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FAQ
Frequently Asked Questions from the Public

Attention Research Scientists:
FAQs on Kepler mission and data releases on MAST
FAQs for the Kepler Guest Observer Program

To miscellaneous recent questions...

A. Kepler Mission Science

  • A1. How will Kepler detect planets?
  • A2. What is the threshold for a detection as a planet transits a star?
  • A3. Do you need several transits to find a planet?
  • A4. Will all the stars Kepler observes have transiting planets?
  • A5. Why can't Earth-size planetary transits be observed from the ground?
  • A6. Don't the stars vary more than the change caused by a transit?
  • A7. What results are expected from Kepler science?
  • A8. When do you think will the first earthlike planet orbiting a Sun-like star be discovered?
  • A9. When will the Kepler Mission Science Office release data on planets detected by transits?
  • A10. When and if we find these exoplanets, what next? Will there be a manned mission to those planets?
  • A11. Can the data from this mission support other research programs?
  • A12. How does the Kepler Mission contribute to the Origins missions SIM and TPF?
  • A13. If Kepler were positioned around a distant star, at what distance could it detect earth? I am curious about how easily the ephemeral “others” might find us, if they exist.
  • A14. Do you think any planets could support life, and if so, what characteristics would they have to have?
  • A15. In the Kepler Planet Candidate Data Explorer, KOI-1902.01 has a planet temperature of -123°F. KOI-401.02 is 212°F? How can planets with such extreme temperatures be classified in the habitable zone?

B. About the Kepler Field of View (FOV)

  • B1. Where is Kepler pointed?
  • B2. Why did you choose the area around Cygnus and Lyra?
  • B3. How far--in miles--are the target stars from Earth?
  • B4. What is the typical distance to the stars where Kepler will find Earth-size planets?
  • B5. How long will it take Kepler to get to its target stars in Cygnus?
  • B6. How does Kepler decide which stars to study?
  • B7. Since Kepler has thruster modules, is it possible to move the spacecraft to look at a slightly different region of space, and what other regions would provide the mission with potential targets to examine?
  • B8. What are alternate names for the Kepler host stars? That would be helpful for amateurs to be able to make finding charts for observing.

C. Kepler: A Photometer

  • C1. What is the main instrument on the Kepler spacecraft?
  • C2. How does Kepler collect starlight to measure transits?
  • C3. How do CCDs (charge coupled devices) work?
  • C4. How many pixels are dedicated to each target star?
  • C5. How are individual target stars imaged by the Kepler CCDs?
  • C6. Does a starlight always land on the same pixels?
  • C7. Do the images of stars overlap on the CCDs?
  • C8. How is the data from the pixels downloaded?
  • C9. Couldn't the mission be done with a smaller photometer and cut the cost?

D. About NASA’s Kepler spacecraft

  • D1. Ball Aerospace & Technologies Corp. is responsible for developing the Kepler flight system. Which other companies contributed to this mission?
  • D2. Who built the 1.4 m (55 inch) primary mirror and what is it made of?
  • D3. How big is the Kepler spacecraft?
  • D4. How much did the Kepler spacecraft weigh at launch?
  • D5. What powers the Kepler spacecraft?
  • D6. Why is the high-gain antenna fixed on the spacecraft rather than on a gimbaled arm?
  • D7. What are the new developments that now make this mission possible?
  • D8. What obstacles had to be overcome in development of the mission?
  • D9. Why not use the Hubble Space Telescope (HST)?
  • D10. Are there other photometry missions or future plans to orbit some more Kepler type missions scanning other parts of the galaxy?
  • D11. What is the mission cost?

E. Launch and Operations

  • E1. What type of rocket was used to launch the spacecraft?
  • E2. How long will the mission last?
  • E3. Where does Kepler orbit? Kepler is in a heliocentric (Sun-centered) orbit.
  • E4. Where is Kepler now?
  • E5. How does Kepler communicate with Earth?
  • E6. What happens in 61 years when the spacecraft is closest to Earth?
  • E7. If the Kepler Mission extends beyond 3.5 years, will the spacecraft be pointed to a different part of the sky?
  • E8. Who manages the Kepler Mission during operations?

F. NASA’s Kepler Mission: history & Johannes Kepler

  • F1. When was the idea to the Kepler Mission born?
  • F2. Why did you name the Kepler Mission after the German astronomer Johannes Kepler?

G. Resources: Photographs, video, imagery, activities, lessons, events

  • G1. Where can I go on the net to see photos taken by the Kepler telescope?
  • G2. How can I locate photographs, graphics, animations and images from the Kepler Mission?
  • G3. How can the public or my classroom be involved?



A. Kepler Mission Science

A1. How will Kepler detect planets?

The Kepler Mission is designed to detect planets as they pass in front of their stars which causes a tiny dip in the stars’ light. See Occultation-Graph animation (QuickTime, 1 MB).

Kepler will look at just one large area of the sky in the constellations Cygnus and Lyra. Over the course of the mission, the spacecraft will simultaneously measure the variations in the brightness of more than 100,000 stars every 30 minutes, searching for the tiny "winks" in light output that happen when a planet passes in front of its star. The effect lasts from about an hour to about half a day, depending on the planet’s orbit and the type of star. The mission is designed to detect these changes in the brightness of a star when a planet crosses in front of t, or “transits the star.” This is called the “transit method” of finding planets. Transits are only seen when a star’s planetary system is nearly perfectly aligned with our line of sight. For a planet in an Earth-size orbit, the chance of it being aligned to produce a transit is less than 1%.

Actual H-alpha image of the sun on the left, with a Jupiter transit superimposed to scale,  as if viewed from outside our solar system.  The image of the sun on the right shows an Earth transit to scale.

A2. What is the threshold for a detection as a planet transits a star?

Transits, which cause dips in a star’s light, are minuscule compared to the brightness of the star, and challenging to detect. For an Earth-size planet transiting a Sun-like star the change in brightness is only 84 parts per million (ppm). That is less than 1/100th of 1%. For a Jupiter-size planet, the transit causes the star light to dip 1 to 2%. The figure shows to scale both a Jupiter transit across an image of our sun on the left and an Earth-size transit to scale on the right. The size of the effect for an Earth is similar to the dimming one might see if a flea were to crawl across a car’s headlight viewed from several miles away.

A3. Do you need several transits to find a planet?

The discovery of a planet is confirmed by observing several transits that have the same depth (dip in star light), duration (time to transit the star), and period (same amount of time between successive transits). A single event that looks like a transit is not enough. It must be confirmed by observing repeated transits.

A4. Will all the stars Kepler observes have transiting planets?

Transits are only seen when a star’s planetary system is nearly perfectly aligned with our line of sight. There is no preferred alignment of the plane of planetary systems. The orbits of the planets can be at any angle to our line of sight. Therefore, the Kepler Mission is designed to look at more than 100,000 stars to find the small percentage that will actually show transits. For a Jupiter-size planet orbiting close to its star, the chance of a transit is about 1 in 10 (10%). For an Earth-size planet in an Earth-size orbit, the chance of it being aligned to produce a transit is less than 1%.

The chance that the orbital plane of a planet is aligned with the observer’s line of sight so that a transit can be seen is equal to the ratio of the diameter of the star to the diameter of the planet’s orbit. (See Characteristics of Transits, specifically the section on "Geometric Probability".) For planets that are very close to their host star (and much too hot for life), the probability of alignment is 10%. For planets in the bigger orbits associated with the habitable zone of their star, the probability is about 1% or less. Therefore the transit method of searching for planes misses 90% of the inner orbit planets and 99 % or more of the planets in habitable zone orbits. That is why the design of Kepler called for a very wide field telescope to be able to observe more than 100,000 stars. If all those stars have close-in planets, about 10,000 (100,000 x .1) would be in the correct orientation to transit. If all those stars have planets in the habitable zone, about 1,000 (100,000 x .01) would be in the correct orientation to transit. Statistically, we can infer that every planet Kepler detects represents hundreds more planets that are out there but not detectable due to inopportune orbital orientation.

When a planet is discovered in an orbit of size S, around a star of size D, the probability that there are other stars with planets in similar orbits is S/D; which is the reciprocal of the probability of alignment. Consequently, each discovered planet can be used to estimate the number of planets that were missed. Doing that calculation for the 2700 planetary candidates already discovered allows us to estimate that the average number of planets per star must be close to one.

A5. Why can't Earth-size planetary transits be observed from the ground?

Crocker Dome
Crocker Dome
There are two major reasons why these observations can't be done from the ground:

  1. The motions in the atmosphere are constantly bending the rays of light from each star into different directions. This is why stars appear to twinkle. If you can see the change with your eye, you already know that the apparent brightness is changing by more than 50% (one stellar magnitude). With a lot of effort and for a very small region of the sky, astronomers have been able to measure changes as small as one part in 1,000 by comparing each star in a group to the whole group. This precision is still not good enough to find Earth-size planets, but should still be okay for detection of giant planets from the ground.
  2. To detect a planetary transit as short as 2 hours out of a year requires measuring the brightness of the stars continuously. You can't blink! That means that you would need to set up dedicated telescopes in many places around the globe, so that there would be at least one of them on the night side of the Earth at all times. But, as the Earth orbits the Sun, the available night sky continuously changes. So there is no one part of the sky that can be continuously monitored throughout the year. In addition, the inevitable bad weather and the moon makes the prospects for ground based observing even that more inefficient. This would end up being a very expensive operation, even if the stars didn't twinkle. To detect Earth-size planets, space is the necessary place to be.

A6. Don't the stars vary more than the change caused by a transit?

Star
Yes, the stars do vary in brightness all the time. In fact it is almost impossible to make a perfectly constant source of light. Fortunately, the stars we are most interested in are stars like our Sun. They vary less than the change in brightness caused by an Earth-size planetary transit on the same time scale as a transit (a few hours).
Our Sun varies over many time scales: There are Maunder minimums, which do not occur for many centuries or longer and have caused "mini ice ages" even as recently as during the 17th century. There is an eleven-year "solar cycle" of minimum and maximum activity. The largest short-term variations are caused by "sun spots" that appear and fade, and rise and set as the Sun rotates with a period of four weeks.
Planetary transits have durations of a few hours to less than a day. The measured solar variability on this time scale is 1 part 100,000 (10 ppm) as compared to an Earth-size transit of 1 part in 12,000 (80 ppm). Even then, most of the variability is in the UV, which is excluded from the measurements by the Kepler Mission . For more information, read about “ Stellar Variability.”

A7. What results are expected from Kepler science?

To estimate possible results for the Kepler Mission science some assumptions must be made:

  • One-hundred thousand main-sequence stars are monitored and variable stars are excluded;
  • On average, two Earth-size or larger planets exist in the region between 0.5 and 1.5 AU. (An AU is an astronomical unit, the average distance from the Sun to the Earth.)
  • About 0.5% of star-planet systems are aligned to allow transit observations of planets in or near the habitable zone of the star. (The habitable zone is the region where water can exist as a liquid on the surface of a planet.)

With those assumptions the number of detections still depends on what size planets are common. We may find

  • About 50 planets if most are about 1.0 Earth radius in size
  • About 185 planets if most are about 1.3 Earth radii in size
  • About 640 planets if most are about 2.2 Earth radii in size
    (Or possibly some combination of the above)
  • About 12% of the cases with two or more planets per system

Most likely results will be some combination of the above. There will also be hundreds of giant planets found and many instances with two or more planets per system. For more on expected results see the Expected Results page.

A8. When do you think will the first Earthlike planet orbiting a Sun-like star be discovered?

Kepler is seeking evidence of Earth-size planets in the habitable zone of Sun-like stars. To confirm an Earth, the science mission team requires a minimum of 3 transits of the same period, depth and duration. In the case of the Sun-like star, the period of the planet would be about the same as the Earth: one year. Thus, it will require a minimum of 3 years (and likely longer) to find an Earth-size planet in the habitable zone, and confirm the observations with 3 transits. Kepler launched in 2009, and the soonest we anticipate announcing an Earth-size planet orbiting a Sun-like star would be sometime in 2012-2013. For more on expected results see the Expected Results page.

A9. When will the Kepler Mission Science Office release data on planets detected by transits?

It is NASA policy to release all scientific information as quickly as consistent with their calibration, validation and meeting mission goals.

Whenever possible, the Kepler team attempts to confirm a planet candidate via radial velocity (spectroscopic) methods from ground-based observatories. The Kepler field of view is in the Cygnus region of the sky, only visible in the summer from the northern hemisphere. For that reason, the year's discoveries are announced well after the summer observing season.

For the data to be released in a form that is of value and that maintains the scientific integrity of the mission, it is released in a processed format, not simply the raw strings of bits returned by the spacecraft. It takes several months for data to be validated and especially for mission integrity, false positive events—ones that look like transits but are caused by other phenomena such as grazing binary stars—must be checked through ground-based observations of the stars.

Data for each 3-month observation period will be made public within one year of the end the observation period. For stars that get dropped from the planet search program, data will be made public within 2 months of their being dropped.

The data from the Kepler Mission is archived at the Kepler archive in the Multimission Archive at STScI (MAST) and is expected to be supported for ten years after the end of mission.

A10. When and if we find these exoplanets, what next? Will there be a manned mission to those planets?

NASA is not contemplating a manned mission to any stars, because it would take so long -- even the closest star, at 4 light-years (LY) distance, would take thousands of years to reach at any speeds we can attain now. The next steps will be to detect light directly from the planet—enough to obtain a spectrum that would tell us what type of atmosphere the planet has, which would give clues as to whether or not the planet actually has life. NASA missions that are being planned that have some of these capabilities are the Space Interferometry Mission (SIM) and the James Webb Space Telescope (JWST).

A11. Can the data from this mission support other research programs?

Obtaining astrophysical information about each star is a natural byproduct of detecting planetary transits. The following are some of the potential uses for these data:



Phenomena

Information Obtained

Stellar rotation rates Variation in rates with stellar type
p-mode oscillations Window on stellar interior:
mass, age, metallicity of stars
Characteristics of solar-type stars Determine what is a "normal" star
Frequency of Maunder minimum Earth climate implications
Stellar activity Star spot cycles, white light flaring,
Paleoclimatology
Cataclysmic variables Pre-outburst activity, mass transfer
Eclipsing binaries Detection of high-mass ratio binaries
Active Galactic Nuclei variability "Engine" size in BL Lac, quasars and blazars


The data from the Kepler Mission is archived at the Kepler archive in the Multimission Archive at STScI (MAST).
For further information, see “ Related Science.”

A12. How does the Kepler Mission contribute to the Origins missions SIM and TPF?

How does the Kepler Mission contribute to  the Origins missions SIM and TPF?
The Kepler Mission contributes in several ways to both the Space Interferometry Mission (SIM) and the Terrestrial Planet Finder (TPF) mission: The Kepler Mission determines the frequency of terrestrial and smaller planets in a larger volume of our Galaxy than available to either SIM or TPF and thereby determines the expected number of planets that either SIM or TPF might observe. If terrestrial planets are rare in the extended solar neighborhood, then the capabilities of both these missions will need to be increased. From the distribution of planets among the different stellar types observed, SIM and TPF will know which types of stars in our immediate solar neighborhood are most likely to have planets. Although neither of these missions will be able to detect terrestrial planets as far away as the Kepler Mission can, the Kepler Mission does identify planetary systems already known to have terrestrial planets. SIM and TPF can observe these systems to determine if there are other larger planets which would not have been seen transiting their parent stars, thereby providing a more complete picture of the composition of planetary systems having known terrestrial planets. The systems found by the Kepler Mission to have terrestrial planets can be examined in the infrared to measure the amount of zodiacal light within each system. If large amounts of zodiacal light are common, then it may be difficult for TPF to image any planets. Results from the Kepler Mission provide the sustained impetus to fund the much more ambitious TPF as stated by the National Academy of Sciences (NAS) decadal survey report Astronomy and Astrophysics in the New Millennium (p. 7), which calls for building the TPF space mission "predicated on the assumption ... that, prior to the start of TPF, ground- and space-based searches will confirm the expectation that terrestrial planets are common around solar-type stars."

A13. If Kepler were positioned around a distant star, at what distance could it detect earth?
I am curious about how easily the ephemeral “others” might find us, if they exist.

Most of the planets Kepler will detect will typically be between about 100pc and 1kpc (1parsec=3.1 light years). So If someone had the equivalent of Kepler in a solar system at say 500 pc they could detect Earth. But... that star would have to be within about 1/2 degree of being along the ecliptic plane on the sky in order for them see see Earth transiting the Sun.

A14. Do you think any planets could support life, and if so, what characteristics would they have to have?

Temperature scale
Actually, that's the main goal of the Kepler mission -- to find planets that could support life. And yes, I think there must be some planets that could support life.

The simplest requirement for a planet to have life (carbon-based like on Earth), is for there to be liquid water. That means the temperature must be above the freezing point of water (0 degrees C or 32 degrees F) and below the boiling point (100 degrees C or 212 degrees F). That leads to other requirements: the planet must stay far enough away from the star, yet close enough to be in that temperature range for liquid water. This zone of distances from a star where liquid water can exist is known as the habitable zone, since that's the region that living things could inhabit.

Earth on a scale
Another requirement is for the planet to have enough air, but not too much air (like the giant planets have). This depends mostly on the planet's size. It must be big enough to have sufficient gravitational pull to hold onto air molecules. At less than 0.8 Earth radii (0.5 Earth masses) the planet would not have enough surface gravity to hold on to a life sustaining atmosphere. That rules out a Mars size planet (about 0.5 Earth radii). At about 2 earth radii (8 Earth masses), a planet would have enough surface gravity to hold onto hydrogen and helium and turn into a gas giant. This depends on composition of the planet, that is fraction of silicates, metals etc. If it's Neptune size (about 4 times Earth diameter) or bigger it's definitely getting to have too much gravity and will hold onto way too much atmosphere. If such giant planets had very large moons---more than half the Earth's diameter---then those moons might support life, if the whole moon-planet system was in the habitable zone of the star.

An informative movie that includes an explanation of habitable zones may be found at TED-Ed - http://ed.ted.com/lessons/a-needle-in-countless-haystacks-finding-habitable-planets-ariel-anbar

A15. In the Kepler Planet Candidate Data Explorer, KOI-1902.01 has a planet temperature of -123°F. KOI-401.02 is 212°F? How can planets with such extreme temperatures be classified in the habitable zone?

The definition of habitable zone is often stated simply as saying temperature must be between 0 and 100 °C. However, actual conditions on real planets (or moons) vary, as we can see in our solar system, where it looks like Europa has liquid water under its icy crust. For that reason, the range of temperatures included in habitable range is expanded some from the nominal values of freezing point and boiling point of water 273-373K (0-100°C). The 212°F temperature you mentioned is the boiling point of water...right at the edge of habitable. Some microorganisms can live at such temperatures, which technically qualifies as habitable.

The calculations for the table start with the power of starlight intercepted by the planet (in units of that received by Earth, known as the solar constant). We further assume that the planet actually reflects 30% of the energy falling on it (an albedo or reflectivity of 0.3). What is absorbed by that planet is balanced by "blackbody radiation" given off by the planet that results in an average equilibrium temperature for the planet. We cannot know the actual ground temperature from Kepler data, but we can calculate an equilibrium planet temperature for an "ideal" blackbody. But that does not take into account other factors like presence of greenhouse gases in the atmosphere or internal heat sources, not to mention actual albedos that can differ radically from 0.3. Greenhouse gases can raise the planet's temperature by anywhere from a few degrees to hundreds of degrees (in the case of Venus e.g.). Greenhouse warming for Earth is 33 K.

Clearly the concept of habitable zone is not an exact science.



B. About the Kepler Field of View (FOV)

B1. Where is Kepler pointed?

Kepler Field of View

The definition of habitable zone is often stated simply as saying temperature must be between 0 and 100 °C. However, actual conditions on real planets (or moons) vary, as we can see in our solar system, where it looks like Europa has liquid water under its icy crust. For that reason, the range of temperatures included in habitable range is expanded some from the nominal values of freezing point and boiling point of water 273-373K (0-100°C). The 212°F temperature you mentioned is the boiling point of water...right at the edge of habitable. Some microorganisms can live at such temperatures, which technically qualifies as habitable.

  1. The field must be continuously viewable throughout the mission.
  2. The field needs to be rich in stars similar to our sun because Kepler needs to observe more than 100,000 stars simultaneously.
  3. The spacecraft and photometer, with its sunshade, must fit inside a standard Delta II launch vehicle. The size of the optics and the space available for the sunshield require the center of the star field to be more than 55-degrees above or below the path of the sun as the spacecraft orbits the sun each year trailing behind the Earth. The Sun, Earth and Moon make it impossible to view some portions of the sky during an orbital year. Thus, Kepler looks above the ecliptic plane to avoid all these bright celestial objects.

These constraints limited scientist to two portions of the sky to view, one each in the northern and southern sky. The Cygnus-Lyra region in the northern sky was chosen for its rich field of stars somewhat richer than a southern field. Consistent with this decision, all of the ground-based telescopes that support the Kepler team’s follow-up observation work are located at northern latitudes. The star field in the Cygnus-Lyra constellations near the galactic plane that meets these viewing constraints and provides more than 100,000 stars to monitor for planetary transits. For additional information, read “ Target Field of View."

B2. Why did you choose the area around Cygnus and Lyra?

We need a region of the sky that is both rich in stars, and one where the Sun does not get in the way throughout the entire orbit of the Kepler spacecraft. Cygnus is far enough north of the plane of Earth' orbit (the ecliptic) that the Sun will not encroach on Kepler's view, yet is in a very star-rich part of our Milky Way galaxy. For additional information, read “ Target Field of View.”

B3. How far--in miles--are the target stars from Earth?

The stars that Kepler is observing are in the range of a few hundred to a few thousand light-years away. One light year is about 6 trillion (6,000,000,000,000) miles.

B4. What is the typical distance to the stars where Kepler will find Earth-size planets?

Kepler target area
Kepler will be looking along the Orion spiral arm of our galaxy. The distance to most of the stars for which Earth-size planets can be detected by Kepler is from 600 to 3,000 light years. Less than 1% of the stars that Kepler will be looking at are closer than 600 light years. Stars farther than 3,000 light years are too faint for Kepler to observe the transits needed to detect Earth-size planets. For further information, read “ Dependencies of Detectable Planet Size.”

B5. How long will it take Kepler to get to its target stars in Cygnus?

The Kepler spacecraft is not traveling to the stars in Cygnus. It will orbit our own Sun, trailing behind Earth in its orbit, and stay pointed at Cygnus starfield for 3.5 years to watch for drops in brightness that happen when an orbiting planet crosses (transits) in front of the star. Cygnus was chosen because it has a very rich starfield and is in an area of sky where the Sun will not get in the way of the spacecraft's view for its entire orbit.

 

B6. How does Kepler decide which stars to study?

The bulk of the stars that were selected are more or less sun-like, but a sampling of other stars were included as well. One of the most important factors was brightness. Detecting minuscule changes in brightness caused by transiting planet is impossible if the star is too dim and/or noisey. Kepler is monitoring stars that are as faint as 16th magnitude, although stars fainter than about 14.5 magnitude are very difficult to perform follow-up observations on. Even for stars fainter than 12th magnitude, they will have to be quieter than the sun or the planets larger than earth to be able to detect transits. Also, there are only a few hundred stars brighter than 9th magnitude.

Another factor in selection was stellar type which is related to the star temperature, size, and mass (see http://en.wikipedia.org/wiki/Stellar_classification). The Sun is a type G2 dwarf star—effective temperature: 5778 K. For stars much hotter (larger and more massive) than the Sun an Earth-size transit is much smaller and more difficult to detect, but can be done if the star is bright and quiet. The graphs below show roughly how many Kepler target stars there are for various temperatures and for various brightnesses.

Distribution of target stars in terms of effective temperature and brightness

 

B7. Since Kepler has thruster modules, is it possible to move the spacecraft to look at a slightly different region of space, and what other regions would provide the mission with potential targets to examine?

Photo of reaction wheel from Ball Aerospace
Photo of reaction wheel from Ball Aerospace

The thrusters are not really for orienting the spacecraft. Orienting the spacecraft is accomplished by reaction wheels, which are special electric motors mounted on the spacecraft that act like specialized gyroscopes. Changes in the motor spin rates result in changes in the spacecraft orientation in different directions without resorting to firing rockets or jets. The motor spin rates are controlled electronically by computer and are essential for altering spacecraft orientation by very small amounts, as needed for keeping the Kepler telescope pointed precisely at it's designated sky target area. The reaction wheels also do the job of rolling the spacecraft 90 degrees every 3 months to keep the solar panels pointed at the Sun.

Use of reaction wheels minimize the amount of fuel needed by the spacecraft. But external forces/torques on the spacecraft, in particular sunlight striking the spacecraft, imparts spin to the spacecraft which the reaction wheels continuously compensate for until they reach their maximum speed (called saturation). The gradual buildup of reaction wheel rotation speed/angular momentum needs to be cancelled once the saturation point is reached. At this point thruster modules are fired (and fuel is used) to spin down (desaturate) the reaction wheels. This happens about every 3 days.

Only 3 reaction wheels are needed to control the 3 degrees of freedom of rotation of spacecraft. But Kepler was provided with 4 reaction wheels, one extra for redundancy in case a wheel fails.

Kepler's Orbit

It's essential that the telescope point at the exact same field of view throughout the mission. That is because it's not sufficient to detect only one planet transit to establish discovery of a planet. Multiple transits are required. And for planets in the habitable zone of a Sun-like star, those transits would only occur every year or so. That's why the mission duration is at least 3.5 years----to find habitable planets around Sun-like stars. If we pointed the telescope somewhere else, we would have to observe this new field of view for 3.5 years or more to reach our science goals. Kepler has shown us, that planets seem to be fairly common, so any other field is likely to show many planet transits such as the current field of view.

Unlike other missions that are observatories designed to look anywhere on the sky, The Kepler hardware (focal plane orientation, sunshade, solar panels, radiator, etc) were designed for this specific star field. The spacecraft could be pointed elsewhere, but sun angle, thermal, power and other things would have to be studied first. The mission design was optimized for this star field which was studied extensively before hand to identify the stars to target. The operations have been tuned to this orientation. Hence, it would be extremely costly and disruptive to the existing science program to point anywhere else on the sky. Also, see FAQ B1: Where is Kepler pointed?

B8. What are alternate names for the Kepler host stars?
That would be helpful for amateurs to be able to make finding charts for observing.

One of the best techniques for finding alternate star names is to use the SIMBAD database at http://simbad.u-strasbg.fr/simbad. Use the "Basic Search" and enter the Kepler name, e.g. Kepler-4, and the result will include other know names (Identifiers).



C. Kepler: A Photometer

C1. What is the main instrument on the Kepler spacecraft?

Kepler CCD Array
The sole instrument aboard Kepler is a photometer (or light meter), an instrument that measures the brightness variations of stars. The photometer consists of the telescope, the focal plane array, and the local detector electronics. Kepler is a 0.95-meter (37-inch) aperture Schmidt-type telescope with a 1.4-meter (55-inch) primary mirror. For an astronomical telescope, Kepler’s photometer has a very wide field of view: it’s about 15 degrees across. It would take 30 Moons lined up in a row to span the Kepler field of view. The photometer features a focal plane array with 95 million pixels. The focal plane array is the largest camera NASA has ever flown in space. For further information, read “Photometer and Spacecraft
Kepler Photometer Cross-Section

C2. How does Kepler collect starlight to measure transits?

See Optical Path Animation (QuickTime, 5.67 MB).

The Kepler spacecraft is a single-purpose Schmidt telescope. It stares continuously at a large field of view (see “Where is Kepler pointed?”) to observe more than 100,000 stars simultaneously. The starlight enters the telescope, reflects from the primary mirror to the focal plane array of 21 modules each with two 50x25 mm 2200x1024 pixel CCDs. The pixels (picture elements) collect the photons of light from the stars. Every 6 seconds, the array “reads out” the number of photons in each pixel to an onboard computer for storage and initial processing. For the selected stars, the data (photon counts) accumulates in an on board computer, and is transmitted to Earth once each month. For more information, see the next question, “How do CDDs (charge coupled devices) work?”

C3. How do CCDs (charge coupled devices) work?

In a CCD, the silicon region is divided electrically into small individual picture elements or pixels with about four hundred elements per cm in each direction, like a very finely divided sheet of graph paper. The free electrons are kept from moving around by permanent channel stops (the vertical lines in the figure) and externally applied voltages (the horizontal lines in the figure). Each pixel can then be thought of as an individual bucket or well that collects electrons.

As shown in the animation, first the CCD is exposed to light from a telescope or camera lens. Overtime this produces an image made up of electrons in the CCD.
To readout an image that has been captured with the CCD requires shifting the information out of the pixels. First, the columns of pixels are all shifted down one row. The bottom row of pixels is shifted into a readout register. Each pixel in the readout register is shifted out to an amplifier and the number of electrons in each pixel are recorded. This produces a series of 1's and 0's that represent the image. This is repeated over and over until all the pixels have been read. The stream of 1's and 0's is then digitally processed to reproduce the image that is later displayed.
In the Kepler Mission the 1's and 0's are recorded onboard the spacecraft and sent to the ground, where the data are processed to look for changes in the brightness of each star that may be caused by a planetary transit. For further information, read “ Photometer and Spacecraft.”

C4. How many pixels are dedicated to each target star?

The image of a star is spread across a “postage stamp” of 30 pixels.

C5. How are individual target stars imaged by the Kepler CCDs?

The star images are spread across a “postage stamp” of 30 pixels. By spreading the light across a set of pixels, Kepler captures all of the photons from a target star without a single pixel saturating (filling up), which would produce faulty data. A second reason for spreading the light across a “postage stamp” of pixels is to compensate for any spacecraft movement. By surrounding the image of the star with sufficient pixels, any tiny movement of the spacecraft will not push the star beyond its “postage stamp” and the mission scientists can be assured that the measurements are accurate.

C6. Does a starlight always land on the same pixels?

Every three months, the spacecraft is reoriented (rotated one-quarter turn) to keep the solar panels pointed toward the Sun. During each 3-month observing period, the starlight does land on the same set of pixels. But after 3 months, when the spacecraft has been rotated, the light from each target star is collected by a new set of pixels on a different CCD. The overall arrangement geometry of the CCDs was designed so that the configuration is nearly identical after each 3-month rotation, except for the center CCD module. For the center module, the stars simply move to a different set of pixels on the same CCD.

C7. Do the images of stars overlap on the CCDs?

More than 100,000 target stars were selected in the Kepler field of view. These stars were chosen so that they do not overlap other stars or more distant background galaxies. The data from each selected star is carefully reviewed to eliminate binary stars, and distant field stars and galaxies. The data for each target star is separate from the data for other nearby target stars.

C8. How is the data from the pixels downloaded?

The Kepler data is stored onboard, and downloaded once per month. It is transmitted from the spacecraft to Earth via NASA’s Deep Space Network. The NASA Deep Space Network - or DSN - is an international network of antennas that supports interplanetary spacecraft missions and radio and radar astronomy observations for the exploration of the solar system and the universe. The DSN currently consists of three deep-space communications facilities placed approximately 120 degrees apart around the world: at Goldstone, in California's Mojave Desert; near Madrid Spain; and near Canberra Australia. This strategic placement permits constant observation of spacecraft as the Earth rotates, and helps to make the DSN the largest and most sensitive scientific telecommunications system in the world. From the DSN, the data flows to the Mission Operations Center in Boulder, CO, then to the Data Management Center in Baltimore, MD, home of the Hubble Space Telescope Science Institute. The raw data is archived at the Data Management Center, and then transmitted to the Kepler Science Operations Center (SOC) at NASA’s Ames Research Center in California. At the SOC, the data is processed and analyzed to produce calibrated light curves. These light curves can reveal the presence of a planet.

Does all of the data collected by the Kepler CCD array get sent back to Earth? No, only the data for selected stars gets sent back to Earth. The Kepler instrument detects light using a set of 21 CCD modules each with two specialized CCDs (the focal plane array) that cover the field of view (105 degrees square). Each “snapshot” produces the equivalent of a 95 megapixels image every 6 seconds. The onboard computer is programmed to keep only the data for the targeted stars, and discard the rest. The onboard computer adds together the 6-second data snapshots into 30 minute observations, and stores that information. The data for non-targeted stars and galaxies is discarded before transmission to Earth.


C9. Couldn't the mission be done with a smaller photometer and cut the cost?

cost-performance curve
A representation of the scientific performance versus project cost is shown in the figure. A well conceived project is at A with maximum possible science per dollar available. Many times, those who fund a program perceive the project to be at B, where costs can be cut without much loss in science; while the science team tries to believe that they are at C, where more science can be achieved at little extra cost. Good clever scientists and engineers might be able to get to point D, but this is unusual. Project managers worth their weight in gold are those who can push toward E, keeping the performance, but saving on cost. Any project headed from A to B A to C or A to F is doomed to be canceled or should be canceled.

For the Kepler Mission to work, 100,000 main-sequence stars must be monitored to a differential photometric precision of 1:50,000 every 6.5 hours. Substantially fewer stars and the results may turn out to be ambiguous. The necessary precision requires recording ten billion photons from each star every 6.5 hours. Thus, a smaller photometer would mean either fewer stars at the required precision or poorer precision for most of the stars and thereby the inability to detect Earth-size planets. Also, the photometer would need to be much smaller before other costs, such as the launch vehicle, would begin to drop significantly.

Our project manager has worked hard at both increasing the performance by increasing the downlink data rate to permit monitoring 100,000 stars (originally we planned to monitor only 5000 stars) and in reducing the cost by changing the orbit to an Earth-trailing heliocentric orbit and thereby eliminating an expensive propulsion stage needed to get to an L2 halo orbit. This also allowed us to use a smaller and less costly launch vehicle. In essence, we have already pushed the cost-performance curve in both the D and E directions.



D. About NASA’s Kepler spacecraft

D1. Ball Aerospace & Technologies Corp. is responsible for developing the Kepler flight system. Which other companies contributed to this mission?

Ball Aerospace & Technologies (Colorado) E2V (England), Corning Incorporated (New York), Brashear LP (Pennsylvania), and……

D2. Who built the 1.4 m (55 inch) primary mirror and what is it made of?

Corning Incorporated (New York) provided the mirror blanks of fused silica for the primary and corrector plate. Brashear LP (Pennsylvania) figured the primary and corrector plate for the Schmidt telescope that is the main instrument. Ball Aerospace and Technologies Corporation (Colorado) constructed the spacecraft.

D3. How big is the Kepler spacecraft?

The overall size is about 2.7 meters (nine feet) in diameter and 4.7 meters (15.3 feet) high.

D4. How much did the Kepler spacecraft weigh at launch?

The total mass at launch was 1052.4 kilograms (2,320.1 pounds) consisting of 562.7-kilograms (1240.5-pounds) for the spacecraft, 478.0-kilograms (1043.9-pounds) for the photometer, and 11.7 kilograms (25.8 pounds) of hydrazine propellant.

D5. What powers the Kepler spacecraft?

Power is provided by four non-coplanar panels with a total area of 10.2 square meters (109.8 square feet) of solar collecting surface area. Combined, the 2860 individual solar cells can produce over 1,100 Watts. Power storage is provided by a 20 Amp-hour rechargeable lithium-ion battery. The spacecraft must execute a 90 degree roll every 3 months to reposition the solar panels to face the Sun while keeping the instrument aimed at the target field of view. See animation.

D6. Why is the high-gain antenna fixed on the spacecraft rather than on a gimbaled arm?

Early in the spacecraft design phase, a decision was made to mount the antenna directly on the spacecraft. There were two reasons: it reduced the mission design and construction cost, and it reduced risk of failure of the arm to extend the antenna after launch.

D7. What are the new developments that now make this mission possible?

Two recent research results have enabled the practicality of the Kepler Mission:

  1. The demonstration that charge coupled devices (CCDs) have the needed photometric performance to make the measurements. All sources of noise (photon shot noise, stellar variability (see above), CCD noise and pointing jitter) when combined must be less than one part in 50,000 (20 ppm); four times less than the effect of an Earth-size transit. The required CCD performance with all the known noise sources has been achieved in recent laboratory measurements along with the detection of Earth-size transit signals (Koch, et al. 2000). Thus, CCDs can be used to simultaneously measure tens of thousands of stars at one time.
  2. Until recently, no one knew what the variations in stellar brightness were on the time scale of a fraction of a day. This information is now available for one star, our Sun. These data indicate that on the time scale of a transit, the variability is typically ten times less than the effect being measured. Fortunately, our Sun is one of the more common stellar types, and we expect other solar-like stars to behave in a similar fashion.

D8. What obstacles had to be overcome in development of the mission?

The main obstacles were (a) technical challenges, to achieve instrumentation and systems operations precise enough to detect transits of Earth-size planets, (b) budgetary restrictions and alterations, and (c) procuring highest quality materials and components. Two CCD manufacturers were initially contracted to make CCDs for Kepler mission and ultimately, one of them was able to produce the quality CCDs required for the mission. Budget constraints caused slight shortening of the nominal mission length from 4 years to 3.5 years and also triggered the decision to replace the gimbaled communications antenna with a fixed one.

D9. Why not use the Hubble Space Telescope (HST)?

Hubble Space Telescope
There are three basic reasons why the HST could not be used to look for planets in the way described here:

  1. The field of view (FOV) of the HST is too small to observe a large number of bright stars. The FOV of the HST is about the size of a grain of salt held at arm's length. There is almost never more than one bright star in the HST FOV at any one time. However, the FOV of the Kepler Mission photometer is about the size of both of your open hands held at arm's length. Or another way of looking at it is, that it is about equal to the size of two "dips" of the Big Dipper.
  2. The brightness of every target star has to be measured continuously, not just once in a while, since one does not know when to expect a transit to happen. The HST is for the use of the entire astronomical community to address thousands of questions and would not be dedicated to just one question requiring continuous use for up to four years.
  3. The HST does not have a specially designed photometer observing over 100,000 stars simultaneously with the precision required for the measurements needed to detect Earth-size transits.

The HST has been used by Ron Gilliland to look for transits of giant planets with periods of only a few days in the globular cluster 47 Tuc, a region of very high star density. No transits were detected.

D10. Are there other photometry missions or future plans to orbit some more Kepler type missions scanning other parts of the galaxy?

Two other photometry missions are MOST and COROT, however, they are considerably less capable than the Kepler Mission, since their primary science mission is to measure the properties of stars. COROT has 1/10 the collecting area for photons, 1/20th the field of view of the sky and stares at a given star field for 1/10 the amount of time that the Kepler Mission stares. MOST is an even smaller mission and less capable for planet detection. MOST was launched on June 30, 2003 and has produced spectacular photometric results on microvariability of stars - asteroseismology. COROT was launched on 27 December 2006. COROT also does asteroseismology, and has found extrasolar planets, though not Earth-size planets in the habitable zone.

May 2013 (after failure of second reaction wheel): There will be no replacement for Kepler. With the data it has already collected but not yet analyzed, Kepler should be able to accomplish all its goals; i.e. determine the fraction of stars in our galaxy that have near-Earth-size planets in the habitable zone of their host star.

Kepler is just the first step in our exploration of the galaxy in the search for life. The second step is the TESS Mission that was chosen by NASA in early 2013 to fly in 2017. It will search all the nearby stars to find the closest stars with planets. After it finishes its task in 2019, NASA may fund a mission to accomplish the third step. That mission will block out the light from the star so that the light from the planet can be analyzed to determine if the planet has an atmosphere, an ocean, and whether the atmosphere has water, carbon dioxide, and oxygen. Our exploration of our galaxy has just begun.

There are no plans to duplicate the Kepler spacecraft and point it toward a different part of the galaxy. Kepler is observing enough samp[ling of stars in the galaxy to be able to deduce the prevalence of planets in the galaxy.

There are new related missions proposed, e.g. TESS (http://web.mit.edu/newsoffice/2011/tess-nasa-award.html) and FINESSE (http://www.jpl.nasa.gov/missions/details.php?id=5996). There's further information at: http://www.nasa.gov/home/hqnews/2011/sep/HQ_11-328_Science_Proposals.html.

D11. What is the mission cost?

The Kepler Mission life cycle cost is approximately $600 million. This includes the design, construction, launch and operation of the spacecraft as well as the scientific analysis of the data. The Mission involves scientists and engineers across the United States, Canada and Europe (see http://kepler.nasa.gov/about/team.html). The Kepler science office is at NASA Ames Research Center.



E. Launch and Operations

E1. What type of rocket was used to launch the spacecraft?

The Kepler spacecraft lifted off March 6, 2009 aboard a Delta II rocket from Cape Canaveral Air Force Station in Florida. Launch occurred at 10:49 p.m. EST.

E2. How long will the mission last?

The Kepler Mission is scheduled to observe for a minimum of 3.5 years. The spacecraft is designed to observe up to 6 years. Generally, NASA’s Science Mission Directorate reviews active missions, and makes the decision to extend missions based upon the opportunity for further scientific discoveries. For more information, read “ Launch Vehicle and Orbit.”

E3. Where does Kepler orbit?

Kepler's Orbit
Kepler is in a heliocentric (Sun-centered) orbit. Kepler’s orbit was chosen to enable continuous observation of the target stars. This requires that the field of view of Kepler never be blocked. For a spacecraft in low-Earth orbit, nearly half of the sky is blocked by the Earth and the obscured region is constantly changing. The most energy efficient orbit beyond Earth orbit is a heliocentric (Sun centered) Earth-trailing orbit. An Earth-trailing heliocentric orbit with a period of 371 days provides the optimum approach to maintaining a stable trajectory that keeps the spacecraft within telecommunications capability. Another advantage of this orbit is that it has a very-low disturbing torque on the spacecraft, which leads to a very stable pointing attitude. The spacecraft must execute a 90 degree roll every 3 months to reposition the solar panels to face the Sun while keeping the instrument aimed at the target field of view. See animation. Not being in Earth orbit means that there are no torques due to gravity gradients, magnetic moments or atmospheric drag. The largest external torque then is that caused by light from the sun. This orbit also avoids the high-radiation dosage associated with an Earth orbit, but is subject to energetic particles from cosmic rays and solar flares. For more information, read “ Launch Vehicle and Orbit

E4. Where is Kepler now?

Kepler has been added to the Jet Propulsion Lab (JPL) Horizon database http://ssd.jpl.nasa.gov/horizons.cgi
Click on "Target Body [change]" and choose Spacecraft > Kepler.

E5. How does Kepler communicate with Earth?

The telecom subsystem will be used for receiving commands and for transmitting engineering, science and navigation data back to Earth. It is designed to operate out to a distance of 96 million kilometers (about 60 million miles). The system uses a parabolic dish high-gain antenna for transmitting, two receiving low-gain antennas and two transmitting low-gain antennas. The system can receive commands from Earth at speeds ranging from 7.8 to 2,000 bits per second, and can send data to Earth at speeds from 10 to 4.3 million bits per second. This transmission capability is the highest data rate of any NASA mission to date.
Telecommunications and navigation support for the mission are provided by NASA’s Jet Propulsion Laboratory (JPL) and NASA’s Deep Space Network (DSN). During the science phase of the mission, Kepler will perform its data-gathering duties automatically. Twice a week, the operations team contacts the spacecraft to assess its health and status and upload any new command sequences. Once per month, the spacecraft stops taking data for one day, re-orientates the spacecraft to point the high-gain antenna at the Earth and downlinks the science data. Every three months, the spacecraft also must be rotated 90 degrees about the optical axis to maintain the maximum exposure on the solar array and to ensure the spacecraft’s radiator is pointing towards deep space. After rotation, the instrument requires a new star pixel map for the 100,000 target stars and the 87 fine guidance sensors stars. (Insert image of spacecraft with antenna labeled--

E6. What happens in 61 years when the spacecraft is closest to Earth?

Like Earth, the Kepler spacecraft orbits the Sun (heliocentric orbit). But, Kepler is in an “earth-trailing” orbit, taking 371 days to orbit the Sun. After 61 years, it will be in the vicinity of the Earth, but not collide. With a smile, Mission Principal Investigator Bill Borucki says “My grandchildren will retrieve Kepler, and place it in the new National Air and Space Museum on the Moon!” Kepler may simply be left in it orbit.

E7. If the Kepler Mission extends beyond 3.5 years, will the spacecraft be pointed to a different part of the sky?

If the Kepler Mission is extended, it will continue to observe the same portion of the sky (same field of view). To confirm a planet, the science mission team requires a minimum of 3 transits of the same period, depth and duration. An extended mission would enable the discovery of planets on longer orbits at greater distances from their parent stars.

E8. Who manages the Kepler Mission during operations?

Mission operations during both commissioning and science operations phases of the mission involve several organizations, including: NASA’s Ames Research Center, Moffett Field, Calif., which will conduct Mission management and operate the Science Operations Center (SOC); The Laboratory for Atmospheric and Space Physics (LASP) at the University of Colorado in Boulder, Colo., which is the site of the Mission Operations Center (MOC); Ball Aerospace & Technologies Corp., also located in Boulder will use its Flight Planning Center (FPC) to provide engineering support; NASA’s Jet Propulsion Laboratory, Pasadena, Calif. will use its Deep Space Network (DSN) for navigation and communication; Space Telescope ScienceInstitute (STScI) in Baltimore, Md. will provide the data management services.



F. NASA’s Kepler Mission: history & Johannes Kepler

F1. When was the idea to the Kepler Mission born?

Bill Borucki and A. L Summers first proposed finding planets with transit photometry in 1984 in an article titled “The photometric method of detecting other planetary systems.” published in Icarus, 58, 121.

F2. Why did you name the Kepler Mission after the German astronomer Johannes Kepler?

NASA’s Kepler Mission was named in honor of Johannes Kepler because he was the first person to describe the motions of planets about the Sun in such a way that their positions could be precisely predicted. He derived three laws of planetary motion from observational data taken by Tycho Brahe. Kepler’s first two laws of planetary motion were published in 1609, 400 years prior to launch. Ten years later, he published his third law of planetary motion, which describes how the orbital period (year) of a planet is proportional to the semimajor axis (distance) from the Sun. The fact that Johannes Kepler derived his laws from data made him the first astrophysicist, and the Kepler Mission honors him for this accomplishment. The Mission also uses Kepler’s third law to determine the size of planetary orbits from the periods discovered by observing repeated transits.

G. Resources: Photographs, video, imagery, activities, lessons, events

G1. Where can I go on the net to see photos taken by the Kepler telescope?

Kepler images are posted on the multimedia section of the Kepler website. Full images of the Kepler starfield are only downloaded every quarter. You can see full starfield images in the Kepler multimedia section under First light and Kepler field of view for each season. Kepler is not designed to get "pretty pictures" like the Hubble Space Telescope, Spitzer, and the planetary missions. The Kepler photometer has especially large pixels, many times the size of those in the Hubble Space Telescope detector. The giant-size pixels make possible the enormous field of view that Kepler has. For routine data acquisition, Kepler is programmed to download information from pixels immediately adjacent to each of those 156,000 stars. This produces extremely pretty light curves, but no "pretty pictures."

G2. How can I locate photographs, graphics, animations and images from the Kepler Mission?

You can find a great variety of images and other materials throughout the Kepler website Multimedia section.

G3. How can the public or my classroom be involved?

The Kepler Mission provides several opportunities for teachers, students and the public to get involved. Kepler’s Education and Public Outreach program includes activities and materials for teaching and learning, and information on public outreach through the Night Sky Network, StarDate, and much more.

RECENT QUESTIONS

Q: I was confused by subject table found at following link: http://kepler.nasa.gov/Mission/discoveries/.The table shows comparison data for the Earth and Jupiter. The temperature data for the Earth & Jupiter do not agree with NASA factsheet for planetary data. In particular, the NASA fact sheet shows the mean temperature of the Earth as +15 deg C or 288 K. Why does the subject discoveries table show the Earth's temperature below the freezing point of water at 255 K ??? Does this mean you are measuring a Global Cooling or just listing some temperature other than mean temperature ? A: The planet temperatures calculated do not take into account atmosphere at all, because the effect of atmosphere is very dependent on composition of atmosphere and we have no way of determining that. If Earth did not have atmosphere, indeed it's temperature would be below freezing point of water. In the case of Jupiter, I have to add the fact that our computed planet temperatures are based on an assumption that the planet is in equilibrium with only radiation from it's star balanced by its own black body radiation. Jupiter has not only an atmosphere to complicate things, but a significant internal heat source as well.