The largest optical mirror ever built for
a NASA mission soared beyond Earth’s
orbit on March 7, 2009, looking not for ET, the Extra-Terrestrial, but rather ET’s home.
The Kepler mission was launched aboard a Delta II rocket from Cape Canaveral
Air Force Base, Florida. Secured inside the rocket was the sole Kepler instrument, the photometer, a 4.6-foot (1.4-meter) diameter, 85 percent, light-weighted mirror eagerly awaiting its opportunity to confirm the existence of other
Earth-size and smaller planets.
Or, maybe not. Either way, scientists say the results will be profound.The Kepler mission is designed with the goal of answering one of humanity’s oldest questions:
Are We Alone?
“If we find a lot of planets in the habitable zone, life is probably ubiquitous in the universe,” said William Borucki, the NASA Amex Research Center scientist leading the mission. If not, Borucki says,
there will be little need for a Star Trek Enterprise, because there’s probably no one to visit
Kepler will precisely measure the light variations from thousands
of distant stars, looking for planetary transits. When a planet passes
in front of its parent star, as seen from our solar system, it blocks a
small fraction of the light from that star, which is known as a transit. Searching for transits of distant “Earths” is like looking for the decrease
in brightness when a moth flies across a searchlight, or a flea crawls
across a car’s headlight. Measuring repeated transits, all with regular period, duration and change in brightness, provides a method for discovering and confirming planets and their orbits – planets the size of Earth and smaller in the habitable zone around other stars similar to our Sun.
Kepler’s aperture is nearly one meter in diameter and Kepler will be the largest Schmidt-type telescope ever launched. Schmidt optics have an unusually large field of view. Kepler’s will be bigger than an open hand held at arm’s length. That compares to the Hubble Space Telescope, which is like looking through a straw held three feet away. The detectors used are charged coupled devices (CCDs) similar to those found in consumer digital cameras. However, unlike an ordinary digital camera with a few megapixels, Kepler has an array of 95 megapixels.
To detect star transits, NASA needed a photometer, or light meter, that can simultaneously measure the brightness variation of over 100,000 stars with a precision of about 20 parts per million (ppm). This precision allows detection of Earth-like transits, which cause a change of brightness of 84 ppm of a solar-like star that lasts from a few hours to about a half of a day. The photometer is so sensitive that planets as small as Mars can be detected when they occur in short-period orbits like many of the giant planets already discovered.
So as not to miss any transits, Kepler will stare at the same field in the Cygnus-Lyra region for the entire mission, three-and-a-half years.
The photometer optics are a modification of the classical Schmidt design. They include a 0.95-meter (37-inch) aperture fused-silica Schmidt corrector plate (lens), and a 1.4-meter (55-inch) diameter ultra-low expansion-glass primary mirror. The scaling factor of the optical design results in 95 percent of the energy from a star being distributed over an area at the focal plane of approximately 7 detector pixels in diameter. The photometer’s primary, 4.6-foot (1.4-meter) mirror is mounted onto three focus mechanisms that may be used in flight to make fine focus adjustments. The focus mechanisms can adjust the mirror’s piston, tip and tilt. While electrical power is required to move the focus mechanisms, they are designed to hold the position of the primary mirror without continuous power.
Space-based mirror applications require materials that can perform to the demanding standards of space research and the extreme environmental conditions of space.
The honeycombed backside of Keplerís primary mirror manufactured by Corning is illuminated as part of the inspection process.
The Kepler Focal Plane Assembly consists
of 42 charge coupled devices.
Ball Aerospace technicians inspect the Kepler primary mirror following
coating at Surface Optics Corporation, San Diego, Calif.