An artist's conception ofthe
Kepler spacecraft. (NASA Photo)
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Backgrounder
Introduction to Kepler
instrument Kepler's mission
overview, scientific objectives
by Ren Haijun
WASHINGTON, March 6 (Xinhua) -- NASA's Kepler space telescope, the world's
first mission to hunt for extra solar Earth-like planets, was launched aboard a
Delta II rocket Friday night from Cape Canaveral Air Force Station in Florida.
After the clock ticked down to liftoff, the Delta II's first-stage main
engine and six strap-on solid rocket boosters ignited, the NASA TV shows.
Three remaining boosters ignited 65.5 seconds later, and the first-stage
main engine continued to burn for 4.5 minutes. The second stage then ignited,
carrying Kepler into a circular orbit 115 miles above the Earth less than 10
minutes after launch.
After coasting for 43 minutes, the second-stage engine fired again,
followed by second-stage shutdown and separation. The third stage then burned
for five minutes.
Sixty-two minutes after launch, the 591-million-U.S.-dollar Kepler
separated entirely from its rocket and reached its final Earth-trailing orbit
around the sun at the altitude of more than 721 kilometers (448 miles), NASA
said. The orbit is similar to that of NASA's Spitzer Space Telescope.
"Everything seems normal," said NASA TV commentator George Diller. "(It's
a) Great Friday night for people to watch a launch from the beach."
"It was just magnificent. It looked like a star was being formed in the
sky," said Bill Borucki, science principal investigator for the mission at
NASA's Ames Research Center at Moffett Field, California. "Everybody was
delighted, everybody was screaming, 'Go Kepler!'"
The spacecraft is designed to find Earth-size planets orbiting stars in
habitable zones -- regions where water could pool on the surface of the planets.
Liquid water is believed to be essential for the formation of life. It will
watch a patch of space for 3.5 years or more for signs of Earth-sized planets
moving around stars similar to the Sun. The patch that Kepler will watch
contains about 100,000 stars like the Sun.
Its instrument is a specially designed telescope of 0.95 meters in diameter
which is called a photometer or light meter, the largest ever flown in space. It
has a very large field of view for an astronomical telescope -- 105 square
degrees, which is comparable to the area of your hand held at arm's length. It
needs that large a field in order to observe the necessary large number of
stars.
The photometer must be space-based to obtain the photometric precision
needed to reliably see an Earth-like transit and to avoid interruptions caused
by day-night cycles, seasonal cycles and atmospheric perturbations, such as
extinction associated with ground-based observing. Results from the Kepler
mission will allow us to place our solar system within the context of planetary
systems in the Galaxy.
"This mission attempts to answer a question that is as old as time itself
-- are other planets like ours out there?" said Ed Weiler, associate
administrator for NASA's Science Mission Directorate at NASA Headquarters in
Washington, before Kepler's liftoff. "It's not just a science question -- it's a
basic human question."
"So far, although we have discovered more than 300 planets (beyond the
solar system), we haven't discovered any Earths," Weiler said.
Using special detectors similar to those used in digital cameras, Kepler
will look for slight dimming in the stars as planets pass between the star and
Kepler. The Kepler's place in space will allow it to watch the same stars
constantly throughout its mission, something observatories like Hubble Space
Telescope cannot do.
"We are very excited to see this magnificent spacecraft come to life when
it reaches space," said James Fanson, Kepler project manager at NASA's Jet
Propulsion Laboratory, Pasadena, California.
After a commissioning period lasting about two months, Kepler will begin
its job of looking for planets. Its isolated perch behind Earth will give the
telescope an unobstructed view of a single, very large patch of sky near the
Cygnus and Lyra constellations.
"We will monitor a wide range of stars; from small cool ones, where planets
must circle closely to stay warm, to stars bigger and hotter than the sun, where
planets must stay well clear to avoid being roasted," said William Borucki, who
has been working on the mission for 17 years.
"Everything about the mission is optimized to find Earth-size planets with
the potential for life, to help us answer the question -- are Earths bountiful
or is our planet unique?"
Kepler will find planets through the transit method. When a planet passes
in front of a star as viewed from Earth, the event is called a "transit".
On Earth, we can observe an occasional Venus or Mercury transit. These
events are seen as a small black dot creeping across the Sun -- Venus or Mercury
blocks sunlight as the planet moves between the Sun and us.
Kepler finds planets by looking for tiny dips in the brightness of a star
when a planet crosses in front of it -- we say the planet transits the star.
Once detected, the planet's orbital size can be calculated from the period (how long it takes the planet to orbit once around the star) and the mass of the star using Kepler's Third Law of planetary motion.
A United Launch Alliance Delta 2 rocket is prepared for the launch of the Kepler Telescope for NASA Kepler, from Pad 17-B at Cape Canaveral Air Force Station in Florida, March 6, 2009. (Xinhua Photo/Reuters)
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The size of the planet is found from the depth of the transit (how much the brightness of the star drops) and the size of the star. From the orbital size and the temperature of the star, the planet's characteristic temperature can be calculated. From this the question of whether or not the planet is habitable (not necessarily inhabited) can be answered.
Since transits only last a fraction of a day, all the stars must be monitored continuously, that is, their brightness must be measured at least once every few hours.
The ability to continuously view the stars being monitored dictates that the field of view (FOV) must never be blocked at anytime during the year.
Therefore, to avoid the Sun the FOV must be out of the ecliptic plane. The secondary requirement is that the FOV has the largest possible number of stars. This leads to the selection of a region in the Cygnus and Lyra constellations of our Galaxy as shown.
"Trying to detect Jupiter-size planets crossing in front of their stars is like trying to measure the effect of a mosquito flying by a car's headlight," said Fanson. "Finding Earth-sized planets is like trying to detect a very tiny flea in that same headlight."
