Introduction
Space exploration has emerged as an important mission for scientists in the 21st century. Two of the major space agencies, NASA and European Space Agency have recognized the significant potential of exploring and looking for transits from space. The NASA has made considerable investments in space exploration programs. One of its most ambitious projects is the discovery program series. These projects comprise of many relatively low-cost and quickly implemented precision missions for exploring planets. The Kepler is the tenth principal investigator-led mission selected in the NASA Discovery program (Koch, 2004). This space mission was designed to look for transits from space and it was successfully launched in March 2009. The Kepler Space Observatory was named for the German Astronomer Johannes Kepler and it was able to successfully accomplish its core objectives until May 2013.
Renee (2010) states that the Kepler Space Observatory might never have become a reality were it not for the works of Johannes Kepler about 4 centuries ago. This German Astronomer was deeply fascinated with the universe and this led him to invent the rules of planetary motion. Kepler’s first published work on planetary orbits was the “Mysterium Cosmographicum” booklet written in 1596. This work attracted the attention of Tycho Brah, a Danish nobleman who had a keen interest in astronomy and had gathered a vast amount of hard data on planetary motions. Brahe asked Kepler to collaborate with him in 1600 in order to create a mathematical model for planetary motions.
By 1605, Kepler had come up with two of his laws of planetary motion, which stated, “Orbits are ellipses with the Sun at one focus, and a planet’s orbital speed varies depending on its distance from the Sun” (Renee, 2010, p.24). Kepler’s third law was discovered in 1618 and it stated, “The orbital periods of the planets were related to their average distance from the Sun” (Renee, 2010, p.24). These three laws are fundamental to astronomy and all significant astronomy innovations consider these laws. Kepler’s third law of planetary motion is used by scientists in the Kepler Mission to determine the semi-major axis for each exoplanet after observing its repeated movements in front of its stars.
Scientific Objectives of the Kepler
The Kepler Mission is NASA’s photometric space-based mission launched into space to detect Earth-like planets. The Mission set out to survey around 150,000 Sun-like stars with the aim of identifying Earth-like planets. This primary goal was supposed to be achieved within a span of 3.5 years. When the Kepler Mission was proposed, the main objectives were highlighted as the exploration of deep space with the aim of identifying planetary systems. A number of scientific objectives were highlighted for the mission. The first was to identify the terrestrial planets that existed in the habitable zone of the huge number of stars that the mission was going to analyze.
Kasting (2010) notes that the Kepler Space Observatory was specifically designed to find habitable planets, which are defined as those planets that are about one-half to ten times the Earth’s mass and exist in the habitable zone. The habitable zone is the region where distance from a star where conditions necessary for survival on a planet, such as liquid water, can be found on the planet’s surface. By analyzing a portion of the Milky Way galaxy, the Kepler Mission seeks to determine how many stars have planets that might be habitable. In addition to finding the Earth-like planets, the Kepler was tasked with determining the pattern followed by these planets in their occurrence throughout the Milky Way.
Once the terrestrial planet has been identified, further observation by the Kepler is needed to determine the orbits of the planet. Planets that have Earth-like orbit shapes and durations are likely to sustain life. Another objective of the mission is to determine the masses, densities, and surface temperatures of these exoplanets. In addition to identifying exoplanets, the Kepler Mission seeks to determine the properties of the stars where these Earth-like planets are found. Kasting (2010) notes that determining the properties of these stars is necessary in order to compare them with the Earth’s Sun.
To achieve its core-objectives, the Kepler was specially designed to be a deep space observatory center. Unlike most satellites that orbit around the Earth, the Kepler established its own orbit around the sun (The Kepler Mission, 2014). The Kepler is equipped with a one-meter Schmidt telescope that has a field of view (FOV) in excess of 100 square degrees. The telescope has an array of 42 Charge Coupled Devices (CCDs) with 95 million pixels.
Cowen (2013) declares that the Kepler Space Observatory is a marvel of engineering due to its ability to remain stable while in orbit. This stability is made possible by the presence of reaction wheels that move at the speed of 1,000 to 4,000 revolutions per minute and ensure that Kepler’s telescope are always pointing at the same location in deep space (Borucki, 2006). The Kepler Mission was supposed to last for at least three and a half years. The 3.5-year timeline for the Kepler Mission was chosen since it would take at least 3 years for the Space Observatory to confirm that a candidate planet is indeed Earth-sized and in the habitable zone.
How this Optical Observatory Works
The Kepler makes use of the photometric method known as the transit to discover planets in deep space. Renee (2010) reveals that the idea for a photometric method to detect Earth-sized planets in the galaxy was first conceived by the space scientist Bill Borucki in 1984. His idea was based on the basic concept that when a planet passes in front of a star, the light intensity diminished. The key function of the Kepler Space Observatory was to detect the “1 part in 10,000” dip in light intensity that happened when a planet orbited in front of its star. The method utilized by the Kepler Mission to identify exoplanets is referred to as the “transit method” since it relies on the changes in a star’s brightness as a planet crosses in front of its star.
The Kepler was designed to be capable of continuous observation of the same FOV through its entire working life. The observation would only be interrupted for the brief duration or a day or less in every three months. A number of considerations were made when choosing the field of view for the Kepler Mission. To begin with, the field had to be viewable for the entire duration of the Kepler Mission. To ensure that the Kepler was not blocked by the Sun at any time during its orbit, the FOV was put above 550 (The Kepler Mission, 2014).
Another requirement was that the area chosen should have a high concentration of sun-like stars. The Kepler needed to observe as many stars simultaneously as possible from its fixed position throughout the mission. Borucki (2006) notes that unlike with most other astrophysics missions that changed their FOV during the course of the mission, the Kepler Mission points to a single FOV for the entire mission. The region in space where Kepler focuses on has two constellations: Cygnus and Lyra. This region has a vast number of stars and is above 550 hence it is not obstructed by the Sun, Earth, and Moon at any point in time making it visible though the entire year.
The Kepler was then to maintain the longest possible continuous observation of the region so that variations in light intensity could be noted and investigated further. Continuous observation of the same region in space is necessary since repeat transits have to be observed before a positive declaration of a candidate planet discovery can be made. NASA (2013) states that a single instance of a dip in star light is not enough to declare that a planet has been discovered. Instead, a number of transit events have to be observed in order to confirm that a planet has been discovered. The Kepler Space Observatory is in an Earth-trailing heliocentric orbit, which enables it to have a continuous view of the selected FOV all through the orbital year.
The probability of observing a transiting planet are reduced significantly by the fact that the planetary system has to be almost perfectly aligned with the line of sight of the telescope in order for the transit to be perceived. Koch (2004) confirms that “the probability for alignment of the orbital plane along the line-of-sight from the observer to the star is relatively small, equal to the ratio of the diameter of the star to the diameter of the orbit” (p.1). For this reason, the Kepler misses up to 99% of the exoplanets that might exist within the area it is focusing on. This is the reason why the Kepler was designed to have a wide field of view (in excess of 100 square degrees). This wide FOV enables the Kepler to observe about 150,000 stars increasing the number of discoverable planets. Even so, scientists estimate that hundreds of candidate planets go undetected for every planet that Kepler detects.
Comparisons have been made between the efficiency of the Kepler mission and ground-based surveys. Stefano (2010) notes that the magnitudes of the lensing signature of the Kepler are in reach of ground-based surveys. This means that it is possible to detect antitransits using ground-based centers such as Pan-STARRS and LSST. However, it would be impossible to detect transits as efficiently as the Kepler can due to a number of limitations suffered by the ground-based surveys.
To begin with, ground stations are exposed to the changing weather conditions and cloud cover on Earth. In addition to this, the movement of the earth around the Sun means that some parts of the sky are invisible from the Earth at certain times of the year. These conditions make it impossible for ground-based observatory centers to maintain the precision pointing that the Space based observatories like the Kepler can maintain. The powerful telescope on the Kepler combined with its location in space makes it best suited to collect the data needed to identify Earth-like planets in distant star systems.
Importance of the findings from Kepler
During the active phase of its operation, the Kepler was able to make a number of important scientific findings. The success of the Kepler mission is evident from the vast number of candidate planets discovered by the mission compared to those discovered through ground-based observations. Prior to the implementation of the Kepler Mission, NASA had only been successful in discovering three candidate planets. Less than a month after its launch, Kepler began to observe thousands of stars in order to discover planets. Using the data that was obtained from the first 10 days of the star monitoring process, astronomers were able to discover five new planets.
As of the June 2010, the Kepler team was able to identify 306 exoplanet candidates after analyzing data obtained from the first 43 days. By the end of Kepler’s mission in 2013, astronomers had discovered a phenomenal 3,500 candidate planets. These planets, which orbit other suns, are in the habitable zone and their size makes them eligible to be candidate planets.
The most important discoveries made by the Kepler mission have been the positive identification of Earth-like planets. Following the detailed analysis of the data obtained from the space observatory, 135 exoplanets have already been confirmed. As the Kepler team continues to go through the vast amount of data collected by the Kepler during its mission, it is expected that hundreds or even thousands of exoplanets will be discovered.
Using the data obtained from the Kepler, scientists are able to construct elaborate profiles of the various candidate planets discovered by the mission. Once the data from the Kepler is transmitted to the Earth, scientists in the Kepler team are able to determine the size of the planet and calculate its distance from its star. It is also possible to determine the mass and surface temperature of some planet candidates by augmenting the data obtained from the Kepler with Earth-based observations.
The power and precision of the Kepler have enabled it to detect Earth-sized planets that are hundreds of light years away from the Earth. Such a feat was previously unaccomplished. The Hubble Space telescope was able to photograph an exoplanet in visible light in 2008. However, the size of this planet was large (estimated to be about three times the mass of Jupiter) and it was at a relatively close distance to the Earth at a distance of 25 light years (Kasting, 2010). The Kepler is powerful enough to detect planets that are in orbit over three thousand light years away from our Sun. The sensitivity of the Kepler’s telescope enables it to detect transits at this great distance.
The Kepler has been able to provide scientists with a rich photometric database that is populated with an enormous number of stars. This information has increased knowledge on star systems. Astronomers are now able to formulate better models of distant star systems using the data obtained from the Kepler mission. The mission has also helped in the identification of white dwarfs within the Kepler FOV. Stefano (2010) states that while the Kepler was designed to discover transits by Earth-like planets, the observatory has discovered multiple hot objects in close orbits around main-sequence stars. These objects are presumed to be remnants of stars (white dwarfs).
The information obtained from Kepler has changed the manner in which astronomers view the universe. Before the mission, astronomers only theorized about the existence of other stay systems and their number was not known. Data from the Kepler has confirmed that other star systems exist in the Milky Way. Findings by the Kepler have made scientists appreciate that there are planets in many other Stars. The Kepler has succeeded in providing scientists with statistics as to how many Earth-like planets may exist in the FOV of the telescope. Future missions will make use of the information obtained from the Kepler to seek out alien life in the identified Earth-like planets.
Recent Discoveries of Exoplanets by the Kepler
The data obtained from Kepler in 2013 contained information on planets whose orbit was close in length to that of the earth (Cowen, 2013). These are important findings since such planets are likely to be in the habitable zone, which increases the probability that they might sustain life. A number of significant discoveries have been made by the observatory in the recent past. On January 2013, NASA announced that a candidate planet, named Kepler-69c had been discovered. This planet is considered a likely habitat to alien life forms since it is Earth-like, exists in the habitable zone, and the planet orbits a star that is similar to the Earth’s Sun.
Another important discovery announced in 2013 was two Earthlike exoplanets that also exist in the habitable zone and orbit stars similar to our Sun. These exoplanets named Kepler-62e and Kepler 62f are presumed to possess liquid water. This means that they might sustain life since scientists suppose that their conditions are viable for sustaining life. In spite of the fact that the Kepler stopped searching for exoplanets in 2013, there is still a huge amount of raw data obtained from the space observatory. It will take a number of years for all the data collected from the Kepler to be analyzed and the results made public.
Impact of the Failure of the on-board Gyroscopes
In May 2013, the Kepler experienced a catastrophic failure of a second on-board gyroscope. This rendered the spacecraft unable to accomplish its primary objectives. Kepler mission scientists were aware of the vulnerability that the on-board gyroscopes on the observatory faced. The Space Observatory had four metal reaction wheels that were needed to keep the station stable. The Mission lost its first wheel in July 2012 but it was able to continue functioning since only 3 wheels were needed and one had been added for redundancy. However, Kepler lost its second wheel in May 2013, making it impossible for the spacecraft to operate since at least three wheels are needed to maintain the spacecraft’s precise orientation (Cowen, 2013).
The failure of the on-board gyroscopes meant that Kepler could no longer be relied upon to engage in the precision pointing that was necessary for the collection of data to find exoplanets. The Kepler was able to accomplish its core objectives by continuously observing the same region in space for an extended period. The gyroscopes were needed to ensure that the telescope could maintain this precise pointing. Without three functioning gyroscopes, the Kepler cannot observe the same FOV for the long period needed to discover transits (Cowen, 2013). It is therefore impossible for the Space Observatory to continue with its primary mission in its current damaged state.
The failure of the on-board gyroscopes on the observatory put the future of the Kepler at risk. NASA had to come up with other objectives to justify keeping the Kepler Space Observatory functioning for coming years. If no new scientific purpose were discovered for Kepler, then the spacecraft would have to be decommissioned. NASA made a public appeal for scientists to present proposals for valuable missions that the Kepler could be commissioned to perform even in its damaged state (NASA, 2013). Scientists responded to this call and issued a number of proposals on the type of alternative missions that the space observatory could engage in.
Future Proposals for Repurposing the Kepler
Following the failure of the second gyroscope-like reaction wheel in the Kepler, NASA engineers contemplated ways to fix the wheels in order to restore the spacecraft to full operation. It would be impossible to send astronauts into space to physically carry out repair work on the damaged wheels since the Kepler is orbiting millions of miles away. In August 2013, engineers reached a consensus that it would be impossible to fix the Kepler’s wheels after detailed analysis showed that the task was impossible (NASA, 2013). Following the decision that no future attempts to restore the Kepler space station would occur, scientists began to reconsider other science programs that the spacecraft can engage in.
One proposed future use of the Kepler is to search for comets and asteroids in deep space. It has also been proposed that the Kepler can be used to provide evidence of supernovas. Studying supernova explosions can be done using the lower photometric precision of 300 parts per million (Cowen, 2013). Even in its diminished form, the Kepler is capable of achieving this precision. The new proposed mission for the Kepler to explore deep space for supernova incidents and other objects such as asteroids and comets has been named K2. This proposal is under review by NASA and it is expected that a decision will be reached by May.
The lack of tracking/stabilization capabilities means that the Kepler cannot discover earth-like exoplanets. However, it can still discover huge exoplanets since these bodies can be discovered through gravitational microlensing as opposed to the transit method. Cowen (2013) notes that the Kepler can still discover planets that are about 3.5 times larger than Jupiter even without its precision pointing capabilities. However, such planets cannot be considered Earth-like due to the massive size.
Conclusion
This paper set out to provide a detailed discussion of the Kepler Space Observatory. It began by articulating the significance of the German Astronomer Johannes Kepler, for whom the observatory is named, on modern day astronomy. From the information provided in this paper, it is clear that the Kepler has made a rich legacy for itself. The spacecraft has helped in the discovery of thousands of candidate planets and from hundreds of earth-like planets have been verified due to the data obtained from Kepler. The success of the Kepler mission has inspired proposals for future missions that have improved capabilities to those of the Kepler. The paper notes that in spite of the loss of its precise pointing capability, the Kepler can still be put to alternative uses. The space observatory can therefore be expected to continue providing scientists with important data for space research for the next few years.
References
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Cowen, R. (2013). The Wheels Come Off Kepler. Nature, 497(1), 417–418. Web.
Kasting, J. (2010). How to Find a Habitable Planet. New Jersey: Princeton University Press. Web.
Koch, D. (2004). Overview and status of the Kepler Mission. Web.
NASA (2013). NASA Ends Attempts to Fully Recover Kepler Spacecraft, Potential New Missions Considered. Web.
Renee, J. (2010). The Kepler spacecraft’s search for other worlds. Astronomy, 38(11), 22-27. Web.
Stefano, R. (2010). Kepler as a Binary Star Mission. AIP Conference Proceedings, 131(1), 196-203. Web.
The Kepler Mission (2014). The Kepler Mission: A Quick Guide. Web.