| Back in late 1998 when I first heard that people were thinking about adding a very large aperture to HST, my initial reaction was, That's ridiculous! For a lot of reasons: We would need at least a six meter diameter mirror to even think about it. It would likely be too heavy for the control system. A light-shield of 9 meters by 15 meters may accelerate the orbit decay enough to require re-boost missions every few months during solar max. A ten-fold increase in collecting area will decrease the field-of-view by about a factor of 3 which will impact finding and tracking guide stars. And, we would have to do wave-front sensing and control; and probably figure and fine line-of-sight control as well. All this to an ancient spacecraft? I mentally filed it away with other absurdities. | But then, I was reviewing reports of the HST Second-Decade Committee
and the UV-Optical Working Group with suggestions of "Key Projects" and
"Legacy Projects" of 100 to 1,000 HST orbits; some of the spectacular results
of the recent very-large ground-based observatories; spectroscopic verification
of the Sloan Survey discoveries of 60-odd new Quasars including the two
highest-redshift Quasars known at z=4.90 and 5.00, Brown Dwarfs, and two
of the three known methane dwarfs, one of which is NOT a companion to an
ordinary star.
And thinking about two of the major goals of the National Priority NASA Cosmic Origins Program: to establish the era of initial galaxy formation, and to |
image Earth-like planets and obtain spectra of their atmosphere
for signs of oxygen and water. And considering that it has long been
suspected, and more recently inferred from Doppler studies, that extra-solar
planets are out there, we just have to be lucky enough to find one with
its orbit plane edge-on so we can confirm it by observing its transit across
our line-of-sight.
The first such observation was made in November 1999; the observation of a planet passing in front of its star, providing direct confirmation of the existence of extra-solar planets that to this date had only been inferred from the wobble of their star. If we could only "see" about two magnitudes dimmer with about a factor of six improved resolution. . . |
| To this statement the question arises, What is the lowest risk, shortest
schedule, highest Science return on investment, lowest cost approach to
this end?
Part of the answer is to make maximum use of existing infrastructure, both in space and on the ground. If there were something to this notion of a Hubble enhancement, we could do the "Key Projects" and "Legacy Projects" planned for up to 1,000 orbits, in days instead of months, at a few magnitudes deeper, with factors of three better resolution! We may even be able to image Earth-like planets around the nearest stars. An eight-meter class enhancement to |
Hubble would yield at least two magnitudes deeper imaging and spectroscopy,
a factor of three increase in spatial resolution, and factors of 12 to
40 in spatial information.
We could do Hubble Deep Fields (HDF) in one or two orbits instead of the 150 orbits required for the original. We could do several per day versus the 10 days for the original. Recall that the HDF was to pick a dark, thought-to-be blank, pin-hole in the sky, and stare at it for two weeks. The thought being to, perhaps, "see" beyond the era of galaxy formation. Well, we did "see" beyond the then-known Universe and, What did we find? Essentially, just more Universe. The galaxy density is roughly what |
would be expected if you just extended out but with interesting nuances.
One, as the redshift increases, we observe an increasing population of
merging galaxies, peculiar galaxies, and irregular galaxies; two, the differential
galaxy counts in both the F450W and F814W filters flatten at the fainter
end; and three, the NICMOS deep images do not reveal a new population of
faint IR galaxies. Taken together, these facts suggest that we may have
already seen into and beyond the era when galaxies first formed.
Unfortunately, we cannot tell because we do not have the sensitivity to "see" deep enough or with the necessary resolution. If we could achieve such an enhancement to HST we could accelerate space science, perhaps a whole decade. |
| Well, we ARE doing active-optics figure and wave-front control on the
very large ground-based observatories, thin-mirror technology has leap-frogged
in just the last year, and now that I think about it, the control system
on HST is operating at grossly de-rated levels relative to other missions.
Yes, it is an old spacecraft, but it is well maintained, everything on it is on-orbit serviceable, except the primary mirror, it has an extraordinary instrument compliment, planned to be even better in servicing missions 3b and 4, it has superb optical performance, and, it can be repaired! |
I concluded that we must take a closer look at this, even if only to determine that it makes no sense, especially if it makes no sense. So I started nosing around for interest. Some early investigators had already looked at it enough to convince themselves that they could configure an eight-meter class primary mirror for shuttle transport but had not gone into the endless list of remaining details. I concluded this issue is so important that I agreed to act as manager for an all-volunteer, preliminary feasibility study, with the mind-set of, Let's define and document why this won't work and get on with our lives. | The study team members just sort of materialized from their personal, academic, and corporate interest in furthering space science and the intrigue of attempting something thought to be not possible. This team was composed of very distinguished scientists, on-orbit servicing and mission-planning specialists, and engineers from academia, government, and industry. After a very accelerated two-month study, the answer is not only, Yes, it is feasible, but the budgetary estimate is of the order of less than half the cost to build a new, non-serviceable, less-capable space observatory! |
The study results are surprising and very interesting.
Feasibility
Study Results Summary
Study Report Section
1., Executive Summary
Study Report Table of Contents
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Jim Crocker