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Comment by obelix_ | Hacker News Reader

5 years to count ~.1 to 1% of the stars in 1 galaxy. What ants we are?

It's not so much that it takes a long time to image all of these stars, but that Gaia is measuring parallax which inherently requires taking measurements for 6 months in order for the Earth to go halfway around the Sun. If you want multiple measurements to reduce your uncertainty, you have to do it in 6 month blocks. 10 measurements per star gives you 5 years.

I'll preface this by saying I know next to nothing about these sorts of measurements. But if you want multiple measurements, isn't time just one variable when measuring parallax? Couldn't we potentially image it from different viewpoints--say, distant satellites relaying messages back for analysis, where time is more a function of the scale of the "imaging web?"

A second satellite could be put in the same orbit around the sun as the Earth, but on the other side of the sun. That would give us stereo vision for the parallax measurements.

But it's cheaper to have just 1 satellite (which is also easier to communicate with because near Earth), and then compare images 6 months apart to get the same data.

A practical problem is that it is hard to communicate with the other side of the sun.

We basically know how to do this. STEREO used lunar gravity assist (check it out, very cool move) to visit Sun-Earth L4 and L5.

On the other hand, Gaia was basically the highest priority scientific mission of ESA and it took 20 years from proposal to launch...

If we could place multiple such instruments in various orbits, it would be helpful. Alas, space science is very much underfunded.

Well, Pan-STARRS1 DR1 (largest sky survey as far as I know) is only 10x bigger with 10.7B objects, so parallax measurement is not a limiting factor for object counts. You are right it is a limiting factor for speed.

Fair question. 150 years ago we were walking. Now we land robots on other planets that run for 10 years. Where will we be after 100 years of Gaia/James Webb?

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