Five EVA sessions: The most complex Shuttle mission ever to repair the Hubble Space Telescope
Andrew Yee, Science North
Press Information Note Nr. 12-93
Paris, France 14 June 1993
FIVE EVA SESSIONS: THE MOST COMPLEX SHUTTLE MISSION EVER TO REPAIR THE HUBBLE
SPACE TELESCOPE
In the opinion of scientists themselves, the ESA/NASA Hubble Space
Telescope is producing really exceptional results that no other instrument
in existence can match. Since the start of its mission in April 1990 it has
been observing planets, stars and galaxies, giving us a greatly improved
overall view of the cosmos. The recently published image of a violent
young star is a further example of the contribution the HST is making to
modern astrophysics. And yet, in December this year, a crew of seven
astronauts, including ESA's Claude Nicollier from Switzerland, will be
taking off on board the US Shuttle to go to work on the HST. Why is this?
Why repair an instrument that is already established as the world's best in
its category?
First, because this mission was planned from the outset. The impression
may have been received that the astronauts' rendezvous with the Hubble
satellite is an emergency mission to correct the optical flaw in the
Telescope's primary mirror. But that is a misapprehension, as the facts
make clear:
"Since design work started on it, back in the 1970s, the Space Telescope
has always been intended to remain in orbit for an operational lifetime of
15 years. That is a relatively long time. Routine service calls by
astronauts have therefore always been one of the key features of the
mission. A series of rendezvous, using the Shuttle, are scheduled at
intervals of three years on average. This year's is the first, with more to
come in 1996/97, 2000, and so on", explains Derek Eaton, HST Project
Manager for ESA.
The structure of the Hubble Telescope is therefore designed to facilitate
the tasks to be performed by astronauts during EVA. It comprises 49
standardised, readily accessible and replaceable modules. On its outer
surface, it has about a hundred footholds and handholds for use by visiting
"space mechanics".
These servicing missions have therefore always been an integral part of
the Space Telescope programme, to fulfil two purposes: to modernise the
scientific instrumentation and to replace any elements found to be
defective. The original plan for the first rendezvous was to replace the
American Wide Field/Planetary Camera (designed in about 1975) with a
more advanced model (1990 technology). After the discovery of the
Telescope's defective vision and a vibration problem on the European
solar arrays, it was decided to take the opportunity to deal with these two
anomalies in addition. Corrective optics will be installed, and the solar
arrays will be replaced sooner than planned.
Secondly, repairs have indeed become necessary. After three years in
orbit, the Telescope has suffered considerable wear, although this was
expected. It has lost three of the original six gyroscopes, leaving it with
the minimum number required for pointing control. Of the three fine
guidance sensors, which serve to "lock on" celestial objects, one is out of
action and the other two are still operating. One of the two electronic
systems for control of solar array deployment is also showing signs of
wear. The deterioration that has occured so far is as yet having little effect
on the programme's overall performance. All the satellite's sensitive
elements were installed in duplicate. But in the case of the gyroscopes,
for instance, there are no useable spares left. In other words, restoring
redundancy on board is becoming a matter of urgency, to make sure that
the mission can continue with maximum reliability and effectiveness.
The third and last justification for December's rendezvous is that it will in
fact be very beneficial scientifically. "The Space Telescope is admittedly
suffering from impaired vision. It cannot see as clearly or as far as
expected. Nevertheless, it has demonstrated its extraordinary capabilities.
It is producing first-class observations day after day, in a limited portion of
the universe. Any improvement is therefore worthwhile, and installation of
the corrective optics is certainly going to be a paying proposition. The
slightest progress -- even if it does not quite attain the standards originally
expected -- will enable us to make significant strides in the quality and
quantity of data and ultimately in our knowledge of the cosmos," maintains
Duccio Macchetto, ESA's man at the Space Telescope Science Institute in
Baltimore, USA, who for 15 years led the European team in charge of
developing the Faint Object Camera. "Moreover," he adds, "the scientific
instrument that will benefit most from the corrective optics will be the FOC,
designed and built by European scientists to explore the most distant
regions of the universe."
With hindsight, the Space Telescope programme approach can be seen to
have been fully justified. Without this crewed mission, it would have been
impossible to correct the two manufacturing faults discovered after the
launch. Worse still, with normal wear and tear on the satellite, the
programme might have run into serious difficulties within the next few
years. Instead, the human presence in space has brought just the right
measure of flexibility to one of the most ambitious space science
programmes of these closing years of the century, while at the same time
actually enabling the payload to be modernised in line with technological
progress on Earth.
The US Shuttle Endeavour will therefore be lifting off in December with
clearly defined objectives, with a two-tier priority ranking:
Top priority:
Change the solar arrays
Replace the Wide Field/Planetary Camera (WF/PC)
Install the corrective optics (COSTAR)
Change some of the gyroscopes
Second priority (according to time remaining available):
Repair a scientific instrument (high-resolution spectrograph)
Increase the capacity of the on-board computer
Carry out other minor repairs (magnetometer, etc.)
The new solar arrays are being delivered this month by ESA to NASA.
WPFC II is more sensitive than the original camera, especially in the
ultraviolet. But COSTAR (Corrective Optics Space Telescope Axial
Replacement), the Space Telescope's "spectacles", is bound to be the
piece de resistance of next December's mission.
COSTAR is the optical system that is to be installed to compensate for the
aberration in the Telescope's primary mirror. The principle according to
which it operates was worked out by a group of American and European
scientists, and it was then selected from among a total of 28 solutions
proposed. It is the size of a telephone booth and will be taking the space
occupied by the scientific instrument of which least use has been made,
the high-speed photometer. It will make corrections for the three
remaining "axial" instruments, the European Faint Object Camera, the
high-resolution spectrograph and the Faint Object Spectrograph. WFPC
II, a "radial" instrument, will contain its own compensation mechanism.
COSTAR is a complex device. It differs from spectacles in that optical
correction is by reflective mirrors rather than transparent glass lenses. It
continas eight tiny aspherical mirrors, the size of a fingernail. The surface
of each has been polished to a precision of one millionth of a millimetre,
which means that the residual surface variation are no higher than a stack
of about 10 atoms! These mirrors will be held by moveable fingers at the
end of a deployable arm and placed, with extraordinary accuracy and
lightness of touch, in front of the instruments to be corrected.
Europe will be playing a substantial role in the repair mission. As Roger
Bonnet, Director of ESA's Science Programme, points out: "First, we have
had a 15% share in the Space Telescope programme from the outset.
That is a considerable scientific and financial input on Europe's part. So
we have a direct interest in improvement of this unique satellite's
performance. Sceondly, our participation includes an extremely important
scientific instrument, the Faint Object Camera, which is severely affected
by the Telescope's blurred vision. So it is very much in out interests for
this problem to be rectified. We have another direct involvement in that
the flexible solar arrays, whicha are affecting the pointing of the Telescope
and have to be changed, were made in Europe, under the Agency's
responsibility. Finally, Europe will be represented on board the Shuttle by
the ESA astronaut Claude Nicollier, who was the obvious choice for this
role, not only on account of his expertise as an astronaut (he is a specialist
in operation of the robot arm) but because he is also an astronomer."
With its very heavy workload, the STS-61 mission promises to be one of
the most sophisticated in the Shuttle's history. It is scheduled to last 11
days, and crew members will be making at least five EVA sorties, an all-
time record. Even the spectacular Intelsat retrieval in May 1992 required
only four. In fact, to be on the safe side, NASA is allowing for two
additional sorties, if neded, so that the total number might be as high as
seven.
In order to bring off this exploit without too much fatigue, the five
extravehicular working sessions will be shared between two alternating
shifts of two astronauts.
The crew will have seven members, all of them with previous experience:
the Commander Richard Covey (aged 47, 3 missions), the Co-Pilot
Kenneth Bowersox (aged 37, 1 flight), the Mission Specialist Claude
Nicollier (aged 49, 1 flight) and the four "space walkers", the Payload
Specialist Story Musgrave (aged 58, 4 flights) and the Mission Specialists
Tom Ackers (aged 42, 2 flights), Kathryn Thornton (aged 41, 2 flights) and
Jeffrey Hoffman (aged 49, 3 flights).
Although he will not be engaged in the EVA work, Claude Nicollier will
have the vital role of operating the Shuttle's robot arm while two of his
colleagues are outside, repairing the Space Telescope.
The detailed mission schedule has not yet been finalised. It will be
published by ESA in October. However, we do already know that the first
two days will be taken up by the launch and the various checks to be
carried out on arrival in orbit. On the third day, Claude Nicollier will
capture the HST with the robot arm. The next five days will be devoted to
the five EVA sessions, during which the solar arrays, the WFPC and the
gyroscopes will be replaced, COSTAR will be installed, and so on. The
HST will be released on the ninth day, the tenth will be taken up by the
checks to be carried out prior to reentry, and the eleventh will see
Endeavour's return to Earth.
The most distinctive feature of the mission is the large number of critical
operations to be carried out in space. Considerable allowance therefore
has to be made for the unforeseen. The mission schedule will be planned
with this in mind, seeking to maximise the chances of success while
retaining the flexibility needed in order to react to circumstances as they
arise. The astronauts themselves are currently engaged in intensive
underwater training, at the space centres in Houston, Texas, and
Huntsville, Alabama. As Story Musgrave commented recently, "The one
thing we know for sure is that there will be surprises. The mission is not
going to pass off exactly as planned. We realise that and so we are
examining all the possible contingencies."
This Information Note is the second in a series of six that ESA is issuing to
the press to provide details of the STS-61 mission scheduled for launch on
2 December 1993 to carry out servicing and repairs on the HST,
explaining the importance of this mission, the aims pursued and the
preparations being made. The first Note (1) gave an account of the
scientific breakthroughs made by Hubble. This one (2) describes the
servicing mission itself in general terms. The subjects of those still to
come will be: (3) ESA's participation in the HST programme, (4) the
astronaut Claude Nicollier, (5) the detailed STS-61 mission schedule and
(6) the Space Telesocpe's future through to the next century.