BRIEF HISTORY OF THE KEPLER MISSION
(W. J. Borucki, 05/22/2010)
The Kepler Mission developed over several decades as a way of answering the question: How frequent are other Earths in our galaxy? In particular, what is the frequency of Earth-size planets in the HZ of solar-like stars? In the last half of the twentieth century, the astrometric and interferometric approaches to finding exoplanets were the favored methods. The surprising discovery by Wolszczan (1994) based on timing of radio pulses from pulsars showed that a wider range of approaches should be considered. A paper by Rosenblatt (1971) provided a quantitative discussion of another alternative method; searching for patterns of transits to get size and orbital period. To be successful, all three approaches depend upon adapting new technology; the underlying principles are well understood. A paper by Borucki and Summers (1984) corrected the detection probability in the paper and pointed out that ground-based observations of at least 13,000 stars simultaneously should be sufficient to detect jovian-size planets, but that the detection of Earth-size planets would require space-based observations. Limitations to the detectability of planets by stellar variations was recognized (Borucki et al, 1985) and discussed more fully by Jenkins (2002).
To examine the technology needed to accomplish transit detection of exoplanets, NASA Ames Research Center (ARC) sponsored a workshop on high precision photometry in 1984 (Proceedings of the Workshop on Improvements to Photometry, 1984). The success of the first workshop encouraged a second workshop (Second Workshop on Improvements to Photometry, 1988) jointly sponsored by ARC and the National Bureau of Standards (now NIST) at Gaithersburg, Maryland in 1987. A wide range of subsystems was discussed including very stable band pass filters, 16-bit analog to digital converters, electronic amplifiers, and detectors. Ion-beam bombardment of band pass filters and the use of silicon diodes were recommended.
To further develop this approach NASA HQ funded the development and testing of proof-of-concept multichannel photometers based on silicon photodiodes. Tests conducted at the NBS and at Ames showed that the diodes had very high precision as expected, but that to reduce their thermal noise, they would need to be cooled to near liquid nitrogen temperatures. Two cooled multichannel photometers were built; the latter was based on an optical fiber feed to the cooled diodes. Erratic transmission of the multimode fibers doomed the latter (Borucki et al.1987 and Borucki et al.1988.).
In 1992, NASA HQ proposed a new line of missions to address questions about the Solar System and that would also consider the search for exoplanets. Proposals for concept studies were invited and discussed at a workshop at San Juan Capistrano, CA. The proposals were for complete missions: science, technical, engineering, management, cost, and schedule were to be addressed. For this opportunity, a team was organized to propose a transit search for terrestrial planets. The proposed concept was called “FRESIP” (FRequency of Earth-Size Inner Planets) to describe its goal.
The review panel found that the science value was very high and would have been supported it had there been proof that detectors existed with sufficient precision and the requisite low noise to find Earth-size planets.
Although the proposal was rejected, members of the team and the science community urged continuation of the ideas and in particular, stressed the wide variety of astrophysics that could be accomplished by observing thousands of stars continuously for a period of years. To explicitly investigate the astrophysics that could be accomplished, a workshop (Astrophysical Science with a Space borne Photometric Telescope, 1994) was held at the SETI Institute in Mountain View, CA. Promising areas of investigations included: solar and stellar physics; including star spots, activity cycle, oscillation, rotation rates, and flares, 2) stellar variability including cataclysmic variables, RR Lyra and d-Scuti stars, and 3) extragalactic objects such as quasars and active galactic nuclei, and 4) acoustic oscillations to investigate the interior of stars and determine their mass, age, and helium abundance.
In 1994, the first flight opportunity for a Discovery-Class mission was announced. The FRESIP Mission was again proposed (Borucki et al., 1996). It proposed a 0.95 meter aperture photometer using CCD detectors to be placed in a LaGrange orbit. CCD detectors were substituted for the silicon detectors because of their capability of tracking many targets simultaneously and their ability to accept many different target patterns. The review panel considered the FRESIP photometer to be a telescope similar to the Hubble Space Telescope (HST) and thus far too expensive to qualify as a Discovery-class mission. The proposal was rejected.
Lab tests to prove that CCD detectors were suitable were funded by small grants from NASA HQ and ARC. The first paper presenting the results of lab tests demonstrating the CCD detectors had the requisite precision and low noise to detect transit patterns of Earth-size transits was published in 1995 (Robinson et al, 1995). The experiment was carried out in the basement of Lick Observatory and used an old 512 x 512 Reticon front-side illuminated CCD in a metal and plastic structure. For many of the simulated stars a precision of 5x10-6 was achieved. Back-side illuminated CCDs, where the light does not pass through the wire traces on its way to the active silicon were expected to have even higher precision. An accidental spilling of liquid nitrogen during the lab tests did not cause loss of precision because the records of centroid movement allowed the motions to be regressed out. In fact it was the mathematical identification and removal of the systematic noise that was the break through step that allowed the intrinsic precision of these detectors to be recognized.
In 1996, the second opportunity to propose for a flight mission was announced. Studies showed that mission costs could be reduced if photometer was placed in a solar orbit rather than a LaGrange orbit because of the reduction of space propulsion systems needed to stay in a LaGrange orbit. At the insistence of several members (Koch, Tarter, Sagan) of the team, the mission name was changed from FRESIP to “Kepler” to honor the German astronomer who developed the laws of planetary motion and the principle needed to calculate optical prescriptions. Both are critical to the operation of the current mission. The mission cost was estimated in three different ways to show that the mission cost could be accomplished for the available budget. The proposal was rejected because no one had every demonstrated that the simultaneous, automated photometry of thousands of stars could be done. The review panel recommended that we build such a photometer to demonstrate the methods to be used. Funding was granted for such a demonstration from both NASA HQ and ARC.
In 1997, the photometer was designed and built. Arrangements were made at Lick Observatory to refurbish the Crocker Dome and to install a radio link between the dome and a receiving station at Ames. Software was written to control the dome and photometer and to analyze the data.
In early 1998, data was being received and analyzed on the simultaneous observations of 6,000 stars in a single field-of-view. Papers describing the results were published in 1999 and 2001(Borucki et al. 1999 and Borucki et al. 2001).
Later that year, the third opportunity to propose for a Discover-class Mission was announced. The review panel acknowledged that science was excellent; that the detectors could provide the necessary performance, and that automated photometry could be done on thousands of stars simultaneously. The proposal was rejected because there was no proof that a photometer with the precision required to find Earth-size planets could be developed that would operate satisfactorily on orbit with the types of noise encountered for such operation.
A proposal was written to build a laboratory demonstration that would incorporate all expected noise sources and impose Earth-size transits on a simulated, but realistic star field.
In 1999, the Kepler test bed was designed, built, and tested (Koch et al. 2000). The results were satisfactory and a report was written and communicated to the review panel chartered by HQ to verify the test bed performance.
In 2000, the fourth opportunity to propose for a Discovery-class mission was announced and Kepler proposed for the fifth time. Kepler was one of three proposals selected from a total of 26 that was allowed to compete by writing a Concept Study Report and demonstrating readiness to proceed.
In December of 2001, Kepler was selected as Discovery Mission #10. Mission development started in 2002 by placing orders for the detectors.
During the years prior to selection, many events helped get the Mission concept accepted. Two major events were the discovery of extrasolar planets by Michel Mayor’s team (Mayor and Queloz 1995) and Geoff Marcy’s team (Marcy and Butler, 1996) and success by several ground based transit search groups (Charbonneau et al. 2000). Once the radial velocity technique had convincingly demonstrated that many exoplanets existed and NASA HQ recognized that the transit technique was proven and that the technology existed that could find Earth-size planets, both the development of the Kepler Mission and a vigorous ground based efforts were funded. In particular, the many years that the Kepler team devoted to convincing the science community, the technical review panels, and NASA HQ officials, helped promote the funding of ground-based transit surveys that are now so successful in finding and characterizing exoplanets. In turn the success of both the radial velocity and transit approaches helped the Kepler Mission to compete against the many excellent proposals received at every AO for a Discovery-class mission.
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Text from article by Borucki, W., Koch, D., Batalha, N., Caldwell, D., Christensen-Dalsgaard, J., Cochran, W.D., Dunham, E., Gautier, T.N., Geary, J., Gilliland, R., jenkins, J., Kjeldsen, H., Lissauer, J.J., and Rowe, J. Kepler: Search for Earth-size planets in the habitable zone. Proceedings IAU Symposium No. IAUS253. 2008. F. Pont, D. Sasselov, and M. Holman, eds.