With Earth based telescopes you get a max distance of 2 au between measurements. While we have multiple space probes well over 100 AU away from us, that's a 2 OoM difference.
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1$\begingroup$ They don’t have telescopes for resolution or precision pointing. $\endgroup$– Jon CusterCommented Jun 19, 2023 at 15:40
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1$\begingroup$ actuallty this astronomy.stackexchange.com/questions/34076/… $\endgroup$– James KCommented Jun 19, 2023 at 18:06
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1$\begingroup$ See astronomy.stackexchange.com/questions/39427/… $\endgroup$– ProfRobCommented Jun 19, 2023 at 19:45
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3 Answers
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The basic reason is that Voyager and friends have cameras optimised for imaging planets, not for doing very precise astronometry.
So why not send a telescope, like that on Gaia, into interplanetary space? There are a number of practical reasons. Gaia produces masses of data, and all this data needs be downloaded to Earth. Since it is close to Earth, it is practical to have a broadband link. But interplanetary probes have very slow download rates. For example, New Horizons took months to download all the data it recorded during its encounter with Pluto. Gaia is powered by solar panels, and these don't work well in the outer solar system, so you'd need an RTG, an this comes with its own set of engineering problems.
And ideally you want to record the apparent position of the each star at multiple different times and from different locations. This helps separate the parallax motion of the star from its actual "proper" motion. You also want to image it with the same equipment since at the ultra-precise imaging that is done by Gaia small differences in the telescope might upset the measurement. This is achieved by taking images of the stars on alternate sides of the Earth's orbit. If you place a probe at at 10 AU (to get a order of magnitude improvement in resolution) you need to wait 16 years for the probe to get to the other side of the sun.
It's also pretty hard to get into such a wide orbit. Escape trajectories are actually easier, but then stars that are "in front" of the probe won't show any parallax at all.
Now these are all practical issues, and at some point in the future it might be judged worth placing an astrometric telescope (or three) in a wide orbit or even having it on an escape trajectory. But the problems are not worth it at the moment.
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Spacecraft such as Pioneer, Voyager or New Horizons are designed to study planets, and their cameras and telescopes to obtain good images of them, not to perform precision astrometry.
Anyway, as a "technology demonstration project" in 2020 NASA executed "The New Horizons Parallax Program", in which the New Horizons spacecraft photographed 2 stars, Proxima Centauri and Wolf-359.
Observatories and amateur astronomers around the world were asked to record both stars with their cameras at the same time as New Horizons did.
The subsequent superimposition of the image taken by New Horizons on the image taken from Earth would then allow the parallactic displacement of the observed stars to be detected, relative to the background of distant stars.
Again, this was a demonstration project intended to popularise astronomy and astronautics; the New Horizons telescope is not designed to measure parallaxes accurately, so this activity has no scientific value.
But it was demonstrated that it was possible, in the future, to design a spacecraft and place it in orbit far away from the Earth to specifically measure parallaxes by recording the same star from the spacecraft and from the Earth at the same time.
In this image below, the comparison between the image of Proxima Centauri recorded by New Horizons and the one recorded from Las Cumbres Observatory, Siding Spring, (0.4 m telescope), Australia.
And this other image is of Wolf 359 from New Horizons and from the Manner 0.6 Meter Remote/Robotic Telescope at the University of Louisville, Arizona USA.
More information can be found at:
The New Horizons Parallax Program
Seeing Stars in 3D: The New Horizons Parallax Program
New Horizons Parallax Images. Images and Formats
Note that if a spacecraft specifically dedicated to precision astrometry were launched into space, it would not be necessary to wait for it to image the same star twice at two different positions to obtain a parallactic angle. It would be sufficient to have another precision telescope on the ground recording the same star at the same time. Similar to what has been done in "The New Horizons Parallax Program".
In an instant we would have as data the distance "$r$" to the spacecraft and the angles "$A$" and "$P$". Basic trigonometry would allow us to calculate the distance to the star "$d$" immediately.
$$d=\dfrac{r \ \sin (A+P)}{\sin P}$$
If the distance "$r$" between the Earth and the spacecraft were much larger than 2 AU, the accuracy of stellar distance measurements "$d$" would be much higher than what is currently obtained from satellites like GAIA located at Lagrange-L2 Sun-Earth. In short, higher accuracy could be obtained in less time.
Best regards.
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$\begingroup$ As long as the spacecraft were not heading towards the star in question. $\endgroup$– ProfRobCommented Jun 21, 2023 at 11:50
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GAIA is a very precisely spin-stabilized observatory (see wikipedia and Gaia's scanning law) which assures that it visits every point in the sky multiple times over a year, while staying at exactly 45 degrees inclined towards the sun (to avoid light being scattered into the cameras, and it initially still had a stray light problem).
You couldn't keep your station and orientation on a interplanetary voyage without large fuel consumption - and as other answers have pointed out, the farther out you go, the longer you need the mission to be, as when you can measure the parallax is tied to your orbital period.
Without the orientation and station-keeping, Voyager-type missions can measure the parallaxes of just a few stars - not worth the effort.
answered Jun 20, 2023 at 16:47
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2$\begingroup$ Station-keeping (remaining on a specific orbit or trajectory) is not really important; as long as the spacecraft's position is measurable. Since parallax measurements are referenced to nearby stars in the same image and not to the absolute pointing direction, all you need for attitude control is a way to avoid blur; either by having a slow enough rate of spin relative to the frame rate, or a rate of spin matched to the clocking rate of the CCD (à la GAIA) $\endgroup$– uhohCommented Jun 21, 2023 at 0:18
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1$\begingroup$ GAIA's scanning law may be important for an all sky survey by a spacecraft that orbits the Sun at 1 AU, but for a deep-space spacecraft like New Horizons (see this answer especially one on an escape trajectory with no periodic motion, you just need a measurement every so often (as you get further and further from the Sun), no scanning laws would be required. And of course objects anywhere near your direction of motion (or the opposite) will have low sensitivity. $\endgroup$– uhohCommented Jun 21, 2023 at 0:23
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1$\begingroup$ @uhoh As usual, your comments miss the point: If you go through the effort of doing parallactic measurements with an interplanetary spacecraft, you don't just want a few stars to be measured precisely. You'd want all-sky, multiple measurements to keep the error bars as low as possible. $\endgroup$ Commented Jun 21, 2023 at 9:23
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1$\begingroup$ @AtmosphericPrisonEscape, for long-baseline parallax measurements, you don't want all-sky measurements: stars in the direction of travel will have negligible parallax, so there's no point in measuring them. You want to optimize for measuring stars perpendicular to the line of travel, where the signal is the greatest. (To get full-sky coverage, you'd send out spacecraft in multiple directions.) $\endgroup$– MarkCommented Jun 22, 2023 at 3:33