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June
25, 2007: NASA's next moon rocket is still on the
drawing board, but already scientists are dreaming up big
new things to do with it.
"The
Ares V rocket will be able to launch missions whose volume
or mass or both can be handled no other way," says Philip
Stahl, an internationally respected optical engineer now at
NASA's Marshall Space Flight Center. Maybe, he says, we should
use it "to launch big space telescopes."
How
big? Consider the following: Ares V will be able to place
almost 130,000 kg (284,000 lbs; 8% more than the Saturn V
rocket of the 1960s) into low Earth orbit. Designed to deliver
cargo to the Moon, the rocket would be large enough to carry
primary mirrors 8+ meters wide. For comparison, Hubble's mirror
measures 2.4 m.
Above:
A 6- to 8-meter space telescope would dwarf the Hubble Space
Telescope. Key missions would include searching for and exploring
earthlike planets in deep space. (NASA)
"How
does a typical astrophysicist work?" Stahl asks. "He
builds a giant telescope on top of a mountain and uses it
for decades, and every few months or years he swaps out instruments
or does other upgrades to keep it going." The
Hubble Space Telescope operates in this fashion, with the
space shuttle doing the servicing and Earth-orbit playing
the role of mountain peak.
But
Stahl wants to go beyond Earth orbit, far beyond, to the L2
Sun-Earth Lagrange point.
A
Lagrange
point is, basically, a parking spot in space. If you put
a spacecraft at a Sun-Earth Lagrange point, it remains in
a fixed position relative to the Sun and Earth. 18th-century
mathematician Josef Lagrange showed that there are five such
points, illustrated in the diagram below.
L1,
located 1.5 million km sunward of Earth, is a good place for
solar observatories. The Solar and Heliospheric Observatory
(SOHO), for example, is there now and enjoys a 24/7 view of
the sun.
L2
lies in the opposite direction, 1.5 million km above the nightside
of Earth. A key advantage of L2 is that the Sun, Earth and
Moon are concentrated in one small part of the sky, giving
any telescope located there a wide and unobstructed view of
deep space. The Wilkinson Microwave Anisotropy Probe (WMAP)
is stationed at L2 and others will eventually join it.
Right:
Earth-Sun Lagrange points. Click on the image to view all
five, L1-L5. [More]
"L2
is a place in space where we want to place a lot of telescopes,"
Stahl continues. So "why don't we treat it as a mountaintop?"
with the telescope's satellite bus providing all the services
of a real mountaintop facility.
Thus,
Stahl, Marc Postman of the Space Telescope Science Institute,
and others within the space science community are thinking
big.
Wish-list
missions for the Ares V range from a 150-meter-wide (492 ft)
radio telescope dish to detect whispers from deep space to
a 5-meter cube of super-pure water encased in light detectors
to assay cosmic rays by their light flashes as they crash
through the water. An optical telescope with a primary mirror
up to 8 m (26 ft.) in diameter could search star populations
in the Milky Way and nearby galaxies for the "fossil
record" of their evolution. It could also hunt for "Earthshine
spectra," faint signs of life in the light reflected
by exoplanets.
The
resolution of the telescope's images would be more than three
times sharper than those of Hubble. More important, the mirror
would see about 11 times fainter than Hubble because the area
of the mirror would be 11 times greater.
Below:
A cutaway diagram of the large monolithic space telescope
shows that most of it is empty space, leaving designers plenty
of margin in equipping the systems and instrument modules.
(NASA)
Until
now, such a mirror was too big to consider. The next-generation
James Webb Space Telescope -- also headed for L2 -- was regarded
as the path for future large space telescopes. Its 6.5-m primary
mirror will consist of carefully folded segments that precisely
align once on station. But future Ares V payload shrouds up
to 12 m (39.4 ft) have been envisioned by NASA planners. That
allows Stahl to consider an off-the-shelf mirror, like the
single-piece, 8-m (26.2 ft) primaries in the ground-based
Gemini telescopes.
While
increasing size, the Ares V would decrease risk. "The
constraints of current launch vehicles place risks on technical
performance, cost, and schedule to get a lot out of a small
package," Stahl explains. The generous size and mass
afforded by the Ares V all but eliminates those constraints
for most payloads.
He
also sees servicing as a key element.
"Why
design for 10 to 15 years?" Stahl asks. "Let's design
so you can swap the instruments periodically and go for 50
years." The bus section -- controls and instruments --
will be small enough that replacements could be sent by smaller
launch vehicles and equipped to replace all the serviceable
components and start a new scientific observing campaign.
In
Postman's words, that would "make L2 the ultimate astronomical
summit."
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Production Editor:
Dr. Tony Phillips | Credit: Science@NASA
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