DISTANT DISCOVERIES
NEXT GENERATION SPACE TELESCOPE
The successor to the Hubble Space Telescope is named the James Webb
Space Telescope in honor of NASA's second administrator.
Hubble allowed us to see like no other telescope ever has. It shared
with us the splendor of the cosmos, such as the birth and death of stars,
the intricacies of other galaxies, and the universe as a toddler.
About eight years from now the James Webb Space Telescope will take
over for Hubble. With a mirror more than twice the diameter of Hubble's,
it will probe even farther into the universe and further back in time.
James E. Webb headed NASA from 1961 to 1968. During his tenure, NASA
developed the Apollo program and launched more than 75 space science
missions, including the first interplanetary explorers. Webb also believed
that a major space telescope should be a priority for NASA.
TRW Inc. of Redondo Beach, California, will design and build the spacecraft
and its 6-meter (20-foot) primary mirror for $825 million.
James Webb Space Telescope will travel to the L2 Lagrange point located
940,000 miles (1.5 million kilometers) from Earth, opposite the sun.
This position will be gravitationally stable and allow the telescope
to use a shield to block radiation from the sun, Earth, and the moon.
This will also allow the spacecraft to remain cool without using a complex
refrigeration system, but the telescope will also be too far away for
shuttle astronauts to service the telescope as they do with Hubble.
With six times the light-gathering area as its predecessor, the new
telescope will be able to view objects that are younger and nearer to
the universe's birth than anything we've ever seen. But unlike Hubble,
which observes in ultraviolet, visible, and near-infrared wavelengths,
the James Webb Space Telescope will concentrate on the far visible to
the mid-infrared range of the electromagnetic spectrum. It will be able
to study the evolution of the first galaxies, probe disks around young
stars for signs of forming planets, examine supermassive black holes,
and make discoveries astronomers haven't yet imagined.
NEXT GENERATION PROPULSION
With space probes going ever farther conventional propulsion systems
are proving way too inefficient. NASA has gone looking for more exotic
ways to fly.
Chemical rockets are slow. They burn their propellant at the beginning
of a flight and then the spacecraft just coasts the rest of the way.
Although spacecraft can be sped up by gravity assist--a celestial crack-the-whip
around planets, such as the one around Saturn that flung Voyager 1 to
the edge of the solar system--round-trip travel times between planets
are still measured in years and decades. A journey to the nearest star
would take centuries if not millennia.
Chemical rockets are fuel-inefficient. Think of driving in a gas guzzler
across a country with no gas stations. You'd have to carry boatloads
of gas and not much else. In space missions, what you can carry on your
trip that isn't fuel (or tanks for fuel) is called the payload mass--e.g.,
people, sensors, samplers, communications gear and food. Just as gas
mileage is a useful figure of merit for the fuel efficiency of a car,
the "payload mass fraction"--the ratio of a mission's payload
mass to its total mass--is a useful figure of merit for the efficiency
of propulsion systems.
With today's chemical rockets, payload mass fraction is low. Even using
a minimum-energy trajectory to send a six-person crew from Earth to
Mars, with chemical rockets alone the total launch mass would top 1,000
metric tons--of which some 90 percent would be fuel. The fuel alone
would weigh twice as much as the completed International Space Station.
A single Mars expedition with today's chemical propulsion technology
would require dozens of launches--most of which would simply be launching
chemical fuel.
In other words, low-performance propulsion systems are one major reason
why humans have not yet set foot on Mars.
More efficient propulsion systems increase the payload mass fraction
by giving better "gas mileage" in space. Since you don't need
as much propellant, you can carry more stuff, go in a smaller vehicle,
and/or get there faster and cheaper. We need advanced propulsion technologies
to enable a low-cost mission to Mars.
Thus, NASA is now developing ion drives, solar sails, and other exotic
propulsion technologies using two basic approaches. The first is to
develop radically new rockets that have an order-of-magnitude better
fuel economy than chemical propulsion. The second is to develop "propellant-free"
systems that are powered by resources abundant in the vacuum of deep
space.
All these technologies share one key characteristic: They start slowly,
They rely on the fact that a small continuous acceleration over months
can ultimately propel a spacecraft far faster than one enormous initial
kick followed by a long period of coasting.
They're all systems with low thrust but long operating times. After
months of continuing small acceleration, you'd be clipping along at
many miles per second. In contrast, chemical propulsion systems are
high thrust and short operating times. After the initial lift off, the
tank is empty.
Leading candidates for the advanced rocket are variants of ion engines.
In current ion engines, the propellant is a colorless, tasteless, odorless
inert gas, such as xenon. The gas fills a magnet-ringed chamber through
which runs an electron beam. The electrons strike the gaseous atoms,
knocking away an outer electron and turning neutral atoms into positively
charged ions. Electrified grids with many holes (15,000 in today's versions)
focus the ions toward the spaceship's exhaust. The ions shoot past the
grids at speeds of up to more than 100,000 miles per hour (compare that
to an Indianapolis 500 racecar at 225 mph)--accelerating out the engine
into space, so producing thrust.
Where does the electricity come from to ionize the gas and charge the
engine? Either from solar panels (solar electric propulsion) or from
fission or fusion (nuclear electric propulsion). Solar electric propulsion
engines would be most effective for robotic missions between the sun
and Mars, and nuclear electric propulsion for robotic missions beyond
Mars where sunlight is weak or for human missions where speed is of
the essence.
Ion drives work. They've proven their mettle not only in tests on Earth,
but in working spacecraft--the best-known being Deep Space 1, a small
technology-testing mission powered by solar electric propulsion that
flew by and took pictures of Comet Borrelly in September, 2001. Ion
drives like the one that propelled Deep Space 1 are about 10 times as
efficient as chemical rockets.
The lowest-mass propulsion systems, however, may be those that carry
no on-board propellant at all. In fact, they're not even rockets. Instead,
they relying on natural resources abundant in space for energy. The
two leading candidates are solar sails and plasma sails. Although the
effect is similar, the operating mechanisms are very different.
A solar sail consists of an enormous area of gossamer, highly reflective
material that is unfurled in deep space to capture light from the sun
(or from a microwave or laser beam from Earth). For very ambitious missions,
sails could range up to many square kilometers in area.
Solar sails take advantage of the fact that solar photons, although
having no mass, do have momentum--several micronewtons (about the weight
of a coin) per square meter at the distance of Earth. This gentle radiation
pressure will slowly but surely accelerate the sail and its payload
away from the sun, reaching speeds of up to 150,000 miles per hour,
or more than 40 miles per second.
A common misconception is that solar sails catch the solar wind, a
stream of energetic electrons and protons that boil away from the Sun's
outer atmosphere. Not so. Solar sails get their momentum from sunlight
itself. It is possible, however, to tap the momentum of the solar wind
using plasma sails.
Plasma sails are modeled on Earth's own magnetic field. Powerful on-board
electromagnets would surround a spacecraft with a magnetic bubble 15
or 20 kilometers across. High-speed charged particles in the solar wind
would push the magnetic bubble, just as they do Earth's magnetic field.
Earth doesn't move when it's pushed in this way--the planet is too massive.
But a spacecraft would be gradually shoved away from the Sun.
Of course, the original, tried-and-true propellant-free technology
is gravity assist. When a spacecraft swings by a planet, it can steal
some of the planet's orbital momentum. This hardly makes a difference
to a massive planet, but it can impressively boost the velocity of a
spacecraft. For example, when Galileo swung by Earth in 1990, the speed
of the spacecraft increased by 11,620 mph; meanwhile Earth slowed down
in its orbit by an amount less than 5 billionths of an inch per year.
Such gravity assists are valuable in supplementing any form of propulsion
system.
Slowing down and stopped presents another problem for propulsion systems.
With chemical propulsion, the usual technique is to fire retrorockets--once
again, requiring large masses of onboard fuel.
A far more economical option is promised by aerocapture--braking the
spacecraft by friction with the destination planet's own atmosphere.
The trick, of course, is not to let a high-speed interplanetary spacecraft
burn up. But NASA scientists say, with an appropriately designed heat
shield, it would be possible for many missions to be captured into orbit
around a destination planet with just one pass through its upper atmosphere.
A hybrid of several technologies could prove to be very economical
in getting a manned mission to Mars. In fact, a combination of chemical
propulsion, ion propulsion, and aerocapture could reduce the launch
mass of a six-person Mars mission to below 450 metric tons (requiring
only six launches)--less than half that attainable with chemical propulsion
alone.
Such a hybrid mission might go like this: Chemical rockets, as usual,
would get the spacecraft off the ground. Once in low-Earth orbit, ion
drive modules would ignite, or ground controllers might deploy a solar
or plasma sail. For 6 to 12 months, the spaceship--temporarily unmanned
to avoid exposing the crew to large doses of radiation in Earth's Van
Allen radiation belts--would spiral away, gradually accelerating up
to a final high Earth-departure orbit. The crew would then be ferried
out to the Mars vehicle in a high-speed taxi; a small chemical stage
would then kick the vehicle up to escape velocity, and it would head
onward to Mars.
As Earth and Mars revolve in their respective orbits, the relative
geometry between the two planets is constantly changing. Although launch
opportunities to Mars occur every 26 months, the optimal alignments
for the cheapest, fastest possible trips happen every 15 years--the
next one coming in 2018.
|
Kahl
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AstroNews
In
coming months early bird observers can spot the Moon’s most spectacular
"hidden" landform. MARE ORIENTALE is a patch of dark lava
flow on the western limb of the Moon. When this part of the lunar surface
is in view, observers can glimpse its massive mountains projecting above
the limb.
