As our world sinks deeper into recession and nations struggle to infuse cash and confidence into their deflated economies, planetary scientists and areospace engineers in the U.S. are preparing to launch the first mission specifically designed to find other planets in the Milky Way similar to our own in size and distance from the stars they orbit.
On March 5, weather and technology permitting, NASA will send the Kepler spacecraft into orbit around the Sun, beginning NASA Discovery mission #10, the latest in the agency's 15-year-old commitment to explore space with "lower-cost, highly focused planetary science investigations designed to enhance our understanding of the solar system." Kepler's purpose is clear:
"The scientific goal of the Kepler Mission is to explore the structure and diversity of planetary systems, with a special emphasis on the detection of Earth-size planets. It will survey the extended solar neighborhood to detect and characterize hundreds of terrestrial and larger planets in or near the 'habitable zone,' defined by scientists as the distance from a star where liquid water can exist on a planet's surface. The results will yield a broad understanding of planetary formation, the structure of individual planetary systems, and the generic characteristics of stars with terrestrial planets."
The price tag is clear by now, too: somewhere between $550 and $600 million. More on this later.
Kepler is a fitting name for this unique craft. German-born Johannes Kepler (1571-1630) is one of the giants of early modern astromony. A contemporary of Galileo and Brahe, Kepler discovered the basic laws governing planetary motion and in doing so created the field of celestial mechanics. He also was among the earliest to defend in print the heliocentric cosmology of Copernicus, and was the first to explain that the Moon was responsible for tides on Earth. His contributions to optics were also important. Because he gave us the ablity to know with mathematical precision the past, present, and future positions of planets, as well as our fundamental understanding of how telescopes and human vision work, it's appropriate that his name is on a spacecraft that relies on an ultra-sensitive light meter and a telescope to determine if planets similar to ours exist in potentially habitable locations elsewhere in the galaxy.
For planetary scientists, size and distance matter. Up to now, the exoplanets discovered outside our solar system have been gas giants, ice giants, or super-hot bodies orbiting close to stars. With Kepler it should be possible to find rocky ones about the same size as Earth orbiting at distances where conditions are more favorable for maintaining liquid water. Where there is water, there may be life.
The Kepler spacecraft was built by Ball Aerospace & Technologies Corp., in Boulder, CO, a major player in space technology which also happens to be a subsidary of Ball Corporation, the venerable company better known to Americans as the manufacturer of Ball glass canning jars. (FYI, the familiar Ball logo is prominently featured on the areospace corp.'s Web site.)
Kepler is highly sophisticated instrumentation and packaging. Its data-gathering capability relies on a .95-meter diameter telescope and a collection of 42 light-sensitive microchips called charged coupling devices (CCDs), together forming the Kepler Focal Plane Array, which will be used to record extremely minute variations in the brightness of stars observed in the Cygnus region of the Milky Way over a period of about four years. The Cygnus region, located in the Orion spur of our galaxy, was selected because this star field isn't obscured by our Sun at any time of the year. The location also avoids occultations, or obscuring transits across the field of view, caused by asteroids and Kuiper Belt objects. In short, Kepler can stay focused on the Cygnus region without interference.
This is important, because Kepler will be measuring the incredibly subtle changes in star brightness that occur when small diameter planets cross, or transit, the face of their orbiting stars. The transit method is how the majority of exosolar planets have been detected, but it also explains why the known extrasolars are so large: on Earth, the observations can't be precise or constant enough to detect smaller orbiting bodies. This is because our atmosphere bends the light entering it (why stars twinkle), and transits by Earth-size or even smaller planets could happen over a very short time frame -- perhaps as little as two hours in the span of a year. Even if it was possible to compensate for light bending and detect Earth-size planets, this would require building dedicated telescopes in multiple locations to avoid the changes in light resulting from Earth's own orbit, not to mention 24/7/365 monitoring. The cost would be many times over what the U.S. will ultimately spend on Kepler.
What makes Kepler relatively cost-effective is its ability to observe a large number of stars from space itself using ultra-sensitive CCDs, something never done before. Initially, Kepler will be looking at about 140,000 stars, but as the projected four-year mission unfolds this number will be narrowed to approximately 100,000. That's still a lot of stars and data to work with. I'm not much of a betting man, but assuming its launch, orbital insertion, and internal systems work properly, I think it's likely that by mission's end, Kepler will have detected many smaller new planets of known size, orbit, and even approximate temperature, including rocky worlds in habitable zones.
In light of the economic trends, it's fair to ask if such results are worth $600m. There are as many opinions about that as there are observers of the project, but one thing is clear: if Kepler wasn't approved back in 2001, prospects for discovering other terrestrial planets in the Milky Way would be on hold indefinitely. This is not to say the project's been immune to cost overruns, sometimes poor managment, and other pitfalls of large and complex contract work. Kepler's had its share of difficulties; the project dramatically faced being canceled in 2005.
I'm looking ahead, though, not back. The possibility of discovering one or more terrestrial planets orbiting in habitable zones in what amounts to our own back yard is more than exciting. If it happens, our understanding of planetary systems will take a dramatic step forward.
Next stop: eBay and Life beyond Earth
Johannes Kepler and the planets known during his lifetime. For editorial use only.
Artist's conception of Kepler in orbit with transiting planet in background. Credit: NASA.
Kepler Focal Plane Array during assembly. Credit: Ball Aerospece & Technology Corp.
Banner bus photo credit: Dorothy Delina Porter.