A recent question asked about the feasibility of using Voyager 2 to detect objects in the Oort cloud. The answers indicate, among other issues, the instruments on-board the Voyagers would likely be insufficient to perform such detection.

However, in theory there is at least one way to detect objects without any specialized equipment: direct collision. The Voyagers are moving very fast, so such a collision would probably render the craft non-functional.

I'd like to separate the "detecting collisions" problem from the various ones raised in the other question, so rather than Oort cloud objects let's suppose one of the Voyagers collides with some previously undetected object tomorrow. In that case, Would we be able to distinguish a crash from other possible failures?

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    $\begingroup$ I would think an occultation would be a better way to discover an object crossed a Voyager's path than a crash. My guess is that a crash would be indistinguishable from other events which might result in an abrupt end of communications. With an occultation, we can guess the distance of the object based on size approximations, using the duration of signal loss. Of course, we can't rule out some kind of instrument malfunction in that case either, but if the signal loss doesn't recur, it is less likely to be instrument malfunction. $\endgroup$
    – called2voyage
    Dec 11 '18 at 15:54
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    $\begingroup$ Of course, occultations and crashes are both unlikely, given how vast the expanse of space is. $\endgroup$
    – called2voyage
    Dec 11 '18 at 15:56

Most likely no.

Voyager downlink communication (via its radio link to NASA's Deep Space Network (DSN) is not continuous. You can check the contact schedule at this Voyager site. If everything looks fine during one DSN contact period, and then at the next contact period there's no signal at all, there are myriads of possible causes, ranging from failure of the radio system or the attitude control system that points the antenna to Earth, to running into something.

If it runs into something, that something doesn't need to be very large. Voyager 1 is receding at nearly 17 km/s, Voyager 2 at ~15.4 km/s, so a BB-sized ice particle could sever the connection to the RTG (the radioisotope battery that powers everything). If rocky or metallic, it could penetrate the multi-layer insulation covering the bus and blast a hole in the main propellant tank...there are multiple soft spots. In any case, all we would know is that when we tried again to contact the craft, we heard nothing.

If, on the other hand, something happened while we were in contact, the radio signal's strength profile would give some clues, though not necessarily definitive. Most failure modes would have the signal die off rather slowly, some faster, some slower. For instance, complete loss of attitude control would have it die off very slowly, with a well-determined profile as the direction to Earth drifts through the HGA's main beam, then into side lobes. Catastrophic rupture of the main propellant tank (very unlikely!) would have it die off in tens of milliseconds, but not between individual bits of data, at the current downlink rate of 160 bps.

There are very few scenarios that would interrupt downlink mid-bit, and collision with something big is one of them. But there is more than one fast-loss scenario. An example might be catastrophic failure of the X-band transmitter's traveling wave tube (TWT), something like breaking the lead to the internal helix coil, though I suppose they would try later to see if they get an S-band signal. A power glitch that smoked both the S-band and X-band TWTs might prevent that. Anyway, there are non-collision events that could interrupt comm mid-bit. They are unlikely, but then again, so is running into something big.

The net result: we wouldn't know for sure.

  • $\begingroup$ Would we know if the trajectory of Voyager was altered by a significantly large comet? I believe another answer stated the largest theoretical comet size to be ~300km (or something along those lines) if this perturbed the path of voyager, but did not sever the connection via impact or anything else, would we be able to know? $\endgroup$ Dec 11 '18 at 17:50
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    $\begingroup$ @MagicOctopusUrn You bet! Such an alteration would change the Doppler shift on the radio signal's carrier, and spacecraft Doppler tracking is exquisitely sensitive to that. We'd know in a heartbeat that something bent our trajectory! $\endgroup$ Dec 11 '18 at 17:57
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    $\begingroup$ What's the smallest significant deviation in trajectory that can be detected? Are we talking in the ballpark of a change in velocity of around ~.00001m/s or something much larger? $\endgroup$ Dec 11 '18 at 18:14
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    $\begingroup$ @MagicOctopusUrn You may be interested in this question, though I don't know that it answers the minimum detectable change. $\endgroup$ Dec 11 '18 at 18:52
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    $\begingroup$ @MagicOctopusUrn, if the Pioneer anomaly is any indication, we should be able to track a change measured in parts per million, or possibly parts per billion. $\endgroup$
    – Mark
    Dec 11 '18 at 23:02

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