By Chris Blades,, Carl Biagetti,,
and Kerrie Bennett Sobering

Since launch in 1990, Hubble has overcome problems and undergone
improvements by means of three amazingly successful servicing missions.
With Servicing Mission 3B (SM3B), scheduled for February, we will soon
again witness the feats of brave astronauts, the heavy lifting power
of the Space Shuttle, and the scientific benefits of an observatory
that can be revisited, serviced, and upgraded in space.

On SM3B, astronauts will install Hubble’s newest science instrument,
the Advanced Camera for Surveys (ACS). The ACS is designed for
ultraviolet and visible light imaging and consists of filters,
dispersers, and three electronic cameras. The ACS Wide Field Camera
(WFC) is a three-mirror optical design with high throughput, wide
field of view, and high sensitivity across the visible wavelengths.
With the WFC, searches for high redshift galaxies and clusters of
galaxies in the early universe will be greatly facilitated. The WFC
is designed to have a tenfold increase in discovery power compared
to the existing Wide Field Planetary Camera 2 (WFPC2), where
discovery power is the ratio of throughput times focal plane area
for the two cameras.

The ACS High Resolution Camera (HRC), also a three-mirror optical
design, is optimized for high resolution and high-contrast imaging,
including a coronagraph to improve near-bright-object contrast. A
typical use of HRC will be diffraction-limited studies of the light
in the centers of galaxies with massive black holes. The third ACS
camera, the Solar Blind Camera (SBC), is a far ultraviolet camera
with a two-mirror optical design. It will be used for faint object
and extended object imaging. With its relatively high throughput,
the SBC will be used to search for quasars, hot stars, and aurora
on Jupiter, as examples. The WFC and HRC have CCD devices, and the
SBC has a photon counting detector.

The ACS is a group effort. It was produced and readied for launch by
Johns Hopkins University (JHU), Goddard Space Flight Center, Ball
Aerospace Corporation, and the Space Telescope Science Institute.
Scientists from universities in the United States and Europe have
contributed to the effort, with most of the science team located at
JHU and the Institute. The Principal Investigator for ACS is Professor
Holland Ford of JHU.

With installation of ACS, the legendary Hubble images are about to get
even better. When first activated, ACS will image standard stars to
verify performance by measuring known light levels. Later, ACS and
a revived NICMOS will make early-release observations (EROs) of
interesting objects, which will be available for public viewing after
three months. The ACS science team’s program will emphasize deep
imaging of the sky to study high red shift galaxies and other
phenomena. Starting with Cycle 11 in mid-July 2002, General Observers
will begin using ACS to record fainter objects and to grasp wider
views of the sky than ever before.

Astronauts on SM3B will perform other installations during five days
of extra-vehicular activity (EVA) beginning two to three days after
launch. The first two EVAs will be dedicated primarily to replacing
the solar array wings. The current solar arrays, which were installed
on SM1 to replace the original arrays, have been degraded by eight
years of exposure to radiation and debris. The new solar arrays will
be rigid and hardier than the two previous sets. They will produce
thirty percent more power, offer increased resistance to temperature
variations, and minimize the atmospheric drag on the spacecraft.

The third EVA will focus on the replacement of Hubble’s Power Control
Unit (PCU), which will require the first powering down of the entire
system since launch. The installment of a new PCU is a central part
of SM3B and will present a special challenge to the astronauts, as
the equipment is not designed to be removed and reset. The power-down
should not last more than six hours.

The ACS claims the beginning of the fourth EVA, after which the
astronauts will install the NICMOS cooling systems (NCS). NICMOS, the
Near Infrared Camera and Multi-Object Spectrograph, has been dormant
since 1997, when its supply of cryogen was depleted. The NCS is an
experimental, super-quiet cooling system, which uses microturbines
that can spin faster than 400,000 rpm. The NCS will re-cool the NICMOS
detectors, revive their infrared sensitivity, and return this unique,
important instrument to duty. The fifth EVA will also focus on NCS and
its companion equipment.

Between EVA four and five, an orbital re-boost will increase Hubble’s
altitude by several kilometers, depending on the amount of fuel
remaining. Despite the thin atmosphere at Hubble’s orbit, atmospheric
drag causes the spacecraft to lose altitude, which would result in
uncontrolled de-orbiting if not corrected. Hubble’s altitude will be
increased during SM3B, as it was in SM1 and SM2, by the creative use
of the Space Shuttle’s thrusters.

The five EVA spacewalks are to be performed by four astronauts, three
of whom are veterans: John M. Grunsfeld, James H. Newman and Richard
M. Linnehan. Michael J. Massimino will be embarking on his first
flight. The commander, two-time shuttle veteran Scott Altman, will
be joined on deck by Duane Carey, also making his first flight, and
Nancy Currie, a three space flight veteran.

Hubble discoveries have inspired and informed scientists and the
public for a decade now. In these times of conflict and uncertainty,
Hubble can seem a metaphor for exploring deeper, transcendent
realities — ones holding no fear but offering hope of new knowledge
and better understanding. It is heartening to reflect that Hubble
will never have been more capable of discovery than it will be in
February 2002 after SM3B. And never has Hubble been needed more as
a touchstone for science in the public interest.