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HUBBLE SPACE TELESCOPE/SERVICING MISSION-01
(HST-SM-01)
OVERVIEW OF MISSION
History
Launched on April 24, 1990, NASA's Hubble Space Telescope was designed to
be the most powerful astronomical observatory ever built. And indeed, HST far
surpasses the capabilities of ground-based optical telescopes for many types of
research. The keys to Hubble's power are its operation in space, far above the
interference of the EarthUs atmosphere, and to the unique instruments it
carries as it orbits the planet. In addition HST was the first observatory
designed for extensive on-orbit maintenance and refurbishment.
While the launch on the Space Shuttle Discovery more than 3 years ago was
flawless, Hubble was not. Two months after HST was deployed into orbit 370
miles (595.5 km) high, Hubble produced a disquieting discovery not about space,
but about itself. The curvature of its primary mirror was slightly - but
significantly - incorrect. Near the edge, the mirror is too flat by an amount
equal to 1/50th the width of a human hair.
A NASA investigative board later determined that the flaw was caused by
the incorrect adjustment of a testing device used in building the mirror. The
device, called a "null corrector," was used to check the mirror curvature
during manufacture.
The result is a focusing defect or spherical aberration. Instead of being
focused into a sharp point, light collected by the mirror is spread over a
larger area in a fuzzy halo. Images of extended objects, such as stars,
planets and galaxies, are blurred.
NASA has been coping with Hubble's fuzzy vision with computer processing
to sharpen images. For bright objects, this technique has yielded breathtaking
detail never seen from the ground. NASA also has been concentrating on the
analysis of ultraviolet light, which ground-based telescopes cannot see because
of the Earth's intervening atmosphere.
To realize the full potential of HST, however, the spacecraft must be
serviced. The telescope mirror itself cannot be fixed or changed. However,
corrective optics can be applied to the HST instruments to compensate for the
aberration, much the same as glasses or contact lenses correct human sight.
The new optics should allow Hubble to accomplish most, if not all, of it's
originally planned objectives.
The mission, though, will accomplish much more than improved vision.
Hubble was designed to spend 15 years in space. Even before the spherical
aberration was known, several servicing missions, including one in 1993, had
been planned so that failed parts could be replaced and others improved with
better technology. This mission will perform that type of servicing in
addition to installing corrective optics.
Endeavour will carry some 16,000 pounds (7,257 kilograms) of servicing
hardware into space. During nearly 2 weeks in orbit around the Earth,
astronauts will use the Shuttle as a kind of orbiting service station from
which they will venture to work on the 12.5-ton (11.34-metric ton) telescope as
it hurtles around the planet at 18,000 miles (28,968 km) an hour.
The crew will spend some 30 hours in space during at least five separate
spacewalk periods, undertaking a series of tasks more complex than any ever
attempted in orbit, to ensure that Hubble remains a viable and productive
national resource throughout its planned 15-year lifetime.
Mission Objectives and Success
The three objectives of the first Hubble servicing mission are to restore
the planned capabilities of the telescope by correcting the optics, to restore
reliability of the spacecraft and to validate that the concept of Hubble on-
orbit servicing is viable.
The top priorities are installation of the replacement solar arrays; two
rate sensing units, one with an electronics control unit; the Wide
Field/Planetary Camera II (WF/PC-II) and fuses; the Corrective Optics Space
Telescope Axial Replacement (COSTAR); at least one new magnetometer; and a new
Solar Array Drive Electronics unit.
For the first servicing mission to be considered fully successful, these
top priority items must be accomplished. In addition, other tasks may be
performed on a time-available basis. The minimum criteria for mission success
are to leave Hubble with three newer-design gyroscope systems and either an
operational WF/PC-II or COSTAR.
First Corrected Image Availability
The first fully corrected Hubble images are estimated to be available 6 to
8 weeks after the servicing mission. This time is necessary for adjustments to
ensure telescope stability and the best possible focus. During this period,
telescope operators on the ground will remotely calibrate the gyros, which help
keep the HST fixed on its targets, and position the corrective mirrors in the
Corrective Optics Space Telescope Axial Replacement (COSTAR) and the Wide
Field/Planetary Camera 2 (WF/PC-II).
COSTAR is being installed to remedy the blurred vision of three observing
instruments on HST. The WF/PC-II is a replacement camera that has its own
corrective optics.
More information on activities after STS-61 necessary to produce a fully
corrected Hubble image can be found in the section on Servicing Mission Orbital
Verification.
Science Accomplishments
Despite the flaw in the primary mirror, the bus-size Hubble still has
been able to gather a wealth of scientific data, most of which would have been
impossible to collect if the telescope did not exist. In the last 3 years, HST
has conducted a variety of scientific investigations that have rapidly expanded
knowledge of what lies beyond the Earth, from the relatively nearby planets in
Earth's solar system to the most distant reaches of the universe.
Hubble's studies have ranged from Earth's neighbor Mars, to the
evolution of stars from birth to death, to the characteristics of galaxies
beyond, and finally to a field known as cosmology, which probes the fundamental
nature of the universe itself.
The following is a small sampler of some of HubbleUs discoveries and
work in progress:
% The Planets
Even prior to the servicing mission, Hubble conducted and continues to
conduct long-term observations of global weather changes on Mars. Hubble has
observed the development of a rare, planet-wide storm on Saturn. The telescope
also resolved, as two distinct objects, the most distant planet in the solar
system, Pluto and its moon Charon. Previously, no telescope had been able to
separate clearly the two bodies.
HST also has been studying long-term weather changes on Jupiter and its
auroral activity. Hubble also has been measuring the extent of the atmosphere
of the Jovian moon Io and also has looked for changes in the satelliteUs
surface.
% Stellar Evolution
Hubble uncovered the strongest evidence yet that many stars form
planetary systems. This evidence was the discovery of disks of dust around 15
newly formed stars in the Orion Nebula, a starbirth region 1,500 light- years
away. Such disks are considered a prerequisite for the formation of solar
systems like Earth's. The HST images confirm more than two centuries of
speculation, conjecture and theory about the genesis of a solar system.
% Star Clusters
HST discovered young globular star clusters at the core of a peculiar
galaxy. The discovery of these stars early in their evolution was the
equivalent of finding a "Jurassic Park" in space.
The space telescope found "blue straggler" stars in the core of
globular cluster 47 Tucanae, providing evidence that some stars "capture"
others and merge with them.
% Gallaxies
HST uncovered circumstantial evidence for the presence of a massive
black hole in the core of the neighboring galaxy M32 as well as the giant
elliptical galaxy M87. Both galaxies have a central concentration of starlight
that probably is shaped by the gravitational field of the black hole. This
implies that massive black holes may be quite common among "normal" galaxies,
perhaps even Earth's.
Hubble yielded direct evidence for galaxy evolution by resolving the
shapes of galaxies that existed long ago. HST revealed that many ancient
spiral galaxies have since disappeared, possibly through fading or collisions
and mergers with other galaxies.
% Cosmology
The space telescope allowed astronomers to take a major first step in
determining the rate at which the universe is expanding. HST detected 27 stars
called Cepheid variables. These stars are "standard candles" for estimating
distances to galaxies. The expansion rate, known as the Hubble Constant, is
one of two critical numbers needed for making a precise determination of the
size, age and fate of the universe.
HST discovered boron, the fifth lightest element, in a very ancient
star. This star would have been one of the earliest formed after the Big Bang
explosion that most scientists believe began the universe. If boron was
produced in the first few minutes of the birth of the universe, it implies that
the Big Bang was not a uniform explosion.
Hubble precisely determined the ratio of deuterium to hydrogen in
interstellar gas clouds. This value shows that the universe has only 6 percent
of the observable matter required to prevent itself from expanding forever.
European Space Agency (ESA) Role in HST
The Hubble Space Telescope is a program of joint cooperation between NASA
and ESA. ESA provided Hubble's deployable solar arrays, the major source of
electrical power, which collects energy from the sun to recharge the
spacecraft's six nickel-hydrogen batteries. ESA's second contribution was the
Faint Object Camera (FOC), which was intended for imaging of the faintest
objects in the visible and ultraviolet spectral regions at very high spatial
resolution. These elements are discussed further in the section addressing
replacement hardware and instruments.
Claude Nicollier, a mission specialist on this flight, is an ESA
astronaut.
SERVICING MISSION ORBITAL VERIFICATION (SMOV)
The purpose of SMOV is to "recommission" HST so that it can begin science
operations as soon as possible following the first servicing mission. This
involves a thorough engineering checkout of all serviced subsystems; optical
alignment and initial calibration of all science instruments; and the phasing
in of astronomical observations. SMOV begins when HST is released from the
Shuttle and is expected to last approximately 13 weeks.
Key Activities During SMOV
% Activation and engineering checkout of the science instruments.
% Optical alignment and focusing of WF/PC-II and COSTAR.
% Initial calibration of WF/PC-II and the COSTAR-corrected science
instruments.
% Early science observations.
Engineering Checkout Activities
% Decontaminate the WFPC II detectors (charge-coupled devices or CCDs) of any
foreign substances by heating the detectors to "drive-off" contaminants.
% Establish proper operating temperature of WFPC II CCDs by monitoring
ultraviolet (UV) light from a calibration star.
% Monitor pressure drop (due to outgassing) until it is safe to turn on high
voltage to the COSTAR-corrected science instruments.
% Determine the effects of the servicing mission on the basic (pre-COSTAR)
optical performance of the science instruments.
Steps in Focusing the Science Instruments
% Check out the first generation instruments and conduct prefocusing tests.
% Adjust the secondary mirror in HST's Optical Telescope Assembly to set
focus for WF/PC-II and correct for residual coma in the Optical Telescope
Assembly.
% Deploy COSTAR arms.
% Adjust COSTAR and WF/PC-II optics and mirrors, including mirror tilt,
coarse adjustment, fine alignment and focus.
Science Instruments Calibration
% A series of tests and measurements to establish the actual performance of
the science instruments in the areas of sensitivity, resolution and detector
response characteristics.
KEY HST SCIENTIFIC GOALS FOLLOWING THE FIRST SERVICING MISSION
% Hubble will determine, precisely, the expansion rate of the universe by
measuring the light curve of Cepheid Variable stars in galaxies out to the
distance of at least 50 million light-years.
Cepheids are pulsating stars that become alternately brighter and fainter
with periods (duration of the states of brightness or faintness) ranging from
10 to 50 days. Astronomers have known for over 50 years that the periods of
these stars precisely predict their total luminous power, which allows their
distance to be measured.
In the expanding universe, the Hubble Constant (Ho) is the ratio of the
recession velocities of galaxies to their distance. (Recession velocity is the
speed at which the galaxy is moving away from Earth.) The age of the universe
can be estimated from the Hubble Constant. The age currently is estimated to be
between 10 and 20 billion years, but a more precise measurement of the Hubble
Constant is required to narrow this range to an accuracy of 10 percent.
% HST will look for the gravitational signature of massive black holes in the
cores of normal and active galaxies. A black hole is a theoretical object that
is so compact and dense, nothing can escape its gravitational field. The HST
spectrographs will measure precisely the velocities of gas and stars orbiting
the center of a galaxy. If the stellar velocities increase rapidly toward the
galaxy center, it would be the signature of a massive, compact central object.
% Hubble will be able to determine the shapes of galaxies that are very
distant. Because remote objects also are relics of the early universe, HST
will be able to study how galaxies have evolved since the beginning of the
universe. Nearby galaxies have spiral, elliptical and irregular shapes,
however, these shapes should have changed over time because the universe is
evolving.
% Hubble will be able to precisely measure the ages of globular clusters by
observing the faintest stars in the clusters. Globular clusters are considered
to be the oldest objects in the universe, and their ages provide insights into
how stars evolve and also provide an independent estimate of the age of the
universe.
HUBBLE SPACE TELESCOPE RENDEZVOUS AND RETRIEVAL
The rendezvous and retrieval operational procedures for the Hubble Space
Telescope will be similar to those conducted on previous missions requiring
capture of a free-flying satellite in orbit.
For the HST mission, Endeavour's crew will perform many orbit adjust burns
to catch up with and retrieve the telescope on flight day three of the mission
using the Shuttle's robot arm.
Once the Shuttle is safely in orbit and the payload bay doors opened, the
space support equipment activation is performed. This includes activating the
flight support system and orbital replacement unit carrier heaters. Other
pre-rendezvous activities will include checkout of the robot arm, the orbiter
Ku-band dish antenna used as a radar system during rendezvous and the ground
command system.
The terminal initiation burn occurs about 2 hours prior to capture at a
distance of approximately 40,000 feet (12,192 m) in front of the telescope.
Several small mid-course correction burns follow before the Commander takes
over manual control of the Shuttle about 1,200 feet (366 m) below and 500 feet
(152 m) behind the telescope.
The orbiter approaches Hubble from underneath, just after orbital sunset.
This approach technique is designed to minimize potential contamination from
the Shuttle's thruster firings.
Prior to capture, a ground-commanded maneuver of the telescope will be
performed to align the grapple fixture on the HST with Endeavour's robot arm.
The size of the telescope maneuver will depend on the angle to the Sun and
ranges from about 70 degrees to 180 degrees.
will be lowered into the payload bay and berthed in the flight support system,
a turntable likened to a lazy susan for its rotation and tilt ability to assist
in the servicing tasks. An electrical cable is remotely attached to provide
orbiter power to the telescope.
COMMANDS TO HUBBLE
Commands to HST are issued from the Space Telescope Operations Center
(STOCC) at Goddard Space Flight Center, Greenbelt, Md., which manages the
orbiting observatory. The STOCC has been the nerve center for Hubble
operations since the telescope was launched. Commands to Hubble are issued
from the STOCC and data gathered by the spacecraft arrive there first.
The STOCC is responsible for most commanding of the HST during STS-61,
although the crew can send a limited number of commands from Endeavour. The
STOCC will send commands configuring the space telescope for retrieval by the
orbiter; integrate commands with crew activities during extravehicular
activispacecraft hardware and perform hardware checkouts and send commands to
configure the space telescope for deployment from Endeavour.