Parralax and the Voyager Probes

One of the ways scientists are able to gauge stellar distances is by parallax: a stereoscopic method by the angle of vision differing six months apart due to the orbit of the Earth.

(ya get all that? :dubious: )

Anyway, what if you used the cameras on the Voyager probes? Surely the distance between the two probes must be hundreds of times larger than the orbit of Earth…and thus provide a parallax of comparable proportion.

If we include other planetary probes like Gallileo, Cassini, Pioneer, etc…the local neighborhood would be mapped out with a high degree of accuracy.

Simply put, the cameras on the Voyager probes weren’t designed for that purpose, and therefore don’t have the necessary resolution. The long baseline certainly helps, but not enough to make up for the lack of resolution.

The current best parallax results do come from a spacecraft, but not from a deep-space craft like the Voyagers. Hipparcos orbited around the Earth, and was able to measure distances by parallax hundreds of times better than any previous effort.

In principle, there’s no reason we couldn’t launch a Hipparcos-like satellite into a Voyager-like orbit, and get better results yet. But that would be expensive, and space funding is scarce enough at the moment, and there are enough other projects which are more worthwhile, that I wouldn’t look to expect it any time soon.

In principle, yes. In practice, I suspect that the Voyager cameras, being designed primarily for planetary observation (and also being 30 years old), won’t beat the systems designed specifically for parallax measurements.

The Voyagers have two cameras: one wide-field (3°) and one narrow-field (0.4°), each producing 800x800 pixel images. This gives a resolution of about 1.9" with the narrow-field camera; since the Voyagers are now at about 100AU, this is equivalent to about a 0.02" resolution at Earth. This is close to the best single-telescope optical measurements from Earth’s surface (interferometric measurements such as those at Keck I+II should be able to do better, though I don’t know if this has been done). Dedicated missions in Earth orbit, like Hipparcos, can do better than that, with an effective parallax resolution of about 0.001".

IIRC Voyager 1 was programmed to measure the density of Saturn’s rings by photographing the strength of light from a star shining behind them: by recording how bright or dim the light was determined the thickness of the ringlets.

Dosen’t that give an indication of the probe’s photographic resolution?
In any case, how much resolution do you need to map point-light sources?

That would tell you something about how accurately the camera can measure intensity (that is, about its dynamic range and time stability), but nothing much about its angular resolution. (But in any case, you can look up the camera specs to find the angular resolution.)

Well, parallax measurements are all about angular resolution. You’re trying to measure the relative angle between a nearby star and a background star; if you can measure this to 1" then you can see Earth-orbit parallax out to about two parsecs. The Voyager narrow-field cameras have pixel sizes of about 1.9", so if they were in Earth orbit they could only see parallax out to one parsec. Since their baseline is 100 times as large, they could instead see parallax out to at most about 50pc.

It’s not just a matter of resolution (although that is true as well when we’re talking about milliarcseconds on a hypotenuse of thousands of light years) but also sensitivity; most distant stars are invisible to the eye and indeed to all but the most sensitive instruments; only big, blustering O and B type stars are discernable beyond a few dozen light years without sophisticated filtering and image enhancement.

Parallax isn’t the only means of determining distance, and indeed, not even the most useful. Comparing spectral emission and absorption lines to apparent magnitude can give a very good estimate of distance (provided you have a good idea of the composition of the intervening interstellar medium). At even greater distances, including where no amount of parallax within the Solar System would give a credible estimate of distance, Cepheid variables give quite a good estimate of distance; indeed, this is how we determined that structures like the Andromeda galaxy were outside of our own Galaxy. We also have pretty good estimates of what the standard redshift is at various distances, so imaging a pulsar and correcting for the redshift gives a good estimate of distance and speed, far better than we could hope to get via parallax (which would be wrong anyway as it would indicate a distance to where the star appears, not where it actually is.) Anne Neville can speak in vastly more detail about this than I can.

Placing dedicated observation satellites at disparate orbits would be of some advantage, but probably not as much as you imagine, and the cost of launching and placing satellites in an extra-Terran permanent solar orbit would be enormous. (You’d have to get them there, and then somehow circularize the orbit, requiring either a hell of a lot of propellant or several swing-by passes.) As Chronos notes, the imaging and data processing systems on the Voyager probes were not designed for this, and while they were state-of-the-art circa 1970, they are as primitive as a U-Matic cassette is to a Blu-Ray disc today. Also, the radioisotope thermoelectric generaters that power the Voyagers are down to a moderate percentage of their original output, while the distance is much greater that the Voyagers (which were essentially intended only to function out to the orbit of Saturn, though the designers clearly had more expansive plans) requiring more power to squirt data back to Earth.

The Voyager probes were an amazing engineering achievement which far outperformed the specified mission goals and provided a vast wealth of novel science data. but they were neither intended nor are they well suited for this proposed task, nor is their a particular need to do so. We have a pretty good idea of distances and speeds of nearly everything in the Local Group save for stuff hidden by dust clouds in the galactic disk (which unfortunately includes the Great Attractor) and parallax isn’t going to give us dramatically better data on that. (See Alan Dressler’s Voyage to the Great Attractor: Exploring Intergalactic Space for more information on this.) There would be some significant benefits to a true widely space interferometer array, but it would be more about increased resolution of relatively nearby objects than more accurate range on very distant ones.


It will have nowhere near a Voyager-like orbit, but ESA’s proposed Gaia mission, due for 2011, will have a solar orbit - it’ll be at the L2 point.

The importance of parallax measurements can’t be dismissed so lightly. Most other distance measurements have to be calibrated against other methods to get absolute results, with the consequence that they ultimately depend on Hipparcos data. Indeed, the major reason for funding Hipparcos was to simplify the calibration of Cepheids, by directly measuring the parallax of enough of them, which in turn was to significantly help reduce the uncertainties (well, at the time, outright discrepencies) in the determination of Hubble’s Constant. Thus even extragalactic astronomers, who are dealing with objects whose parallax will never be measured, are always implicitly partially dependent on parallax methods when they quote absolute distances.

I was about to say the same thing, but to be fair, there are a couple of distance-measurement techniques which don’t rely on parallax. The moving cluster method could, in principle, be used far past the distance at which parallax is useful (your effective baseline is the distances between stars), but I think that the Hyades are the only cluster suitable for use with it. The light shell from an observed supernova is a standard ruler, whose distance you can determine by measuring its angular size (I’ve only heard of this being used with SN1987a, but I’m not certain that’s the only one). And if you have a resolved binary, you can use the Doppler shift curve to determine the orbital radius, and use that as a standard ruler (useful to the extent that the stars are further apart than your parallax baseline).