Kepler space telescope

NASA spacecraft for exoplanetology (2009–2018)

Kepler
Kepler in orbit
Artist's impression of the Kepler telescope
Mission typeSpace telescope
OperatorNASA / LASP
COSPAR ID2009-011A Edit this at Wikidata
SATCAT no.34380
Websitewww.nasa.gov/kepler
Mission durationPlanned: 3.5 years
Final: 9 years, 7 months, 23 days
Spacecraft properties
ManufacturerBall Aerospace & Technologies
Launch mass1,052.4 kg (2,320 lb)[1]
Dry mass1,040.7 kg (2,294 lb)[1]
Payload mass478 kg (1,054 lb)[1]
Dimensions4.7 m × 2.7 m (15.4 ft × 8.9 ft)[1]
Power1100 watts[1]
Start of mission
Launch dateMarch 7, 2009, 03:49:57 (2009-03-07UTC03:49:57) UTC[2]
RocketDelta II (7925-10L)
Launch siteCape Canaveral SLC-17B
ContractorUnited Launch Alliance
Entered serviceMay 12, 2009, 09:01 UTC
End of mission
DeactivatedNovember 15, 2018 (2018-11-15)
Orbital parameters
Reference systemHeliocentric
RegimeEarth-trailing
Semi-major axis1.0133 AU
Eccentricity0.036116
Perihelion altitude0.97671 AU
Aphelion altitude1.0499 AU
Inclination0.4474 degrees
Period372.57 days
Argument of perihelion294.04 degrees
Mean anomaly311.67 degrees
Mean motion0.96626 deg/day
EpochJanuary 1, 2018 (J2000: 2458119.5)[3]
Main telescope
TypeSchmidt
Diameter0.95 m (3.1 ft)
Collecting area0.708 m2 (7.62 sq ft)[A]
Wavelengths430–890 nm[3]
Transponders
BandwidthX band up: 7.8 bit/s – 2 kbit/s[3]
X band down: 10 bit/s – 16 kbit/s[3]
Ka band down: Up to 4.3 Mbit/s[3]
← Dawn
GRAIL →
 

The Kepler space telescope is a defunct space telescope launched by NASA in 2009[5] to discover Earth-sized planets orbiting other stars.[6][7] Named after astronomer Johannes Kepler,[8] the spacecraft was launched into an Earth-trailing heliocentric orbit. The principal investigator was William J. Borucki. After nine and a half years of operation, the telescope's reaction control system fuel was depleted, and NASA announced its retirement on October 30, 2018.[9][10]

Designed to survey a portion of Earth's region of the Milky Way to discover Earth-size exoplanets in or near habitable zones and estimate how many of the billions of stars in the Milky Way have such planets,[6][11][12] Kepler's sole scientific instrument is a photometer that continually monitored the brightness of approximately 150,000 main sequence stars in a fixed field of view.[13] These data were transmitted to Earth, then analyzed to detect periodic dimming caused by exoplanets that cross in front of their host star. Only planets whose orbits are seen edge-on from Earth could be detected. Kepler observed 530,506 stars and detected 2,778 confirmed planets as of June 16, 2023.[14][15]

History

Pre-launch development

The Kepler space telescope was part of NASA's Discovery Program of relatively low-cost science missions. The telescope's construction and initial operation were managed by NASA's Jet Propulsion Laboratory, with Ball Aerospace responsible for developing the Kepler flight system.

In January 2006, the project's launch was delayed eight months because of budget cuts and consolidation at NASA.[16] It was delayed again by four months in March 2006 due to fiscal problems.[16] At this time, the high-gain antenna was changed from a design using a gimbal to one fixed to the frame of the spacecraft to reduce cost and complexity, at the cost of one observation day per month.

Post launch

The Ames Research Center was responsible for the ground system development, mission operations since December 2009, and scientific data analysis. The initial planned lifetime was three and a half years,[17] but greater-than-expected noise in the data, from both the stars and the spacecraft, meant additional time was needed to fulfill all mission goals. Initially, in 2012, the mission was expected to be extended until 2016,[18] but on July 14, 2012, one of the four reaction wheels used for pointing the spacecraft stopped turning, and completing the mission would only be possible if the other three all remained reliable.[19] Then, on May 11, 2013, a second one failed, disabling the collection of science data[20] and threatening the continuation of the mission.[21]

On August 15, 2013, NASA announced that they had given up trying to fix the two failed reaction wheels. This meant the current mission needed to be modified, but it did not necessarily mean the end of planet hunting. NASA had asked the space science community to propose alternative mission plans "potentially including an exoplanet search, using the remaining two good reaction wheels and thrusters".[22][23][24][25] On November 18, 2013, the K2 "Second Light" proposal was reported. This would include utilizing the disabled Kepler in a way that could detect habitable planets around smaller, dimmer red dwarfs.[26][27][28][29] On May 16, 2014, NASA announced the approval of the K2 extension.[30]

By January 2015, Kepler and its follow-up observations had found 1,013 confirmed exoplanets in about 440 star systems, along with a further 3,199 unconfirmed planet candidates.[B][31][32] Four planets have been confirmed through Kepler's K2 mission.[33] In November 2013, astronomers estimated, based on Kepler space mission data, that there could be as many as 40 billion rocky Earth-size exoplanets orbiting in the habitable zones of Sun-like stars and red dwarfs within the Milky Way.[34][35][36] It is estimated that 11 billion of these planets may be orbiting Sun-like stars.[37] The nearest such planet may be 3.7 parsecs (12 ly) away, according to the scientists.[34][35]

On January 6, 2015, NASA announced the 1,000th confirmed exoplanet discovered by the Kepler space telescope. Four of the newly confirmed exoplanets were found to orbit within habitable zones of their related stars: three of the four, Kepler-438b, Kepler-442b and Kepler-452b, are almost Earth-size and likely rocky; the fourth, Kepler-440b, is a super-Earth.[38] On May 10, 2016, NASA verified 1,284 new exoplanets found by Kepler, the single largest finding of planets to date.[39][40][41]

Kepler data have also helped scientists observe and understand supernovae; measurements were collected every half-hour so the light curves were especially useful for studying these types of astronomical events.[42]

On October 30, 2018, after the spacecraft ran out of fuel, NASA announced that the telescope would be retired.[43] The telescope was shut down the same day, bringing an end to its nine-year service. Kepler observed 530,506 stars and discovered 2,662 exoplanets over its lifetime.[15] A newer NASA mission, TESS, launched in 2018, is continuing the search for exoplanets.[44]

Spacecraft design

Kepler in Astrotech's Hazardous Processing Facility
Interactive 3D model of Kepler
Interactive 3D model of Kepler

The telescope has a mass of 1,039 kilograms (2,291 lb) and contains a Schmidt camera with a 0.95-meter (37.4 in) front corrector plate (lens) feeding a 1.4-meter (55 in) primary mirror—at the time of its launch this was the largest mirror on any telescope outside Earth orbit,[45] though the Herschel Space Observatory took this title a few months later. Its telescope has a 115 deg2 (about 12-degree diameter) field of view (FoV), roughly equivalent to the size of one's fist held at arm's length. Of this, 105 deg2 is of science quality, with less than 11% vignetting. The photometer has a soft focus to provide excellent photometry, rather than sharp images. The mission goal was a combined differential photometric precision (CDPP) of 20 ppm for a m(V)=12 Sun-like star for a 6.5-hour integration, though the observations fell short of this objective (see mission status).

Camera

Kepler's image sensor array. The array is curved to account for Petzval field curvature.

The focal plane of the spacecraft's camera is made out of forty-two 50 × 25 mm (2 × 1 in) CCDs at 2200×1024 pixels each, possessing a total resolution of 94.6 megapixels,[46][47] which at the time made it the largest camera system launched into space.[17] The array was cooled by heat pipes connected to an external radiator.[48] The CCDs were read out every 6.5 seconds (to limit saturation) and co-added on board for 58.89 seconds for short cadence targets, and 1765.5 seconds (29.4 minutes) for long cadence targets.[49] Due to the larger bandwidth requirements for the former, these were limited in number to 512 compared to 170,000 for long cadence. However, even though at launch Kepler had the highest data rate of any NASA mission,[citation needed] the 29-minute sums of all 95 million pixels constituted more data than could be stored and sent back to Earth. Therefore, the science team pre-selected the relevant pixels associated with each star of interest, amounting to about 6 percent of the pixels (5.4 megapixels). The data from these pixels was then requantized, compressed and stored, along with other auxiliary data, in the on-board 16 gigabyte solid-state recorder. Data that was stored and downlinked includes science stars, p-mode stars, smear, black level, background and full field-of-view images.[48][50]

Primary mirror

Comparison of primary mirror sizes for the Kepler telescope and other notable optical telescopes.

The Kepler primary mirror is 1.4 meters (4.6 ft) in diameter. Manufactured by glass maker Corning using ultra-low expansion (ULE) glass, the mirror is specifically designed to have a mass only 14% that of a solid mirror of the same size.[51][52] To produce a space telescope system with sufficient sensitivity to detect relatively small planets, as they pass in front of stars, a very high reflectance coating on the primary mirror was required. Using ion assisted evaporation, Surface Optics Corp. applied a protective nine-layer silver coating to enhance reflection and a dielectric interference coating to minimize the formation of color centers and atmospheric moisture absorption.[53][54]

Photometric performance

In terms of photometric performance, Kepler worked well, much better than any Earth-bound telescope, but short of design goals. The objective was a combined differential photometric precision (CDPP) of 20 parts per million (PPM) on a magnitude 12 star for a 6.5-hour integration. This estimate was developed allowing 10 ppm for stellar variability, roughly the value for the Sun. The obtained accuracy for this observation has a wide range, depending on the star and position on the focal plane, with a median of 29 ppm. Most of the additional noise appears to be due to a larger-than-expected variability in the stars themselves (19.5 ppm as opposed to the assumed 10.0 ppm), with the rest due to instrumental noise sources slightly larger than predicted.[55][46]

Because decrease in brightness from an Earth-size planet transiting a Sun-like star is so small, only 80 ppm, the increased noise means each individual transit is only a 2.7 σ event, instead of the intended 4 σ. This, in turn, means more transits must be observed to be sure of a detection. Scientific estimates indicated that a mission lasting 7 to 8 years, as opposed to the originally planned 3.5 years, would be needed to find all transiting Earth-sized planets.[56] On April 4, 2012, the Kepler mission was approved for extension through the fiscal year 2016,[18][57] but this also depended on all remaining reaction wheels staying healthy, which turned out not to be the case (see Reaction wheel issues below).

Orbit and orientation

Kepler's search volume, in the context of the Milky Way
The motion of Kepler relative to Earth, slowly drifting away from Earth in a similar orbit, looking like a spiral over time

Kepler orbits the Sun,[58][59] which avoids Earth occultations, stray light, and gravitational perturbations and torques inherent in an Earth orbit.

NASA has characterized Kepler's orbit as "Earth-trailing".[60] With an orbital period of 372.5 days, Kepler is slowly falling farther behind Earth (about 16 million miles per annum). As of May 1, 2018[update], the distance to Kepler from Earth was about 0.917 AU (137 million km).[3] This means that after about 26 years Kepler will reach the other side of the Sun and will get back to the neighborhood of the Earth after 51 years.

Until 2013 the photometer pointed to a field in the northern constellations of Cygnus, Lyra and Draco, which is well out of the ecliptic plane, so that sunlight never enters the photometer as the spacecraft orbits.[48] This is also the direction of the Solar System's motion around the center of the galaxy. Thus, the stars which Kepler observed are roughly the same distance from the Galactic Center as the Solar System, and also close to the galactic plane. This fact is important if position in the galaxy is related to habitability, as suggested by the Rare Earth hypothesis.

Orientation is three-axis stabilized by sensing rotations using fine-guidance sensors located on the instrument focal plane (instead of rate sensing gyroscopes, e.g. as used on Hubble).[61] and using reaction wheels and hydrazine thrusters[62] to control the orientation.

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