The search for other potentially habitable planets is about to get serious.
NASA gave the final necessary green light today to the Kepler mission. It will be the first spacecraft devoted to the search for Earth-sized planets around other stars and an important step toward finding life elsewhere in our galaxy, if it exists.
The space agency also approved the Dawn mission, which will orbit the two largest asteroids in our solar system.
Both missions are part of NASA's Discovery program. The decisions mean the missions will now be funded and planning and construction can begin. Each is slated for launch in 2006.
Search for another Earth
The Kepler satellite will orbit the Sun and study some 100,000 stars for four years, looking for planets that are similar in size to Earth and in similar orbits around their stars. Only planets with orbits in this so-called "habitable zone," where it is not too hot and not too cold, could have liquid water, scientists say. And liquid water is seen as a necessary ingredient for life as we know it.
"This will be the first mission that ought to be able to produce a census of Earth-sized planets in the habitable zones around other stars," said David Morrison, a member of the mission's science working group at NASA's Ames Research Center, which will oversee the project.
Current ground-based searches typically find only very large planets that orbit very close to their host stars and almost certainly could not support life. About 80 of these so-called extrasolar planets have been found. Most are larger than Jupiter.
Earth-sized planets can't be detected from Earth using current technology.
Seeing stars
Kepler won't actually see any planets. Instead, it will use a recently developed and unique technique to search for planets by watching the stars around which they orbit.
"Kepler will stare continuously at 100,000 stars to measure the tiny drop in brightness that occurs when a planet crosses in front of the disk of the star as seen from Kepler's position in space," Morrison explained in a telephone interview.
Planet hunters call the technique the transit method and have used it to look for extrasolar planets with Earth-based telescopes. By noting how much the star dims and for how long, scientists calculate the approximate size of the planet that must be blocking the light.
Kepler's telescope will have a 0.95-meter (37-inch) aperture.
Search for life
Morrison said Kepler will do more than find planets.
"This is a mission that addresses one of the most fundamental issues of astrobiology -- whether other inhabitable planets are frequent in the universe."
If Kepler finds many Earth-sized planets, as researchers expect it might, then a whole new view of the universe will unfold. Earth will suddenly not seem such a special place.
The spacecraft will not reveal whether there is actually life on any of the planets it finds.
Still, Morrison said Kepler stands to be one of NASA's most exciting missions, with headline-making discoveries.
The spacecraft will lay some groundwork for future missions, too. NASA hopes to put ever-more powerful telescopes into space that would be even better equipped to spot and then study Earth-sized planets. Eventually, NASA officials hope to probe the atmospheres of such planets for signs of chemicals that are only produced when life is present.
In perhaps a decade or more, NASA planners would like to photograph one of these planets, assuming they exist.
Dawn mission
The Dawn mission will make a nine-year journey to orbit the two most massive asteroids known, Vesta and Ceres. The robot will orbit from as high as 800 kilometers (500 miles) to as low as 100 kilometers (about 62 miles) above the surface.
Both asteroids are in the main asteroid belt, between Mars and Jupiter, but scientists say they are very different from each other.
Ceres has a surface that contains water-bearing minerals. It may hold a very weak atmosphere and frost, scientists say. Vesta, on the other hand, is thought to be dry, having been resurfaced by lava flows. It may have an early magma ocean like Earth's Moon, researchers think. Like the Moon, it has been hit many times by smaller space rocks.
Dawn will weigh and measure the asteroids and examine their craters. It will also work to determine what they are made of and how magnetic they are. All this information should help scientists better understand how our solar system formed and evolved.
Dawn will be managed by NASA's Jet Propulsion Laboratory.
The cost
Kepler and Dawn were selected from 26 proposals made in January 2001. Both will be under the $299 million limit for Discovery Program missions, which in the past have included NEAR and Mars Pathfinder.
NASA developed the Discovery Program as the cornerstone of its "Faster, Better, Cheaper" approach to space flight that has brought inexpensive successes -- such as NEAR's landing on asteroid Eros -- over the past few years, along with a handful of failed missions, including three at Mars.
"Kepler and Dawn are exactly the kind of missions NASA should be launching, missions that tackle some of the most important questions in science yet do it for a very modest cost," said Ed Weiler, associate administrator for space science at NASA Headquarters in Washington.
advertisement
