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The Advantages of a Maneuverable, Long Duration,
High Altitude Astrophysics Platform
R.A. Fesen
Dartmouth College
Department of Physics and Astronomy
The Main Point
A high-altitude, balloon-borne optical telescope could
generate high resolution images on a par with HST.
But do it at a tiny fraction of the cost.
The Hubble Space Telescope
The Hubble Space Telescope’s high angular
resolution at optical wavelengths has made it a
extraordinarily powerful research tool in many
sub-fields of Astronomy.
HST is NASA’s best known spacecraft with its
images seen worldwide.
But Hubble is a relatively small telescope.
What makes Hubble so great? There are much larger
telescopes in the world.
The primary mirror of Keck I. Thirty-six hexagonal segments are joined to form the
10-m mirror. (Notice the person on the crane in front of the mirror to set the scale.)
The twin Keck I and II telescopes are the largest optical telescopes in the world.
The Hubble Space Telescope doesn’t rank among the World’s 25
largest telescopes
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11.8m (2 x 8.4m) LBT (05)
10.4m GTC LaPalma (05)
2 x 10.0m Keck I and II
10.0m SALT (05)
9.2m HET
8.3m Subaru
4 x 8.2m VLT (ESO)
8.1m Gemini North (40%)
8.1m Gemini South (40%)
6.5m MMT
2 x 6.5m Magellan
6.0m BTA
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2.4m Hubble
5.0m Hale
4.2m WHT
4.2m SOAR (30%)
4.0m CTIO (100%)
3.9m AAT
3.8m UKIRT
3.8m KPNO (100%)
3.6m ESO
3.6m CFHT
3.6m Telescopio Galileo
3.5m WYIN (40%)
3.5m ARC
3.5m NTT
Hubble’s Main Advantages:
1. High resolution optical imaging: FWHM = 0.03-0.05’’
2. UV/Optical spectra at sub-arcsec angular resolution
3. UV sensitivity: 1160 – 3000 Angstroms
4. Low background in the near IR (1.0 – 2.5 microns)
Hubble’s Main Advantages:
1. High resolution optical imaging: FWHM = 0.03-0.05’’
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UV/Optical spectra at sub-arcsec angular resolution
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UV sensitivity: 1160 – 3000 Angstroms
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Low background in the near IR (1.0 – 2.5 microns)
The value of High Angular Resolution is obvious.
The value of High Angular Resolution is obvious.
14,000 ft
250 miles
We place telescopes on high mountains.
But wouldn’t it better better from an airplane?
A picture taken from 51,000 ft aboard the Concorde
Stratospheric Unmanned Aircraft: UAVs.
There’s currently considerable interest in UAVs in the
military and in the general aerospace community, mostly
in aircraft-type vehicles like Predator and Global Hawk.
Predator B (Altair)
Global Hawk
But these platforms are poorly suited for most science
missions. They are expensive, not very stable, and have
short flight times.
Just how high up do you have to go to avoid all
clouds and stormy weather and start having
space-like astronomical observing conditions?
A photo taken from the window of a TR-1 (U2)
aircraft from an altitude of around 75,000 ft.
Lake Tahoe, CA
View of Oregon from a
U2 flying at 70,000 ft.
The cloud deck is more
than 30,000 ft below.
There is simply no
weather above 60 kft.
And the sky gets very
dark.
Stratospheric Winds vs Altitude
Nearly all HAPs are designed to operate in the 60-70 kft
altitude range where wind speeds tend to be the lowest.
National Weather Service
The sweet spot in altitude for an airship serving as an astronomical
observing platform is 65 - 85 kft.
Altitude
0 km
0 ft
Pressure Density Temperature
1013 mbars 1.2 kg/m3
+15 C +60 F
4.4 km 14,000 ft
620 mbars 0.82 kg/m3
-11 C +10 F
12.2 km 40,000 ft
185 mbars 0.31 kg/m3
-57 C
-71 F
15.2 km 50,000 ft
115 mbars 0.19 kg/m3
-56 C
-69 F
18.3 km 60,000 ft
70 mbars 0.12 kg/m3
-56 C
-69 F
19.8 km 65,000 ft
55 mbars 0.088 kg/m3 -56 C
-69 F
21.3 km 70,000 ft
45 mbars 0.075 kg/m3 -56 C
-69 F
24.4 km 80,000 ft
28 mbars 0.042 kg/m3 -52 C
-62 F
25.9 km 85,000 ft
22 mbars 0.034 kg/m3 -51 C
-60 F
27.4 km 90,000 ft
17 mbars 0.027 kg/m3 -49 C
-56 F
SOFIA
HA Airship
Altitude:
41 kft
Primary Mirror Diameter:
Image quality:
2.5 m (~ HST )
80 kft
0.5 m
~ 3” @ 5 microns
~2.5” @ 5 microns
> 15 microns
> 0.3 micron
Diffraction image quality:
Wavelength regime:
0.3 – 1600 microns
0.3 -1.0 microns
Number of observing hrs per yr:
960 hrs (~ 8 hrs per night)
~3000 hrs
Development/Construction Cost:
$482M
?
Est. Operations costs per year:
~$50M
small; $2-3M
Even at an altitude of 14 km (46 kft),
nearly a mile above where NASA’s
SOFIA 747SP will fly, there are still
many strong atmospheric absorption
features between 5 – 10 microns.
And these can vary with time.
But nearly all these features go away
when flying at 28 km (90 kft), with little
gained by flying higher than this.
Airship
SOFIA
Mauna Kea
A 70 kft High-Altitude, Station-Keeping LTA Platform
• Advantages:
• Offers spacecraft-like optical imaging capabilities.
• No ground-site to purchase or develop.
• No LDB/ULDB “no-fly” zone worries.
• No weather interference; robust target scheduling.
• Little atmospheric extinction; superb photometric conditions.
• Locations near the Equator offers both N & S hemisphere
target viewing.
• True horizon-to-horizon observing is possible.
• Little scattering of moonlight; i.e., largely darktime observing.
• Simple line-of-sight 24/7 communications to platform.
Several commercial telecommunication firms together with a
well funded US military project are aiming at making
autonomous, high-altitude, lighter-than-air (LTA) vehicles
which can maneuver and station-keep for weeks to months.
Such platforms may be a reality in a few years.
A 0.5 m (20-inch) telescope mounted on such a
high-altitude platform could generate highresolution optical images superior to any groundbased facilities.
HELIOS is a solar powered, propeller-driven, ultralightweight
aircraft reached an altitude of 96,800 ft in August 2001.
Built by AeroVironment
DoD is spending $40M just on a 9 month PDR study for
a design of a long-duration, station-keeping airship.
It must fly unmanned autonomously at 70,000 feet for 1 – 6
months. Payload weight: 2 tons with 10 – 15 kWatts of
power available to the payload. Fly at mid-latitudes.
NORAD and MDA liked Lockheed-Martin’s design, an
wanted to spend $50M on a 2-yr CDR program, followed by
$9M for construction of a full-scale prototype.
The telescope could be mounted in
between the two airships, allowing for
nearly unobstructed viewing.
A 70 kft High-Altitude, Station-Keeping LTA Platform
• Engineering Obstacles
• Platform engine and payload power requirements; “Sprint &
Drift” mode.
• LTA envelope fabric strength and UV + ozone durability.
• Launch and recovery procedures.
• Lightweight telescope + precise pointing and tracking system.
• Ability to slew telescope quickly without re-positioning the
airship.
One could go even higher than 70 kft, but…
The higher up one goes, the larger the LTA vehicle needed.
The heavier the payload, the larger the LTA vehicle needed.
And the bigger the airship, the harder it will be to fly/push
against the stratospheric winds in order to station-keep.
A High-Altitude Astronomical Observatory
would be basically a “Hubble-Junior”
We know that if we go up to 100 kft, the image
quality will be space-like. However, it is unlikely in
the near future to build a high altitude science
platform that can station-keep at these altitudes.
So:
1. Can we do excellent science at 65-75 kft?
2. Can we make optical observations during the
day? That is, could we operate 24/7 ?
3. How dark is the sky at 65 kft at night and day?
A High-Altitude Astronomical Observatory
1. Sky brightness measurements
2. How does the sky brightness overhead
change with altitude
3. Is it possible to observe during the day? Is
the sky dark enough? Can one image stars
during the daytime at 65+ kft ?
4. Payload stability.
Station Keeping: Sprint and Drift
wind
Rc
upwind position
station
Rd
V
station - desired position over ground
upwind position - nav algorithm
attempts to maintain this position
Rd - drift radius, determined by
wind/speed equation
Rc - control radius for navigation
control algorithm
Vehicle V drives for station until within
distance Rd, then upwind position becomes
new target. V continues at desired ground
speed to upwind position until within Rc
then desired ground speed becomes zero.
Now Flying:
A small prototype (but science-sized!) is already flying.
South Korea’s Station-Keeping HA Airship Program by
the Korean Aerospace Research Institute & Worldwide
Aero Corp. Size: 50 m x 12 m
The telescope could be mounted in between the two
airships, allowing for nearly unobstructed viewing.
Summary:
A High-Altitude Astronomical Airship Platform
• Besides space-like optical imaging capabilities, an Astro-HAP offers:
• No ground-site to purchase or develop.
• No weather interference; robust target scheduling
• Little atmospheric extinction; superb photometric conditions
• True horizon-to-horizon observing is possible.
• Little scattering of moonlight; i.e., longer darktime observing runs
• Simple line-of-sight communications 24/7
• Locations near the Equator offer both N & S hemisphere target viewing
• Engineering Obstacles
• Platform engine and payload power requirements; “Sprint & Drift” mode
• LTA envelope fabric strength and UV + ozone durability
• Launch and recovery procedures
• Lightweight telescope + precise pointing and tracking system
• Ability to slew telescope quickly without re-positioning the airship
A High-Altitude, Astronomical Platform
• Advantages:
• No ground-site to purchase or develop.
• No weather interference; robust target scheduling
• Little atmospheric extinction; superb photometric conditions
• True horizon-to-horizon observing is possible.
• Little scattering of moonlight; i.e., longer darktime observing runs
• Simple line-of-sight communications running 24/7.
• Engineering and Environment Obstacles
• Platform engine and payload power requirements
• LTA envelope fabric strength and UV + ozone durability
• Night-time battery, fuel-cell capacity
• Launch and recovery procedures
• Lightweight telescope + precise pointing/tracking system
• Ability to slew telescope without re-positioning the airship
SOFIA: Stratospheric Observatory For Infrared Astronomy
Altitude:
Primary Mirror Diameter:
Image quality:
41 kft
2.5 m (~ HST )
~ 3” @ 5 microns
Diffraction image quality:
Wavelength regime:
> 15 microns
0.3 – 1600 microns
Number of observing hrs per yr:
960 hrs (~ 8 hrs per night)
Development/Construction Cost:
$482M
Est. Operations costs per year:
~$40M
How high up do you have to fly?
A photo taken from the window of a TR-1 (U2) aircraft
from an altitude of around 75,000 ft.
SOFIA: Stratospheric Observatory For Infrared Astronomy
Altitude:
Primary Mirror Diameter:
Image quality:
41 kft
2.5 m (~ HST )
~ 3” @ 5 microns
Diffraction image quality:
Wavelength regime:
> 15 microns
0.3 – 1600 microns
Number of observing hrs per yr:
960 hrs (~ 8 hrs per night)
Development/Construction Cost:
$482M
Est. Operations costs per year:
~$40M
Instead of a balloon gondola arrangement which blocks out part of the sky, a high-altitude
scientific airship platform might employ a double-hulled catamaran design.
A solar powered, superpressure Airship platform
Platform Communications Corp.
But…
The higher up one goes, the larger the LTA vehicle needed.
The bigger the payload mass, the larger the LTA vehicle
needed. And the bigger the airship, the harder it will be to
fly/push against the stratospheric winds.
So…
How high do you have to get to get Hubble-like images?
Can a LTA vehicle be maneuverable (i.e. station-keeping)
and have a long duration flight time?
Can a lightweight, moderate-size telescope be built to fly on
a LTA Airship and be stable enough for precise pointing and
image tracking over much of the sky?
Now: How do you do this?
…leverage prior & current UAV Research.
 Many technology challenges have been solved. A Hybrid Airship concept leverages
many prior NASA projects: e.g., ULDB balloon fabrics, HA airship designs (SWRI), HA
solar powered aircraft (AeroVironment), thin-film solar arrays (ITN Energy Systems), &
“Blind Pointer” (NASA/AMES).
 DoD has fast-tracked $90M (PDR/CDR) for a similar but far larger & much more
powerful airship design for use by NORAD and Missile Defense Agency being
developed by Lockheed-Martin. Military Reqs: 4000 lb payload & 10-15 kilowattts of
payload power; to be located at mid-latitudes with flight durations of at least 6 months
up to 5 yrs.
Can one really build a HA Airship for astronomy?
• The platform must:
• Be stable enough for subarcsec resolution images
• Be able to view most if not much of the sky
• Carry a relatively large, lightweight telescope
• Require no expendables (e.g., LN2)
Lightweight Telescope and Pointing/Guiding System
Telescope and Optical Imaging Camera:
• Ball Aerospace’s 1.3m Beryllium AMSD mirrors (10.3 kg/m^2) or a silicate
graphite mirror
• CCD camera + thermal electric cooler (no dewar; no LN2). Ambient air
temperature ( -55 C)
• An array of four 8128 x 8128 CCD with 0.1’’/pixel would provide a FOV of nearly
30 arcmin square. But no dewar needed! If scale was set at 0.2”/pixel, image would
be 1 degree square.
• Telescope + CCD camera + filter wheels: <100 kg.
Lightweight Telescope and Pointing/Guiding System
Stability and Precise pointing:
• NASA Ames “Blind Pointer” – 10 arcminute accuracy
• NASA/Univ. Wisconsin Starfinder: ~2 arcsec accuracy
• Commerical image stabilization platforms/software
• Tip/tilt secondary mirror for fine guiding and image control
A High Altitude Science Platform would leverage several
technological developments from prior NASA and DoD projects
• LTA Vehicle (Balloon) fabrics:
• ULTRA Long Duration Balloon Project (NASA/Wallops)
• Airship design/hardware, and command and control software
• Army’s Sounder project (SWRI/DoD)
• High Altitude propulsion: (lightweight motors and efficient props)
• Pathfinder and Helios (AeroVironment and NASA/ERAST)
• Lightweight Photovoltaic cells (Solar Cells)
• Pathfinder and Helios (AeroVironment and NASA/ERAST)
• Lockheed-Martin’s DoD high altitude airship (e.g., United Solar
Ovonic’s ultra-light, amorphous, thin-film triple junction PV
modules)
There has long been interest in building such a vehicle from various governmental agencies and the
telecom industry.
The first real attempt was made by Raven Industries in the late 60’s with a project called High Platform
II. This solar powered 20 knot, 136 lb aerostat had control surfaces attached to its Mylar hull and was
capable of operating for more than 6 months at 70,000 ft.
This experimental solar powered airship was
flown successfully at 70,000 ft. Note the
solar cell array on the nose.
The gondola provided the support for the
mechanical components of the propulsion
system.
Summary:
1. Optical imaging would greatly raise the visibility of NASA’s balloon
program with the public and in the science communities.
2. A HA airship with a 1-2 meter astronomical telescope would generate space quality data and could be
built by leveraging many technology developments from previous NASA and on-going DoD programs.
3. Development costs could be rather modest relative to the potential
science impact.
4. A maneuverable, long duration, high altitude platform has applications outside of Astronomy, e.g.,
Earth Science, Atmospheric Science.
Supplemental slides
Large, ultra lightweight mirrors
are now available
The VLT 1.1 meter secondary mirrors are Beryllium mirrors.
Each has a total weight 51 kg with mirror assembly.
The 0.9 m SIRTF mirror is also made of Beryllium. The whole
Ritchey-Chretien design telescope weights less than 50 kg.
VLT
SIRTF
The high cost of spacecraft development, deployment, and operations have kept current
UV/Optical/IR space observatories down to just a handful.
•
•
•
•
2.40 m Hubble Space Telescope (end ~2007)
0.85 m SIRTF/Spitzer Space Telescope (end before 2009)
0.75 m (4 x 0.35 m) FUSE (end ~2006)
0.50 m Galex (end ~2006)
Q2) Can one build a maneuverable, long-duration LTA
science platform suitable for astronomical imaging?
2001 NASA Study:
Cross Enterprise Technology Development Program
A high Altitude, Station-Keepping Platfor for Astronomy
Winds are
slowest around
altitudes near
16 - 20 km.
StratSat is being built by ATG and is intended to remain
airborne for up to five years at 60,000 ft, undertaking narrow
and broadband communications as part of a much larger
global communications and surveillance network.
It will have a solar array and a 450 hp diesel engine as a
propulsion unit. Overall length is 656 ft (200m). Completion
of the first vehicle is expected in 2004/2005.
HELIOS is a solar powered, propeller-driven, ultralightweight
aircraft reached an altitude of 96,800 ft in August 2001.
Built by AeroVironment
HA Airships are also being looked at as communication platforms
The cost of building, launching, and establishing communication links for
conventional communication satellites, plus the expenses and environmental
issues with numerous cell phone towers have lead some telecommunication
companies to look closer to Earth for cheaper solutions.
Company
Country
SkyStation
USA
ATG
UK
Wireless ISG
Japan
Platform Wireless
USA
Platform Name
Sky Station
StratSat
Stratospheric PS
Airborne Relay Comm