longer lived isotope, U-234) Anyway, the half-life of Pu-238 is about 88 years, so you'd expect it would take about that long for the heat output of the RTG to drop by half.

However, the radioactivity of the RTG fuel source also degrades the thermocouples used to generate electricity over time, and this reduces the power output on top of the decay of the fuel.

Additionally, as the amount of Pu-238 remaining falls the amount of heat generated by it also falls, lowering the temperature of the RTG and reducing the Carnot efficiency of the device, though only marginally.

I was curious to see how accurate the displayed ephemeris are (http://voyager.jpl.nasa.gov/Scripts/distance.js) given that they are known very precisely indeed.

For instance, ranges to Saturn are known to better than 1km, and Mars to better than 10m (http://tmo.jpl.nasa.gov/progress_report/42-178/178C.pdf, e.g., fig. 1 for Mars), and we don't even get to take direct measurements of those bodies as we can for the Voyagers.

Alas, the js seems to simply be interpolating between two known values, at a spacing of 1 year (86400s):

   var epoch_0 = 1373990400;
   var epoch_1 = 1374076800;
    [...]
   var dist_0_v1 = 17396281625.1305;
   var dist_1_v1 = 17395807835.8111;
   current_dist_km_v1 = ( ( ( current_time - epoch_0 ) / ( epoch_1 - epoch_0 ) ) * ( dist_1_v1 - dist_0_v1 ) ) + dist_0_v1;

I'm not sure how to square this linear progression over a 1-year span with the comment in the OP that "Because Earth moves around the sun faster than Voyager 1 is traveling from Earth, the distance between Earth and the spacecraft actually decreases at certain times of the year."

As dschleef said, the epoch span is one day. You will notice that dist_1_v1 is less than dist_0_v1. This fixes the problem you noticed, leading to a reasonably correct, shrinking value for the distance between Voyager and the Earth.

My understanding is that we have been able to take direct radio interferometery distance measurements of Mars since the Viking and other landers were there.

That sounds right. But, to get the numbers in the www page, you can't just measure where it is. You have to about predict where it will be, which requires forward integration of a motion model for all the mass in the solar system. The paper I referenced cites Mars range measurements, and related measurements, as a check on this forward integration.

Planetary radar is another option for a ground truth check even if you don't have a radio source (like Viking) on or near the planet in question.

86400s is one day, not one year.

Thanks -- apparently I have not been getting enough sleep. That explains how they can get distances to increase, then decrease (as noted nearby in this thread).

Maybe take an average of the difference between the nearest point and the farthest then settle for the middle?

I like to think that some day in the future, we will recover them both and place in a museum here on Earth.

Or we'll build the museums around the spacecraft, perfectly co-moving. Visitors will be able to walk around them and admire their antique workmanship as they obliviously continue their steady journey.

What an excellent tribute; I hope we have the technology to do this within our lifetimes.

"With today's technology, would it be possible to launch an unmanned mission to retrieve Voyager I?" http://what-if.xkcd.com/38/

What I find mind boggling is the fact that Voyagers are not even at a full light day distance yet. Things in Astronomy are often expressed in distances of light years. Unimaginable vast distances away from us.

Wow. I had no idea they were moving that fast (relative to us).

Yeah, seeing those KM increase so fast really drives it home.

I don't understand why they would ever be getting closer to Earth. Wouldn't they be launched at a time of the year when Earth is moving in the direction the they are supposed to go in order to give them maximum speed?

A very simple way to explain it: imagine you are on a playground and there is a merry go round right in front of you. A kid jumps in it and it starts spinning. At the same time you start to slowly walk away in a straight line. If you keep looking at him, you will see that the distance between you two varies (he gets closer, then farther, then closer back again) due to the rotation of the merry go round. In this example, the merry go round is the solar system, the Sun is at its center, fixed in this frame of reference, the kid is the Earth and you are the Voyager probe. Hope it helps!

They don't fly in a straight line away from Earth:

http://solarsystem.nasa.gov/multimedia/display.cfm?IM_ID=214...

You cannot really go in straight lines in the solar system due to orbital mechanics. Whatever you do, you're in orbit about something, even if the orbit is a hyperbolic escape trajectory and you're never returning. So the probes were not launched in the same direction they're currently traveling.

I believe the Voyagers didn't gain solar escape velocity at launch; they were sent to Jupiter via a regular Hohmann-like (half-ellipse) transfer orbit and used the gravity assist there to actually kick them to an escape trajectory.

"Because Earth moves around the sun faster than Voyager 2 is traveling from Earth, the distance between Earth and the spacecraft actually decreases at certain times of the year."

The Voyager probes used to be moving much faster, but as they've fought the gravity of the Sun they've slowed down, that's the price they pay for leaving the Solar System. The result is that now the Earth moves faster than the Voyager probes, relative to the Sun.

Cool. But I was hoping to see a speculative article about why we haven't seen other systems' Voyager equivalents.

Aside from the speed, even if an alien voyager passed right through our solar system we would probably miss it unless it were really bright/loud for some reason.

Although Voyager has traveled an incredible distance, and is moving at incredible (to us) speeds, it is a tiny spec that hasn't even truly left our solar system yet. By my math it would take it more than 10,000 years to travel a single light year away, and the closest star to Sol is 4 times as far.

I'm afraid that if we see an actual vessel from another system, it will be a flying saucer capable of warp speed, and not an early space probe. ;)`

A von Neumann probe should be less of a challenge than faster than light travel, so I think a slow moving ship is more likely.

https://en.wikipedia.org/wiki/Self-replicating_spacecraft

And exactly how often do you notice a stray molecule exhaled by the person standing next to you drifting across your field of vision?

Because, given the relative scales of the things involved (solar systems and space probes), likening Voyager to a molecule is giving it orders of magnitude more ability-to-be-noticed than it's due.

Molecules don't emit radio signals that anyone ca intercept.

The replies to my comment might have been some of the points in such an article. :)

I haven't really though about the speed of moving of the Voyager probes before, but I noticed it's km's/s which I didn't expect (not really into astronomy and physics). But I also noticed the page does the distance calculation offline, so I decided to just measure the distance on a second interval and derive the speed. I think their algorithm isn't accurate, since I am getting a reading of about 33 km/s and it seems that the actual speed is around 17 km/s. Here is my script:

   var lastDistance = parseInt($("#voy1_km").text().replace(/(\sKM)|(,)/g, ""));
   var interval = setInterval(function() {
      var newDistance = parseInt($("#voy1_km").text().replace(/(\sKM)|(,)/g, ""));
      var diff = (newDistance - lastDistance);
      console.log("Speed: " + (diff * 3600) + " km/h");
      console.log("Speed: " + (diff) + " km/s");
      lastDistance = newDistance;
   }, 1000);