David Koch Discusses the Kepler Mission from NASA
Emerging Research Front Commentary, October 2010
Dr. David Koch accepting an honor award for the Kepler team from Lewis S. G. Braxton, III, NASA Ames’ deputy center director. View larger image in the tab below. Photo by NASA Ames.
Article: Kepler Mission Design, Realized Photometric Performance, and Early Science
Authors: Koch, DG et al.
David Koch talks with ScienceWatch.com and answers a few questions about this month's Emerging Research Front paper in the field of Space Science.
Why do you think your paper is highly cited?
The topic of exoplanets has become a hot topic in astronomy and has received widespread public interest. The central subject of the paper, the Kepler Mission, is the one mission currently capable of detecting Earth-size planets in the habitable zone around solar-like stars. The mission has become the touchstone against which planet detection is measured.
This paper was designed to be an overview of the mission, giving the background on the mission design, such as why we selected to point at a particular place in the sky, summarizing the top level instrument and mission parameters, for example, the spectral bandpass and the reasons for its selection, and also referring to a host of more specialized papers in particular those on the data processing and scientific interpretation processes used to arrive at the results.
Hence, the paper is intended to be a standard reference for the mission, both as an introduction to its design, as well as a source of key parameters for those who are interested in analyzing and interpreting the results.
This is truly a team effort as seen from the long list of co-authors. It is a highly integrated team of scientists, engineers, programmers, and managers from NASA, industry, and academia.
Does it describe a new discovery, methodology, or synthesis of knowledge?
The paper describes both early results and the methodology used to produce the results. The principal purpose of the Kepler Mission is to find Earth-size planets in the habitable zone of stars. This cannot be done from the ground. (The habitable zone is the range of distances from a star where liquid water could exist on the surface of a planet.)
"For millennia we humans have been pondering the question: "Are there other worlds like ours?" The results from Kepler will not be able to identify inhabited worlds - planets with life, but will answer the question as to whether Earth-size planets are common or rare in the habitable zone of other stars. Even a null result would be significant, implying our having to rethink our place in the Universe."
The approach is to measure sequences of transits, that is, the slight dimming of the stellar brightness caused by a planet as it passes in front of (transits) its parent star as viewed from our solar-system. For an Earth-Sun analog this is a change in brightness of only 84 parts-per-million (ppm). The transit duration lasts anywhere from about an hour to half a day, depending on the size of the star, the orbit of the planet and its inclination along our line-of-sight.
The paper first shows that the required precision for detecting Earth-size planets has been realized and then summarizes the first several planet discoveries made with Kepler as well as astrophysical results extracted from the data.
Precise, contiguous, and long-term photometry of a vast number of stars is a new regime of exploration in astronomy. Much of astronomy has been dedicated to obtaining better spatial and spectral resolution at all wavelengths to unravel the mysteries of the cosmos. But there are many facets that are understood by exploring the time domain, such as eclipsing binary stars, pulsars, X- and gamma-ray bursts and novae, to name just a few.
Searching for transits has pushed this research field into a heretofore unexplored realm. To detect a sequence of Earth-analog transits requires both precision and patience. To detect a sequence of at least three transits of a planet in an Earth-like orbit requires continuous monitoring of the set of stars for at least three years. This can't be done from the ground due to the effects of the atmosphere and weather on precision and the day-night and lunar cycles on contiguity.
Aside from operating in space, the key to precision is stability and the key to the success of Kepler is maintaining everything as near to constant as possible for as long as possible. One needs to essentially eliminate any variations in the time domain of a transit that can impact the precision. Things like heating and cooling, stray-light, gravity gradient variations, and ionizing radiation effects occurring with a 100-minute period as happens in low-earth-orbit are the major reasons for Kepler's operating in a very benign heliocentric orbit.
The observing protocol is changed only every three months when the spacecraft is rotated 90 degrees about the star field to keep the solar array pointed towards the Sun and the cooling radiator for the charge-coupled devices (CCDs) pointed to deep space as the spacecraft orbits the Sun. Once per month there is a break in observing to download the scientific data.
The stability of the observations has been so good, that to our surprise, we can eliminate many false positive signals due to background eclipsing binary stars by observing the shift in the centroid of the light from the star during a transit. In many cases we are able to measure the location of the centroid of the star's image on the CCDs to a small fraction of a millipixel, that is to one-millionth of one degree on the sky.
A background eclipsing binary generally causes a shift of tens of millipixels in the centroid, whereas a planetary transit of a star generally causes a shift of only a few tenths of a millipixel or less. Achieving this level of precision is a tribute to the entire development team, especially those at Ball Aerospace and Technologies Corp. and the data processing team at NASA's Ames Research Center.
Would you summarize the significance of your paper in layman's terms?
The paper provides a top-level discussion of the reasoning behind the design of the Kepler Mission. With all the possibilities for performing an experiment, one has to make many choices and prune off alternate approaches to arrive at what one hopes is the optimum solution within the constraints of the system. Analysis of the data from the first 43 days of operation demonstrates that the photometric precision needed to detect Earth-size planets has been achieved.
This precise photometric capability also enables many aspects of stellar variations to be explored and understood for the first time either by "mining" the data or by explicitly selecting certain stars to be observed at a one-minute cadence. This has led to new results for oscillating and pulsating stars. Measurement of pressure-mode (p-mode) oscillations in stars allows one to derive the radii, masses, and ages of the stars. Analysis of stellar pulsations allows one to understand the interior of stars. An overview of these new results is presented.
How did you become involved in this research, and how would you describe the particular challenges, setbacks, and successes that you've encountered along the way?
My entire career as a scientist has been spent designing, developing, and operating astrophysics missions and analyzing the data. Bill Borucki at NASA Ames had been working on the concept of detecting Earth's around other stars for many years. When NASA announced its intent to create Discovery missions, Bill asked me in 1992 if I would work with him on developing a concept for a photometric space mission.
We knew the mission would be feasible because of two then recent developments: 1) measurements from the Solar Max Mission showed that the variability of the Sun on the timescale of a transit was low enough to allow for detection of Earth-like transits and 2) large format CCDs were becoming available. (Kepler has a focal plane of 95 mega pixels.)
We submitted formal proposals to NASA's Discovery program Announcement of Opportunities in 1994, 1996, 1998, and 2000 in competition with about 30 to 40 other proposals. Each time we proposed anew, we addressed the weaknesses identified by the review process of the previous proposal.
"The principal purpose of the Kepler Mission is to find Earth-size planets in the habitable zone of stars. This cannot be done from the ground."
Our greatest challenge was demonstrating the concept and convincing our peers that it was feasible. Following the rejection of our 1998 proposal, we received sufficient funds to develop an end-to-end system technology demonstration of the instrument concept using a flight-like CCD, incorporating effects such as spacecraft jitter. But we and no one else had ever been able to simulate and detect an Earth-like transit signal at 84 ppm.
The breakthrough came when I came up with the concept, using nanotechnology, of simulating transits by changing the amount of light coming through the pin holes in a star plate. The feat was accomplished by causing a fine wire placed over the pin holes to expand by 12 nm. This was done by running a small current through the wire, causing it to heat and thereby expand.
This allowed us to produce Earth-like transit signals for selected stars while viewing a simulated star field of more than a thousand stars with a wide range of brightness, just like the real sky.
With this new tool we were able to probe our instrument performance and hone our detection methodology, demonstrating transit detections with a one-sigma noise of 20 ppm for a 12th magnitude solar-like star. Having done this, we were able to make the first cut in the 2000 proposal cycle and win down selection in Dec. 2001.
The Kepler Technology Demonstration continued to be a useful tool during development. We tested a single string of proto-flight CCDs and electronics with it and learned essential things about the behavior of the hardware that required revisions to the flight electronics.
Throughout the proposal and the development processes leading to launch, we succeeded in areas that often are a death sentence to projects:
1) We stayed focused on our one objective, resisting the inclination to expand our objectives and add on new capabilities for what is often believed to be at little or no cost. Our goals and objectives are virtually unchanged even today from our 1996 proposal.
2) Unlike what happens so often in protracted cases, our scientific goal of detecting Earth-size planets and measuring their frequency was not overcome by events and rendered no longer cutting-edge science. Rather, Kepler has become the touchstone for detection of terrestrial planets. Recall that no exoplanet around a normal star had been detected before 1995 and today there are nearly 500 known exoplanets, but only a few of which are small enough to be called super-earths.
Where do you see your research leading in the future?
We expect to determine the frequency of exoplanets, especially Earth-size planets and how this varies with stellar parameters including binary stars, orbital parameters, planetary size, and the multiplicity of planets in a system. These results will lead to a better understanding of planet formation theory and how our Solar System came to have its present properties. These results will also be used to guide the development of future missions to detect and measure the composition of exoplanet atmospheres.
As a by-product of the search for planets, Kepler is providing a photometric archive of time-series data that is unprecedented in terms of contiguity, precision, duration, and the number of stars monitored. The archive has already proven useful to over 400 scientists from around the world who have organized themselves into over a dozen working groups to conduct astrophysical investigations of the Kepler data and investigate many aspects of stellar variability.
Perhaps they will be able to answer such diverse questions as: How common are stars like the Sun? In the realm of paleoclimatology, how often do Maunder minimums occur? Such minima are associated with mini-ice ages, which occur every few centuries, but haven't occurred on Earth since the early 17th Century. The study of stellar behavior is not purely theoretical, since understanding stellar behavior is relevant to understanding the habitability of planets, including our own.
Do you foresee any social or political implications for your research?
For millennia we humans have been pondering the question: "Are there other worlds like ours?" The results from Kepler will not be able to identify inhabited worlds—planets with life, but will answer the question as to whether Earth-size planets are common or rare in the habitable zone of other stars. Even a null result would be significant, implying our having to rethink our place in the Universe.
David Koch, Astrophysicist
NASA Ames Research Center
Moffett Field, CA, USA
KEYWORDS: INSTRUMENTATION: PHOTOMETERS; PLANETARY SYSTEMS; SPACE VEHICLES: INSTRUMENTS; STARS: STATISTICS; STARS: VARIABLES: GENERAL; TECHNIQUES: PHOTOMETRIC, EARTH-SIZE PLANETS; SOLAR-TYPE; STARS; HAT-P-7B.