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This is a follow-up building on Does Voyager 1 do anything to be more observable?

Perhaps no later than 2025 AD, the Voyager twins are expected to become mute; the RTGs will no longer provide enough electricity to power their instruments.

After that, anyone interested in locating either twin will have to rely on the last reported heading and velocity. There is no reason to believe it's path will change, though more definite knowledge of it's course and velocity would be helpful — but the availability of such information is not a reasonable expectation.

At that point, how would one track either twin?

E.g. Optically by Kepler or it's descendant

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The Voyagers will not deplete their fuel for centuries. Within a few more years, however, the rest of the involved system will degrade to the point that the voyagers can no longer transmit.

IR telescopes will be able to spot them, provided they don't have a course change, as the Plutonium in the RTG continues to decay, and it's decay products continue to decay, for many years, probably at least a century.

Wikipedia shows that the RTG has about 2400W of thermal power, and the reason the three RTG's are out on the boom was to prevent excess heating of the rest of the probe. Internally, the RTG's are about 1000° C, and cooling slowly. Given the half-life of 87 years for the Plutonium, it will still be about or above 400°C in the 2050's. The junction couplings are likely to corrode to non-function before then. The relatively high temperature will generate a strong IR signature via the principle of blackbody radiation. Still, it's only a 400W signature at launch.

Given the distance regime (101-124 AU) where it's at, the IR signature should be detectable for at least the next 50 years, and probably considerably longer, tho', thanks to the Inverse Square Law of EM Radiation, the IR signature is a tiny source. You'll need to be looking for it.

The distance regime also means that optical is not particularly useful. The light from the sun is too dim to be readily detected with current telescopes, tho' a hubble image might be doable with sufficient exposure time. At 101.8 and 124 AU, they receiver 1/10363 and 1/15376 the energy as an object near earth. And with only around 13 square meters of cross sectional surface area, most of which is angled, that's VERY little being reflected back.

http://voyager.jpl.nasa.gov/mission/weekly-reports/
http://en.wikipedia.org/wiki/MHW-RTG
http://en.wikipedia.org/wiki/Plutonium-238

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  • $\begingroup$ Just the kind of response I hoped for. A follow-up; could spectroscopy detect the Pu emissions? $\endgroup$
    – Everyone
    Commented Aug 25, 2013 at 8:02
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    $\begingroup$ Probably not - Pu-238 emits alpha and heat, and the alpha is low energy. Further, there's insulation mass that will absorb some of the tiny radiation from other sources. Almost all the radiation is going to be infrared, via blackbody radiation. $\endgroup$
    – aramis
    Commented Aug 25, 2013 at 8:09
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    $\begingroup$ What is "tho'"? $\endgroup$
    – JYelton
    Commented Sep 13, 2013 at 18:49
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    $\begingroup$ Tho' is a contraction of though. $\endgroup$
    – aramis
    Commented Sep 15, 2013 at 8:21
  • $\begingroup$ One other thought - 400°C is the same range as some of the detected brown dwarves... color T8 brown dwarfs. 1000°C is T2... but thousands of times closer (and thus millions of times higher magnitude per unit luminosity.) If we can spot a brown dwarf at 100 LY, we should be able to spot a 3 billion times smaller cross section object the same temp at 1/63,000 the distance... $\endgroup$
    – aramis
    Commented Jan 2, 2015 at 3:13

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