HST10X, The Installation
Mission
aJames H. Crocker, bThe HST10X
Study Team, c*Donald A. Dufford
aBall Aerospace & Technologies Corp.,
P. O. Box 1062, Boulder, CO 80306-1062
bSee the Team Members List in the REFERENCES
section
cJohns Hopkins University, Dept. of
Physics and Astronomy, Baltimore, MD 21218
ABSTRACT
The Space Sciences of Astronomy, Astro-Physics, and Astro-Biology could
be advanced by ten years, perhaps more, if a faster, cheaper, better way
than an entirely new spacecraft could be found to implement an 8-meter
class observatory in space. Why 8 meters? Recent science results such as
the Hubble Deep Field (HDF)1 and other observations from the
very large ground-based observatories suggest that to achieve the two prominent
Space-Science goals of establishing the era of initial galaxy formation,
and imaging and spectroscopy of Earth-like planets requires at least two
magnitudes deeper imaging and a factor of six better resolution than anything
now in existence or planned for UV/Optical wavelengths. The UVOWG Final
Report2 lists agonizing details of critical science objectives
toward these goals, agonizing because we cannot achieve them from the ground
even with the four 8-meter mirrors of the VLTI. An 8-meter class space
telescope will provide about 2.5 magnitudes deeper imaging and a factor
of 6.5 better spatial resolution than the best we have now, HST. This paper
describes a feasibility study for augmenting HST with 8-meter class optics.
The results are very interesting, and surprising.
Keywords: Space Science, Origins of Life, Big Bang,
Earth-like Planets, Astronomy, Astro-Physics, Astro-Biology, Hubble Space
Telescope, HST10X, Hubble Space Telescope Optics Performance Enhancement.
INTRODUCTION
This paper is one of two companion papers on the subject of HST10X,
an investigation into the feasibility of augmenting the Hubble Space Telescope
(HST) with an Optics Performance Enhancement (OPE) to achieve of order
ten-fold improvement. This paper emphasizes the preliminary feasibility
study3 results and the Feasibility Reference Mission (FRM) for
astronaut installation. The other paper by Holland C. Ford, HST to HST10X:
A Second Revolution in Space Science, emphasizes the science that will
be enabled by this enhancement.
1. The HST10X study
1.1. Overview
1.1.1. Background
The Space Sciences could be advanced by ten years, perhaps more, if
it were feasible to augment HST with optics to provide of order ten-fold
increase in collecting surface area. We could do things like DETECT EARTH-LIKE
PLANETS ORBITING NEARBY STARS at distances up to 10 parsecs; we could perform
spectroscopic examination of Earth-like planets to search for atmospheric
oxygen, a certain sign of life, out to about 5 parsecs; we could obtain
additional Hubble Deep-Field (HDF) observations AT 15 PER DAY instead of
the 150 orbits in 10 days for the original, at five times the angular resolution;
the "Key Projects" of 100-1,000 orbits and the "Legacy Projects" of 500-1,000
orbits proposed for HST's second decade could be done in days instead of
months; we could confirm, or deny, the HDF implication that we have already
seen into and beyond the era when galaxies first formed. Until recently
such an endeavor was unthinkable. However, with the evolution of new technology
such as lightweight optics, ultra stable materials, active damping structures,
active mirror figure controls, and significant evolution of Extra-Vehicular
Activity capabilities demonstrated by HST and designed for the International
Space Station (ISS) missions, it is now reasonable to, at least, consider
significant optical enhancements to HST.
With this evolution in technologies, the possibility is very real that
HST could be augmented with new optics providing two to three magnitudes
deeper imaging and a factor of six improved resolution through a ten-fold
increase in collecting surface area, an HST10X.
This possibility is so compelling, a team of very distinguished scientists,
on-orbit servicing and mission-planning specialists, and engineers from
academia, government, and industry agreed to conduct an all-volunteer,
preliminary study to assess the feasibility of such an enhancement to HST.
1.1.2. Study Goal
The goal of the study was to search for at least one technically feasible
approach to HST10X as well as the scientific discovery potentials and origins
objectives (imagining power) that such a system could produce in the 2006
to 2008 time frame. The period of this initial feasibility study was two
months and it was conducted in mid 1999.
1.1.3. Study Summary
The study was conducted by teams of specialists in the various areas
in question. The results of these investigations are summarized in the
subsections noted below .
The Science that would be enabled, Section 1.2.
The Cargo/Installation Mission including stowage for transport by the
Orbiter and detailed astronaut operations, Section 1.3.
An Optical Assessment of the various mirror materials and configurations
studied that may be suitable, Section 1.4.
A Structural Dynamics Assessment of the impact of the additional mass
on the HST structure, Section 1.5.1.
A Thermal Systems Assessment of the impact on existing systems and
instruments, Section 1.5.2.
An assessment of the impact on Fine Guidance Sensor (FGS) operation,
Section 1.5.3.
A Control Systems Assessment of the impacts on slew and control performance
and the orbital decay issue, Section 1.5.4.
A mission analysis of the feasibility of adding a stationkeeping system,
Section 1.5.5.
1.1.4. Study Limits
The depth of investigation for this study was limited to that necessary
to confirm, or deny, feasibility. This was done to minimize the duration
of the study. One result of this limit was that the work by the separate
teams was done in parallel without benefit of results from the other teams.
The next step would be the iteration process between the teams to establish
consistent assumptions and requirements.
1.1.5. Study Conclusion
It is clear from the results of the investigations that augmenting HST
to an 8-meter class observatory is feasible, will have much less detrimental
impact than previously thought, and will cost of order less than half that
of a new spacecraft. Such an augmentation could advance by years achievement
of two of the major goals of the National Priority NASA Cosmic Origins
Program4. Operations costs could be reduced
since HST10X will produce more science in one year than HST would in several
years, and by using "Key-Project" and "Campaign" observing modes.
1.2. Science Enabled by the HST10X Enhancement
The Science Team was asked to define the science that might be enabled
by a ten-fold enhancement to the optics of the Hubble Space Telescope (HST10X).
They assumed an 8.4-m aperture, afocal telescope in front of the existing
HST optics. The science enabled by such an enhancement is only beginning
to be considered but will certainly include:
DETECTION OF EARTH-LIKE PLANETS ORBITING NEARBY STARS at distances
up to 10 parsecs. Furthermore, it will enable spectroscopic examination
of Earth-like planets to search for atmospheric oxygen, a certain sign
of life, out to about 5 parsecs. The best wavelength to look for earth-like
planets is between 0.6 & 0.8 micrometers which is right in the middle
of the HST band pass.
Obtain additional Hubble Deep-Field (HDF) observations in one orbit
AT 15 PER DAY instead of the 100 orbits in 14 days for the original, at
five times the angular resolution. A one-orbit F814W HST10X exposure
will be deeper than the original HDF and have 14 times more spatial information
per galaxy. HST10X will allow measurement of the spectra and spectra-energy
distributions of high-redshift galaxies that cannot be reached with
the Keck and VLT telescopes.
The "Key Projects" of 100-1,000 orbits and the "Legacy Projects" of
500-1,000 orbits proposed for HST's second decade could be done in days
instead of months.
Confirm, or deny, the HDF implication that we have already seen into
and beyond the era when galaxies first formed.
HST10X will extend the Hubble Telescopeís extraordinary capability of
measuring Cepheid distances from the present 20 Mpc to 100 Mpc. These measurements
will determine H0 (averaged over 100 Mpc) in one step, thereby avoiding
the systematic errors introduced by secondary distance indicators. The
Cepheid distances will map large scale flows within 100 Mpc and delineate
the large scale distribution of dark matter that drives the flows.
HST10Xís large increase in spatial resolution (3.5 to 6.5) and spatial
information (12 to 40) will capitalize on HSTís stunning discovery of proto-planetary
disks in the Orion nebula to study the formation of stellar and planetary
systems. HST10X may actually be able to detect the signature of planets
forming in the proto-planetary disks.
By using faint quasars as UV background sources, HST10X could make tomographic
maps of the full "cosmic web" of the filamentary distributions of hot (shocked)
and warm (photoionized) baryons left over from the epoch of large-scale
structure formation.
In the companion paper by Holland C. Ford, HST to HST10X: A Second Revolution
in Space Science, we quantify the performance of HST10X, compare that performance
to the performance of present and planned HST instruments, and give examples
of outstanding science programs that can be undertaken with HST10X.
1.3. The Installation Mission
The study team considered many system configurations during the two-month
study and identified several workable ways to transport and deploy an 8-meter
telescope in front of the existing HST. The approach selected to demonstrate
feasibility is based on using as much existing space-qualified hardware
as possible to save cost and reduce the associated risks of schedule, cost,
and mission success.
The installation mission is in four EVA days following routine lift-off,
rendezvous, capture, and berthing:
Day 1. Remove the forward 3.5-m section of the existing Old Light-Shield
(OLS).
Day 2. Install a New Light-Shield (NLS).
Day 3. Prepare and install the New Mirror Assembly (NMA).
Day 4. Install replacement Science Instruments.
The Extra Vehicular Activity (EVA) work required is similar to that proposed
for the Space Station and has been reviewed by astronaut consultants and
space-system servicing engineers and found to be acceptable.
Conclusions: There is team consensus that, from a technical
standpoint, this job is feasible and could be launched as early as 2006
if proper funding were made available.
Mission details are presented in Section 2, The Installation Mission.
1.4. Optical Assessment
The Optics Assessment Team was asked to identify methods that appear
feasible to enhance HST to an 8-meter class observatory; and, select one
configuration for a Feasibility Reference Mission (FRM); keeping in mind
the two key science goals of identifying the era of initial galaxy formation
and imaging extra-solar Earth-like planets.
Several mirror designs were considered. The round, two-fold design was
selected for the feasibility reference mission but, depending on the material
selected for flight, other designs could be considered. The round two-fold
design will have the best optical performance, especially with 5000 actuators
on the back plane. A ray-trace diagram of the "add-on" telescope is shown
in Figure 1.4.1. The two-fold mirror concept is shown in Figure 1.4.2.
The afocal telescope concept is shown in Figure 1.4.3. The enhancement
components are shown stowed for transport in the Orbiter Cargo Bay in Figure
1.4.4.
Figure 1.4.1. Ray-trace of "add-on" Telescope
.
The University of Arizona (UA) design selected for this study will weigh
between 15 and 20 Kg/square meter, have about 5000 correcting actuators,
and use an existing VLT spare mirror blank. The round design can be built
in the existing UA facility, which has built several 8-meter mirrors, or
could be done by REOSC or Contraves. The grind and polish of this 8-meter
mirror would be done just like any of the ground-based mirrors such as
VLT, Subaru, or Gemini. Once the mirror is polished it will be turned over
onto a fixture that will support the mirror while all but 2 mm of glass
is ground away. The 5000 actuators add complexity but have a nice advantage
of providing the capability to control wavefront and, in the case of the
Secondary Mirror, control line of sight. The schedule includes a plan to
build dummy mirrors out of Borosilicate for initial test to save schedule
and minimize risk to the flight mirrors. Others can do the initial grind
on the VLT blanks while UA is working on the test-dummy. |
|
|
| Figure 1.4.2. The Two-fold Mirror Concept |
Figure 1.4.3. The Afocal Telescope Concept |
|
|
Figure 1.4.4. HST and Enhancement Components in the Orbiter
Cargo Bay
|
Conclusions: Several mirror designs were considered
and found acceptable for this application. The round, two-fold design was
selected for the feasibility reference mission because it will have the
best optical performance, and VLT blanks are available.
1.5. Technical Reports
1.5.1. Structural Dynamics
The HST10X Structural Dynamics Assessment was conducted to investigate
the mass properties, interaction with the Pointing Control System, thermal
deformation, and potential line-of-sight jitter of the Hubble Space Telescope
with the HST10X enhancement. The dynamic mathematical model of HST developed
after the first servicing mission (SM-1) was modified with added large
mirrors and light-shield to approximate the properties of the example HST10X
configuration.
Conclusions: The increased Moment-of-Inertia (MOI) will
require increased slew times and more reaction-wheel torque but within
acceptable margins, light-shield resonance may require minor changes to
the control law, pointing jitter can be mitigated with the line-of-sight
control
of the secondary mirror if necessary, and the enhancement will add about
3.500 pounds to the current HST weight budget of approximately 24,000 pounds
which is about 15%.
1.5.2. Thermal Assessment
Conclusions: In general, the results of the thermal assessment
are that thermal effects will be small or can be mitigated easily, with
the exception of the solar arrays. The solar-array operating temperature
will be elevated by about 20 degrees C due to the space-view reduction
from the New Light-Shield (NLS). This can be mitigated by the addition
of supplemental solar arrays on the bottom of the NLS which will, at least
partially, offset this temperature rise. Adding reflective surfaces to
channel the heat back into space will provide additional relief. Making
the new arrays wider than currently planned will further reduce the temperature
rise.
1.5.3. Fine Guidance Sensor Operation
The issues assessed were the FGS encoder resolution for guide-star acquisition,
the field-of-view reduction, and the ability to maintain fine-lock with
the amplified jitter as measured by the FGS.
Conclusions: First, the FGS will still acquire its target in
the usual sequence of Search, Coarse Track, and Fine Lock. Second, the
FGS field of view will be reduced from 69 square arcminutes to approximately
6.3 square arcminutes, while the faint end of the guide- star candidates
improves from 14.5 to 17 v-magnitudes. Third, jitter as measured by the
FGS, will be amplified by a factor of 3.3 and requires additional consideration.
Finally, a trade-off study needs to be addressed by the astronomy community
to assess how much the improved faint guide-star capability will offset
the reduced FOV.
1.5.4. Pointing and Control
1.5.4.1. Control Systems
The Moment-of-Inertia (MOI) increase of approximately a factor of two
will increase the torque required of the control system proportionally.
Because the Reaction-Wheel torque is fixed, control bandwidth must be decreased
by one over the square root of the MOI change. This will increase response
time and slow maneuvers. The requirement to slew 90 degrees in 18 minutes
will have to be relaxed unless actuator torque can be increased.
Conclusions: For HST10X, it appears the control system will
provide acceptable science and slew performance with minor changes to the
controller.
1.5.4.2. Orbit Decay
Conclusions: The larger New Light-Shield (NLS) and the solar
maximum in 2010 combine to accelerate the rate of orbit decay to the extent
that either Stationkeeping or a re-boost mission will be required.
The minimum science-floor altitude coupled with the maximum density (minimum
altitude) allowable for a 90 degree slew in 50 minutes require that something
must be done to keep HST above approximately 300 nautical miles altitude
out beyond year 2012. One option is adding an Ion-Engine Stationkeeping
System (IESS) and, aside from the added solar array power needed, this
appears to be a reasonable solution for the problem.
1.5.5. ION Engine Station Keeping System
Conclusions: There is a readily available Stationkeeping System
that can provide sufficient boost for stationkeeping, acceptable science
efficiency versus boosting ratio, and adequate fuel margin for end-of-life
ascent. There are convenient attachment points that are structurally acceptable.
The large-diameter threaded holes in the existing HST trunnion sockets
will make ideal mechanical mounts for the ion thrusters.
1.6. The HST10X Investigation Team List
The References section contains a list of the Team Members with phone
numbers, addresses, and EMAIL addresses.
1.7. Example Schedule
Section 3 is an example schedule for design, development, and launch
of the HST10X enhancement mission resulting from this study. Note
there are some areas where work could be accomplished in parallel allowing
some schedule compression.
2. The Installation Mission
2.1. Mirror Configuration for the Cargo Bay
One major challenge was to design a folding 8-meter class mirror that
will fit in the Orbiter Cargo Bay with room for the Flight Support System
(FSS) required for servicing. The selected mirror configuration is a table-top
fold (two folds) of the 8-meter New Primary Mirror (NPM), the New Secondary
Mirror (NSM) placed inside the fold of the NPM, and the combination mounted
in an existing Space Lab Pallet (SLP). The selected mirror configuration
mounted on the SLP, the New Light-Shield (NLS) support structure, and the
light-shield will all fit in the Orbiter Cargo Bay. In addition, there
is space for a Science Instrument Protective Enclosure (SIPE) mounted to
the Flight Support System (FSS) in a way that allows transporting both
a refurbished Fine Guidance Sensor (FGS), or a replacement Radial Science
Instrument (SI) in its place; and an Axial SI, probably a Coronagraph.
We believe all of this will fit in the Orbiter Cargo Bay, fall within the
weight restrictions, and allow extra fuel for a re-boost. The "Mission
Sequence" section below contains Figures of the Orbiter with the mission
cargo on-board and the Installation Sequence.
2.2. Installation Mission Overview
The example installation mission is in four EVA days following routine
lift-off, rendezvous, capture, and berthing:
Day 1. Remove the forward 3.5-m section of the existing Old Light-Shield
(OLS).
Day 2. Install a New Light-Shield (NLS).
Day 3. Prepare and install the New Mirror Assembly (NMA).
Day 4. Install replacement Science Instruments (SI).
The Extra Vehicular Activity (EVA) work required is similar to that proposed
for the Space Station and has been reviewed by astronaut consultants and
space-system servicing engineers and found to be acceptable.
A major challenge for the installation will be containment of the bolts,
nuts, and washers that attach the section of the old light-shield to be
removed and other debris-containment measures. A large part of a full EVA
day is allocated to removing and retaining these items.
Once the upper 3.5-m section of the OLS is removed, zip nuts and quick
connections will be used to install the NLS lower structure which supports
the deployable NLS. The NLS is deployed radially only at this time and
will be deployed axially later, after the new Primary and Secondary Mirrors
are in place.
Before the New Primary Mirror (NPM) is installed, the new-mirror-assembly
(NMA) interface-ring is installed, it is clamped onto the existing Optical
Telescope Assembly (OTA) structure at the top ring that supports the Old
Secondary Mirror. The NMA snaps into place on this interface ring with
four bolts and the New Light-Shield is deployed axially, completing the
assembly.
2.3. Mission Sequence Description
A mission sequence was designed to test and demonstrate that unpacking,
configuring, and temporary stowage have been properly accounted for in
the design of the Orbital Replacement Unit (ORU) pieces and the layout
of the System Support Equipment (SSE) to accommodate the ORUís and the
HST within the confines of the Orbiter cargo bay. The lift-off mass total
for the payload is 19,725 pounds. Orbiter kits such as the Standard Mixed
Cargo Harness and retention latches for the New Mirror Assembly (NMA) Carrier,
FSS, and Old Light Shield (OLS) (for return to Earth only) are booked against
Orbiter vehicle mass properties.
2.3.1. EVA Day 1, Remove Old Light-Shield (OLS) and Install Ion Thrusters
On EVA Day 1, tilt HST to the 43.8 degree position. Prepare the OLS
for removal and stowage by replacing the old scuff plate with Kit #1 which
serves as the Grapple Fixture (GF) pin and subsequently the slide-out longeron
trunnion. Stage the Keel trunnion Kit #2.
Translate the NLS cylinder package from the back of the FSS to the jettison-capable
parking fixture on the NMA Carrier.
Unbolt the OLS from the HST at 106 places. Install Kit #2. Stow the
OLS into the starboard and keel trunnion Shuttle latches via RMS operation,
and motor those latches closed. Deploy the port trunnion from Kit #1 and
motor that trunnion latch closed as well.
While in a mode where rotating HST is convenient, install the ion thrusters.
The large diameter threaded holes in the existing HST trunnion sockets
make ideal mechanical mounts for the ion thrusters. P601 is co-located
with the starboard socket and circumference harnessing could route power
to the other ion thrusters, bussed off of the first installation.
2.3.2. EVA Day 2, Install New Light Shield (NLS), but do not deploy
axially
On EVA Day 2, remove the truss packages from the faces of the NMA Carrier
and install them in a dispersed fashion around the OLS unoccupied flange,
utilizing the FSS Rotator for access. Remove the NLS furled base from the
SIPE mandrel and install the rods individually to the truss work, again
utilizing the FSS Rotator. Remove the parked NLS cylinder package from
the NMA Carrier fixture and cord-lock it in-place on HST, centered over
the 125" flange hole. Send the command to deploy the radial component of
the NLS cylinder package and adjust the cord locks appropriately to center
the flat cylindrical array of Astromasts onto the base. Fasten the Astromast
canisters to the base at appropriate truss interfaces and inspect the serve
of the Multi-Layer Insulation (MLI) for probability of clean axial deployment.
During the overnight period following EVA Day 2, and preceding Day 3,
the augmentation solar array power Functional Test will be completed.
2.3.3. EVA Day 3, Prepare and install New Mirror Assembly (NMA)
On EVA Day 3, elevate the scissors jack to enable petal positioning.
Fold down and latch the starboard petal.
Two EVA personnel will now work as EVA-RMS (stationed on the robot arm)
and EVA-FF (Free Floater). EVA-RMS works in conjunction with the Remote
Manipulator System Operator, who is conducting Intra-Vehicular Activity
(IVA).
Having gained access to the NMA/OTA Interface-Ring via starboard petal
positioning, remove the interface ring from the launch position on the
NPM latches. IVA RMS Operator/EVA-RMS team will now translate the ring
to the tilted HST flange interface (actually the OTA spider). Clasp the
ring to the spider using the four OTA Clamps.
For the NSM/Mast, EVA-FF will unclasp the NSM/Mast assembly from the
slotted-tub devices and the IVA RMS Operator/EVA-RMS team will translate
it up and over the port wing. While staged in this manner, EVA-FF will
fold down and latch the portside petal of the NPM. The IVA RMS Operator/EVA-RMS
team will then translate and rotate (by hand) the NSM/M in-board to match
up the latch sets to the NPM. EVA-FF engages the four latches which anchor
the NSM/M gussets to the mirror base. The New Mirror Assembly is now assembled.
The IVA RMS Operator/EVA-RMS team now maneuvers EVA-RMS to the underside
of the New Mirror Assembly (NMA) to enable grasping of the underside handholds
most suited for translation to HST. EVA-FF releases the single-fastener
clamp holding the NMA to the Carrierís tub. Closing of the scissors jack
is not necessary for landing.
While the IVA RMS Operator/EVA-RMS team translates the NMA to HST, EVA-FF
translates their person to the flange as well. Meeting at the flange, the
IVA RMS Operator/EVA-RMS team positions the NMA with EVA-FF assisting in
fine-motion detail. After nesting the clamp halves, EVA-FF closes the single
fastener clamp, gaining access via standard hand-tool extensions. Electrical
connectors are mated and the command to activate the axial-deploy portion
of the NLS is issued.
2.3.4. EVA Day 4, Swap SI's
On EVA Day 4, swap the new Axial SI for NICMOS and swap the new Radial
SI for FGS #3. These are standard SI replacements as conducted on SM-1,
SM-2, and SM-3A/B. Co-location of NICMOS and FGS #3 expedite this operation.
2.3.5. Deploy HST
Following overnight Functional Tests of the wavefront correcting actuators
and the new SI's, HST10X is deployed via the RMS.
2.4. Mission Sequence Pictorial
3. HST10X Example Schedule
References:
1. The Hubble Deep Field (HDF) , http://www.stsci.edu/ftp/science/hdf/hdf.html
2. UV-Optical Working Group (UVOWG) White Paper, (Mike Shull, Chair)
titled, The Emergence of the Modern Universe: Tracing the Cosmic Web (Shull
et al. 1999: see http://casa.colorado.edu/~uvconf/UVOWG.html
3. Hubble Space Telescope, Optics Performance Enhancement, Preliminary
Feasibility Study Summary Report, http://www.pha.jhu.edu/groups/hst10x
4. The Origins Program, http://origins.jpl.nasa.gov
5. HST10X Study Team
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Jim Crocker
* Don Dufford
