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When initial searches for a Kuiper Belt object for New Horizons to fly to after passing Pluto did not find good targets, the Hubble telescope was used, and it resulted in the current targeted flyby for 2019. Initial searches used ground based telescopes. When the search risked running out of time without a good target, the Hubble was brought in to help.

According to what you read about the current generation of large earth based telescopes that use adaptive optics, those telescopes have a much larger angular resolution and light gathering area than the Hubble. So why was the Hubble better capable of finding a good target?

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Part of the answer may be that the current generation of adaptive optics telescopes only do adaptive optics in infrared, at least if KBOs are best observed in visible light which I don't know but I've moved that to a separate question.

Regarding everyone suggesting that atmospheric absorbtion is to blame, how does that square with this: The 8.3m Subaru telescope (which was one of the telescopes used in the ground search) has a light collecting area of 53m2. The Hubble has a collecting area of 4.5m2. So atmospheric absorbtion would need to be 91.5% for them to collect the same amount of light. Sure atmospheric absorbtion is high for some infrared wavelengths, but surely not that high over all the relevant wavelengths.

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    $\begingroup$ Given that 2014 MU69 has an apparent magnitude of 27, I would guess that atmospheric absorption (rather than turbulence) killed the signal for ground-based observers. $\endgroup$ – Carl Witthoft Sep 1 '17 at 13:01
  • $\begingroup$ I think Carl is right. The limiting magnitude of even the best ground-based telescopes just barely touches on 27 (if it even does). Adaptive optics are really good at taking out atmospheric seeing effects, but they can't take out absorption effects. $\endgroup$ – zephyr Sep 1 '17 at 14:15
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    $\begingroup$ How does that square with the much bigger light collecting area of ground telescopes? See also the edit to the question. $\endgroup$ – JanKanis Sep 1 '17 at 21:36
  • $\begingroup$ Nothing to do with atmospheric absorption, which is minimal at visible and near-IR wavelengths. $\endgroup$ – Rob Jeffries Sep 6 '17 at 21:18
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I suspect it's a combination of two things:

  1. Stable, guaranteed high-resolution imaging across the entire field of view, something not possible with ground-based adaptive optics;

  2. Very low background in the optical for HST (Hubble), versus a very high background for ground-based AO in the near-infrared.

Most adaptive optics systems are only able to correct in a small region (the "isoplanatic patch") around a bright "guide star" (say, half an arc minute in radius at most); even with artificial laser guide stars, you still need a moderately bright guide star for so-called "tip-tilt" correction. This means you can only do adaptive optics searches in limited parts of the sky.

HST, on the other hand, delivers high-resolution imaging throughout its entire field of view (several arc minutes across), all the time, regardless of where it's pointed.

To make matters worse, the trajectory of New Horizons is close to the Galactic plane, so there are lots of faint background stars. This makes it harder to pick out possible Kuiper Belt objects, making a very accurate and stable point spread function (such as that of HST) even more important.

These searches are best done in the optical, to minimize the sky background. The absence of atmospheric sky glow (mostly scattered light from the Sun and Moon) for HST makes it easier to quickly detect faint sources like KBOs. The fact that adaptive optics systems, as you and Rob Jeffries note, work almost entirely in the near-infrared, where the atmospheric background is much higher, makes it even worse for them.

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  • $\begingroup$ Good points about the guide star and field of view. $\endgroup$ – Rob Jeffries Sep 6 '17 at 21:28
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Adaptive optics only mitigate the air turbulence that blurries the images - and even that is only a partial recovery.

All other issues remain. Air absorbs various wavelengths. Air has a certain amount of glow from various sources (light pollution, etc) which masks faint objects. Etc.

There is no real substitute for a large telescope operating in vacuum.

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I think you have hit the nail on the head in your question. KBOs are seen in reflected sunlight and they are incredibly faint, since the amount of reflected light reaching the earth goes as the inverse fourth power of their distance from us (see my answer to this question on trying to view Oort cloud objects).

To see such objects requires deep imaging observations with low levels of background contamination. That background is minimised by having images with an extremely small point spread function (PSF) - the kind of PSF that can only be achieved by space-based telescopes or ground-based telescopes using adapative optics.

However, the solar spectrum of course is strongly peaked in the visible region and ground-based adaptive optics systems are not effective in this wavelength range (ground-based AO works in the near-infrared, but in addition to the KBOs being intrinsically fainter at these wavelengths there is also the issue of background noise contributed both by the Earth's atmosphere and the telescope itself). Therefore the Hubble Space Telescope is the instrument of choice.

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The magnitude of these Kuiper Belt Objects is incredibly small, to begin with. The atmosphere distorts stars normally and scatters light even on the clearest of nights. In addition to that, these closer objects can be found with infrared detection. The atmosphere absorbs infrared wavelengths extremely well, which makes space based observations a necessity. The Hubble telescope also detects ultraviolet, visible, and near infrared making that an ideal telescope for these small Kuiper Belt Objects.

Atmospheric absorption of various wavelengths

To deal with the atmospheric absorption, ground based telescopes such as the Subaru telescope get built on mountains so that there is less atmosphere to look through and the chance of cloud cover. However, the problem with the search for KBO was that it needed to be done in a short period of time so that New Horizons could be directed to it with less fuel. Hubble is ideal for that because it can look at the objects in the right direction all day while ground based telescopes could only do that at night assuming it is a clear enough night to see those objects. Normally Hubble is reserved for the most exclusive scientific projects and searches because of how its data quality is amazing. New Horizons had already cost so much that it was worth using a bit of time searching for its next destination instead of just ground based telescopes.

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    $\begingroup$ HST is in low Earth orbit. It cannot observe objects continuously. Absorption in the visible and near-IR is of order 10-20% and has nothing to do with it. $\endgroup$ – Rob Jeffries Sep 6 '17 at 21:20
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This [1] article suggests that one of the advantages that Hubble and other space telescopes have is that they can better image very faint objects because they don't have to deal with atmospheric glow. Adaptive optics don't help with that, and a larger collecting area also collects more background glow. Also the atmospheric glow is intenser in infrared than in visible light.

Other listed differences: Ground telescopes can't make as accurate brightness measurements due to atmospheric turbulence (AO apparently doesn't help with that); ground telescopes can have better angular resolution due to larger sizes; ground telescopes can use bigger, heavier, better spectrographs than are practical in spacecraft.

[1] Introduction to Adaptive Optics and its History, Claire Max, at Center for Adaptive Optics, U. Calif, 2001

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