The final farewell to the Kepler planetarium

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The final farewell to the Kepler planetarium







Oddly enough, there was a time not too distant in which humanity did not know if planets existed around other stars. That did not deter some visionaries from proposing the construction of a space telescope to discover planets through transits, that is, the decrease in the brightness of the star that occurs when the planet passes by. The idea was crazy, because this technique favors the detection of very large planets that rotate very close to their star. But, as everyone thought they knew, giant planets are only found in very far orbits. In spite of everything, a team of researchers led by William Borucki had the audacity to challenge the scientific community and proposed in 1992 the FRESIP space telescope to detect extrasolar planets. A large part of that same scientific community lacked time to make fun of Borucki, a physicist with extensive experience in NASA but who was not an astrophysicist and did not even have a PhD. Little did it matter that Borucki, along with Audrey Summers, had presented his method to discover planets at an early date like 1983. At that time few people paid attention to him and in 1992 things had not changed much. FRESIP (Frequency of Earth-Size Inner Planets) was nonsense that no one in their right mind would support, despite its low cost. And, indeed, it was. NASA rejected the FRESIP proposal for being too ambitious.



Kepler Space Telescope (NASA).



But NASA did not have two factors. The first and fundamental was that, against all odds, the giant planets yes that they can orbit close to their stars, as Didier Queloz and Michel Mayor discovered with surprise in 1995 when they discovered 51 Pegasi b, the first planet detected around a star in the main sequence. This discovery multiplied Kepler's scientific potential by several orders of magnitude. The second factor was the extraordinary stubbornness and determination of Borucki. Borucki I knew that his idea was good and, especially since 1995, that his telescope would be able to detect numerous exoplanets. In 1996 FRESIP changed its -horroroso- name to Kepler at the suggestion of astronomers such as Carl Sagan or Jill Tarter.



Bill Boricki, Kepler's father (NASA).



The Borucki planet-hunting telescope was rejected four times by NASA (in 1992, 1994, 1996 and 1998). And not without reasons. The space agency doubted that the detectors were sensitive enough, that the telescope was fairly stable or that the natural variability of the stars could detect planets using a space telescope. But Borucki and his team improved their original design until, finally, the mission was approved in 2001 as part of the low-cost Discovery mission program. Although this program was designed to give preference to planetary probes, Kepler's potential was too high to pass up.



Design of FRESIP (NASA).



Kepler, the tenth Discovery mission, took off on March 6, 2009 using a Delta II 7925-10L rocket from the 17B ramp at Cape Canaveral, almost twenty years after Borucki proposed FRESIP for the first time. Kepler was a 1,052.4 kg space telescope with a primary mirror 1.4 meters in diameter and 0.95 meters effective aperture with a silver coating. The mission was carried out by JPL, with Ball Aerospace as the main contractor. Being a Schmidt-Cassegrain type telescope, the optical tube had a corrective lens 0.95 meters in diameter at its end. The only instrument consisted of 42 CCD detectors, grouped in pairs, which were cooled to -85 º C and covered by a sapphire lens. The detectors were sensitive to the wavelength range from 420 to 850 nm. These detectors would serve as an exquisitely accurate photometer - 10 parts per million - with the aim of simultaneously measuring the brightness of about 170,000 stars located in a 105º field of view in the Cygnus and Lyra region. In this field there are about four and a half million stars, but Kepler would only observe the brightest, that is, those with a magnitude no higher than 14.



Launch of Kepler in 2009 (NASA).





Parts of Kepler (NASA).





Kepler optical tube (NASA).



Each CCD resistant to radiation had dimensions of 5 x 2.5 centimeters and 2,200 x 1,024 pixels, so that, in total, Kepler had 95 million pixels, a record for the time. As a good photometer, Kepler did not get proper images, but curves of light. In fact, the focal plane was slightly out of focus (in about 10 seconds of arc) with the aim of increasing the photometric accuracy. The data was recorded half an hour home, with the exception of those corresponding to 512 stars whose brightness was measured every minute as a calibration. Kepler had to continuously observe the star field for at least three and a half years to check the frequency of planets around other stars, with special emphasis on the frequencies of small, terrestrial rocky planets.



The mirror during construction (NASA).





Kepler primary mirror (Ball Aerospace).



The telescope was placed in a solar orbit with a period of 373 days, following the Earth in its trajectory around the Sun. In this way it was guaranteed that the telescope could point continuously to the same area of ​​the sky, hence the region of The chosen observation was close to the galactic equator, but far from the ecliptic (where the sun passes). The downside of Kepler's observation technique is that most of the stars in the field were too far away to be accurately observed by terrestrial observatories, so confirmation of most of the discovered worlds would be impossible. But it did not matter, because Kepler was a statistical mission, that is, the important thing was never to obtain information about specific planets, but to estimate how common are the exoplanets in our Galaxy depending on their size and the type of star.



Kepler optics (NASA).





Support detectors (NASA).





Kepler CCD detectors (NASA).



Kepler had dimensions of 4.7 meters long and 2.7 meters in diameter and was powered by a panel of 10.2 square meters capable of generating 1,100 watts. The ship's brain was a RAD750 processor, a version of the PowerPC chip used in other NASA missions such as the MRO Martian probe. For his mission to be a success, Kepler had to aim at the same area of ​​the sky with an accuracy of nine milliseconds of arc. For this, I would use, as usual, a minimum of three flywheels of inertia. Unlike other missions, Kepler needed to operate at least three flywheels perfectly to fulfill his primary mission. The telescope was launched with only four handwheels - which were used continuously to reduce the workload of each unit - due to budget limitations and the high weight of them. The steering wheels revolved at a speed of between one thousand and four thousand revolutions per minute (it was tried to keep the revolutions to the minimum) and they worked using the information provided by four sensors of precision located in the corners of the photometer, fourteen solar sensors and two stellar sensors, as well as two inertial guidance units. Eight thrusters powered by 11.7 kg of hydrazine would be used to "unclog" the flywheels periodically and for attitude control maneuvers that fall outside the capabilities of these, as well as to maintain a minimum angle with the Sun of 55 °. In order to communicate with the Earth, located at an average distance of 96 million kilometers, Kepler used a main antenna and four low gain, with a capacity to send between 10 and 4.3 megabits per second and receive between 7.8 and 2,000 bits per second.



Kepler Telescope (NASA).





Kepler before the launch (NASA).





Kepler before the launch (NASA).





Solar panels from Kepler (NASA).



Kepler's "first light" took place on April 8, 2009, but the first planets would not be announced until January 4, 2010. As expected, the first five planets-from Kepler-4b to Kepler-8b- they were "boring" hot jupiters like 51 Pegasi b. Kepler worked exquisitely, but the data reduction was anything but simple. As mission critics had predicted, errors in telescope pointing and variation in detector sensitivity, as well as stellar spots and other phenomena, conspired to generate background noise that was very difficult to filter. And then we had to eliminate the false positives, that is, the transits produced by brown dwarfs or other stars in multiple systems, in addition to the noise caused by background stars that could not be resolved in the photometric data. An authentic computer nightmare. But the main problem faced by the mission was stellar variability. The stars in Kepler's field of vision had turned out to be, on average, more unruly than predicted by the theoretical models and previous observations. This meant that Kepler would need much more than three and a half years to complete his mission and estimate the frequency of terrestrial planets around other stars. In other words, more transits of a small planet would now be necessary to confirm its existence (in general, six instead of three). For this reason, NASA soon expanded Kepler's primary mission until 2016.



Kepler's orbit around the Sun (NASA).





Kepler observation field (NASA).



2011 would be the first year in which Kepler managed to attract the attention of the media. On January 10, the discovery of Kepler-10b was announced. Although it was an inferno with oceans of molten lava, it was the first rocky planet detected by the space telescope. On September 15, it was the turn of Kepler-16b, the first circumbinary exoplanet of the mission (that is, a world that rotates around two stars of a binary system at the same time). On December 5 the media went crazy with the announcement of Kepler-22b, the first planet of Kepler located in the habitable zone. Although it is probably a minineptuno and not a rocky planet, Kepler-22b was an appetizer of the data avalanche that was to come. On April 18, 2013 Kepler's team published the discovery of Kepler-62e and Kepler-62f, the first probably rocky planets located in the habitable zone detected by the mission. However, it was not until April 17, 2014 when Kepler detected Kepler-186f, the first terrestrial-sized rock planet located in the habitable zone (in this case, a red dwarf).



Artistic recreation of Kepler-22b (NASA).





Kepler-22b light curve observed by Kepler (NASA).



But while the scientific part of the mission was developing successfully, the same did not happen with the technique. On July 4, 2012, Kepler lost one of the inertia wheels (number 2). The telescope was therefore left with only three flywheels of inertia, the minimum necessary to point to the same area of ​​the sky and detect terrestrial-sized planets. The mission team crossed their fingers so that no other flyer failed before completing the primary mission, but this time they ran out of luck. On May 14, 2013 the second flyer (number 4) stopped working and the primary mission ended abruptly. As a result, Kepler was no longer able to discover the smaller and potentially more interesting exoplanets, such as exotierras (terrestrial planets located in the habitable zone). A mission of 700 million dollars had ended due to the failure of a component of 200,000 dollars. Other similar flyers had presented problems in several missions that carried them, such as the NASA Dawn probe. Kepler's team tried to solve the problem in 2007, but by then the telescope was ready for launch. In 2008, however, the four flyers were sent to the construction company, Ithaco Space Systems, for in-depth review. Possibly without this additional maintenance the primary mission of Kepler would have ended much earlier.



Two of Kepler's four reaction wheels (in black) (Ball Aerospace).





Detail of one of the flywheels of inertia (NASA).



In spite of everything, the results of the primary mission were impressive and have radically changed our understanding of the planets in the Galaxy. The original data were processed using all the tricks learned in this last decade of exoplanetary studies and on June 19, 2017 the final catalog of Kepler's primary mission was published, which contains important differences with the one announced shortly after the failure of the second steering wheel of inertia The figures are overwhelming. Kepler has discovered 4,034 candidates for planets, of which 2,335 have been confirmed. Think wisely. When Borucki came up with the FRESIP proposal, he was not known any extrasolar planet. Today the vast majority of exoplanets discovered have been by Kepler. Of those more than two thousand confirmed planets, only between two and twelve are potential exotierras (in 2013 it was thought that they were close to thirty), but they are too far away for them to be analyzed in detail by the current terrestrial telescopes. Of course, one can not help but think that it is a shame that Kepler did not carry five or six flyers of inertia instead of four.



The diversity of the Kepler planets (NASA / Ames Research Center).





Frequency of planets depending on their size (NASA / Ames Research Center / CalTech / University of Hawaii / B.J. Fulton).





Potentially habitable terrestrial planets discovered by Kepler (NASA).



But Kepler did not die in 2013. His primary mission was over, but it was still a space telescope about a meter otherwise perfectly functional. NASA asked the scientific community for ideas to continue taking advantage of Kepler. The winner was the K2 proposal, which would use the radiation pressure of sunlight to aid in the aiming of the telescope. That is, the light would serve as luck of third flywheel of inertia (although with less precision than one of truth). In return, the telescope should maneuver with respect to the Sun and, therefore, could no longer point to the same region of the sky. The K2 mission began in May 2014. Kepler would change stellar field every three months or so, which has allowed the discovery of a few hundred planets - most of them giant - and the acquisition of new data on stellar variability, supernovae and minor bodies Of the solar system.



K2 Mission of Kepler (NASA).



The K2 mission had the days counted from the beginning because the telescope had to use its hydrazine propellants to keep aligned with the Sun. In principle it was expected that the mission would last between two or three years, but, once again, Kepler has surpassed the more optimistic expectations. On October 19, 2018 Kepler went into hibernation mode when the reserves of hydrazine were depleted. Without fuel it was impossible to continue with the mission and on October 30, NASA terminated Kepler's observations. Thus ended nine years of prolific activity that have changed our way of seeing the Universe. Fortunately, Kepler's successor mission, TESS, is already in orbit. However, TESS has not been designed to discover exotierras, so, even if you discover one or another, you will have to wait for PLATO to have the real substitute of Kepler.

Nearly ten years after its launch, Kepler's legacy is not just those nearly 2,600 planets, but knowing that virtually all stars in the sky have worlds around, some of them Earth-like. And all as a result of Borucki's obstinacy about thirty years ago. Goodbye, Kepler, and thanks for everything.



Bill Borucki in 2015 (NASA).

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