1. Executive Summary
1.1. Overview
1.1.1 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 report describes a feasibility study for augmenting HST with 8-meter class optics. The results are very interesting, and surprising.

1.1.2. Background

An 8-meter class enhancement to HST would provide the capability to do things like DETECT EARTH-LIKE PLANETS ORBITING NEARBY STARS at distances up to 10 parsecs and perform spectroscopic examination of Earth-like planets to search for atmospheric oxygen, a certain sign of life, out to about 5 parsecs; and 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.

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.

1.1.3. 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. This initial feasibility study was conducted in mid 1999 and was limited to two months.

1.1.4. Study Teams

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 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.

1.1.6. 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. 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.

1.1.7. Study Results Summary

Is it feasible? The short answer is yes.

The Science that would be enabled is stunning.  It is detailed in Section 2.

The study mission design retains full functionality of HST and all current and planned instruments, as well as a full fall-back contingency.

A primary enabling factor is the design of the existing light-shield structure. It is designed to be separated at a location that both gives access to the existing mirror support structure for attachment of the new mirror assembly, and to attach the new light-shield in a completely non-destructive process.

Space-servicing and astronaut operations specialists find the example mission acceptable.

There are three 8.2-meter mirror blanks left over from the VLT that will be suitable for this application. The use of one of these could reduce the time, cost, and risk of the enhancement.

The added mass and Moment-of-Inertia (MOI) shift are well within control system margins. The added mass would be of order 3,500 pounds, approximately 15% of the current HST, and the MOI shift is of order 1.25 meters.

Structural dynamics analysis found no severe impact.

Thermal effects will be inconsequential except for the solar arrays which will operate about 20 degrees C warmer due to reduced space view. This is not prohibitive but could be mitigated by several low-cost processes if desired.

The FGS's will work. The reduced FOV will be mitigated by the capability to use guide stars 2.5 v-magnitudes dimmer.

The increased orbit decay rate will require either a re-boost mission during the solar max. period or a stationkeeping system. There is a readily available stationkeeping system with demonstrated operation on other missions that is included in the weight budget.

The design, development, and construction could be done in a five- to eight-year time frame, perhaps less depending on funding.

The cost will be of order less than half that of an entirely new spacecraft and observatory.

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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 150 orbits in 10 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.

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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.

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 installation mission is in four Extra Vehicular Activity (EVA) days following routine lift-off, rendezvous, capture, and berthing:

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 3, The Installation Mission.

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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.

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.

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.

 Figure 1.4.1, A ray-trace diagram of the "add-on" telescope, click here.

 Figure 1.4.2, The two-fold Mirror Concept and
 Figure 1.4.3, The Afocal Telescope Concept, click here

And all this will fit in the Orbiter Cargo Bay as shown in Figure 1.4.4.

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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.

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

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. Recall that the solar arrays will be replaced in the normal maintenance mission planned for 2003.

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.

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.

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

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

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.  Team List

 Section 6 is a list of the study Team Members.

1.7.  Example Schedule

Section 7 is an example schedule for design, development, construction, and launch of the HST10X mission resulting from this study. Note there are some areas where work could be accomplished in parallel allowing some schedule compression.