I've been reading about the Hipparcos Satellite recently and it seems it was the first big telescope with precision-measuring devices to accurately pinpoint a star's location in the sky.

I know Hipparcos was launched in 1989, and Hubble didn't come till 1990 (and its optics weren't corrected till 1993), but I thought Hubble was planned a lot earlier and could do such things as star mapping.

Am I wrong? Can't Hubble accurately measure star positions? And it seems to me that Hubble's large mirror could do it better than Hipparcos.

The only things I can think of:

  1. Maybe Hubble was too busy booked by other scientists wanting to study other things like the planets, or searching for asteroids. Maybe they auction time on it and other interests have been willing to pay more?

  2. Maybe Hipparcos was designed differently to see stars better. I know Hubble can only view the visible and slightly ultraviolet light, and maybe some near infrared too. But that seems enough to view the stars and map their location. Did Hipparcos need to view other parts of the spectrum for some reason?

EDIT: I'm asking this question not because I want to sour-grapes NASA or ESA or anything. I'm trying to understand why different telescopes are apparently needed for similar things. Just looking at Hipparcos compared to Hubble, they look very different. But I could have sworn Hubble is capable of doing the same kind of star mapping.

EDIT 2: My understanding is that Hubble has Control Moment Wheels so it can very finely adjust and maintain its attitude (direction). This, plus Hubble's apparently much bigger light gathering power, seem to imply that it can do the same job as Hipparcos.


2 Answers 2


As @Vedant Chandra says, Hipparcos was dedicated to astrometry. To do this properly, you need to make sure your measurements are accurate. The pointing accuracy of your satellite is a nice specification, but a system that does not rely on this can achieve more accurate measurements.

To do this, Hipparcos looks in two directions at once. The telescope combines two images from sky regions 58 degrees apart, and measures star positions relative to each other. This eliminates the satellite itself as a source of errors.

Hubble was made to be pointed at a tiny region of sky for a long time, to observe faint objects. Hipparcos, on the other hand, spins continuously and measures stars as they move across its field of view. It also looks at a much larger area of the sky than Hubble.

So, yes, Hipparcos was necessary and its measurements could not have been made by Hubble.

  • $\begingroup$ Thanks this is what I needed. It makes me wonder what two Hipparcos satellites could do, if placed in Mars' orbit at opposite points. $\endgroup$
    – DrZ214
    Commented Jun 12, 2015 at 14:28
  • 1
    $\begingroup$ @DrZ214 That would not help much. What you need is high angular precision between the two observed regions. Also, ESA has recently launched an improved Hipparcos-like satellite called Gaia, which can map stars with 7µas precision. $\endgroup$ Commented Jul 6, 2015 at 15:00

The Hipparcos satellite was solely devoted to astrometry. It did not take 'regular' images, it used an eccentric Schmidt telescope to overlay images onto a grid. It finally resulted in a catalogue of stars, not an archive of images. It's purpose was to detect and find the position of stars in the sky.


An 'image' taken by Hipparcos. As you can see, it's more of a measurement than an image. (Courtesy Wikimedia Commons

The Hubble space telescope was exactly what it says on the box; a telescope. It consisted of five scientific cameras and a Ritchey–Chrétien telescope. It's function is to take advanced images of distant worlds in incredible detail.


Image of the Butterfly Nebula, taken by Hubble. (Courtesy Wikimedia Commons)

As you can see, their functions were completely different. Hipparcos was far more accurate when it came to astrometry, and Hubble could not 'replace' it in that aspect.


Whilst it is true that even Hubble can perform astrometric measurements, this is not it's main function. Hipparcos was specifically designed to measure the sky, and Hubble was specifically designed to image it. Hubble's FGS system was installed primarily to aim it with high precision, not to perform astrometry.

As such, Hubble's astrometry has a median accuracy of $±0.0003 arcseconds$, whilst Hipparcos has a median accuracy of $±0.0001 arcseconds$.

  • $\begingroup$ But can you explain why Hipparcos was far more accurate? My understanding is that Hubble has Control Moment Wheels so it can very finely adjust and maintain its attitude (direction). This, plus Hubble's apparently much bigger light gathering power, seem to imply that it can do the same job as Hipparcos. $\endgroup$
    – DrZ214
    Commented Jun 12, 2015 at 6:42
  • $\begingroup$ I've added the details @DrZ214 $\endgroup$ Commented Jun 12, 2015 at 7:42

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