Olá!
O Cisne é uma constelação imensa
e muitíssimo arrebatadora, eu diria,
porque se apresenta enquanto um verdadeiro e imenso pássaro
voando com suas asas inteiramente abertas, esticadas,
projetadas rumo norte e rumo sul, cabeça ereta e honrosa,
e cauda ainda mais honrosa
pois que é representada por sua estrela Alpha, Deneb
(deneb é um vocábulo que quer dizer cauda, rabo).
Não podemos nos esquecer que Deneb uma das três estrelas
que perfazem o chamado Grande Triângulo do Verão
(ou do Norte, para nós, do hemisfério sul,
pois que acontece nos tempos do nosso Inverno!).
juntamente com Altair, estrela-alpha Aquilae, e Vega, estrela-alpha Lyrae.
A bem da verdade, o Grande Triângulo do Verão
acolhe - entre Cygnus e Lyra - aquilo que denominamos como Zona Habitável
onde a Sonda Kepler veio observando, buscando e encontrando
potenciais Planetas habitáveis!
(Veja sobre estas descobertas acessando
http://www.nasa.gov/mission_pages/kepler/multimedia/images/kepler-field-of-view-photo.html
Kepler Mission Star Field
An image by Carter Roberts of the Eastbay Astronomical Society in Oakland, CA, showing the Milky Way region of the sky where the Kepler spacecraft/photometer will be pointing. Each rectangle indicates the specific region of the sky covered by each CCD element of the Kepler photometer. There are a total of 42 CCD elements in pairs, each pair comprising a square. Credit: Carter Roberts / Eastbay Astronomical Society.
http://www.nasa.gov/mission_pages/kepler/multimedia/images/fov-kepler-drawing.html
Kepler's Field Of View In Targeted Star Field
Artist's Rendering Of Kepler's Target Region In The Milky Way
An artists rendering of what our galaxy might look as viewed from outside our Galaxy. Our sun is about 25,000 light years from the center of our galaxy. The cone illustrates the neighborhood of our galaxy that the Kepler Mission will search to find habitable planets. Credit: Jon Lomberg.
Next: Close-up view of the same region
Next: Close-up view of the same region
A. Kepler Mission Science
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%.
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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.
http://kepler.nasa.gov/Mission/faq/
A14. Do you think any planets could support life, and if so, what characteristics would they have to have?
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.
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.
B4. What is the typical distance to the stars where Kepler will find Earth-size planets?
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.”
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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.
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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
http://kepler.nasa.gov/Mission/faq/
B. About the Kepler Field of View (FOV)
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Kepler will look at just one large area of the sky in the constellations Cygnus and Lyra. The star field for the Kepler Mission was selected based on the following constraints:
- The field must be continuously viewable throughout the mission.
- The field needs to be rich in stars similar to our sun because Kepler needs to observe more than 100,000 stars simultaneously.
- 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."
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.”
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?
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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.
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.
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?
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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.
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.
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?
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?
http://kepler.nasa.gov/Mission/faq/
D9. Why not use the Hubble Space Telescope (HST)?
There are three basic reasons why the HST could not be used to look for planets in the way described here:
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- 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.
- 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.
- 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.
http://kepler.nasa.gov/Mission/faq/
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.
http://kepler.nasa.gov/Mission/faq/
http://kepler.nasa.gov/images/MilkyWay-Kepler-cRoberts-1-full.png
