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Encyclopedia of the Cosmos
Yervant Terzian, is "The David C. Duncan Professor in the Physical Sciences", in the Department of Astronomy and Space Sciences at Cornell University. He was Department Chairman from 1979-1999.
His fields of expertise are the physics of the Interstellar Medium, Galaxies, and Radio Astronomy.
He is a Research Professor with the National Astronomy and Ionosphere Center and member of the International Astronomical Union, the International Union of Radio Science, the American Astronomical Society, and the Hellenic Astronomical Society, among other memberships.
He has been a Scientific Editor of The Astrophysical Journal (1989-1999).
In 1984, he received the Clark Distinguished Award for Excellence in Teaching. In 1996, he was appointed Director of NASA's New York Space Grant Program to enhance science education.
He has been awarded Honorary Doctor of Science degrees from the University of Indiana (1989), the Yerevan State University in Armenia (1994), the University of Thessaloniki in Greece (1997), and from Union College (1999).
In 1990, he was elected Foreign Member of the Armenian Academy of Sciences. In 2001 he was elected a Fellow of the American Association for the Advancement of Science.
In 2002 he was elected Chairman of the US Consortium of Universities and Institutes to construct the Square Kilometer Array giant radio telescope.
Yervant Terzian is the author or co-author of more than 200 scientific publications and the editor of six books, including "Carl Sagan's Universe".
Famous Astronomer
Johannes Kepler (1571-1630) had studied astronomy long before he met Tycho: he favored the Copernican world-view and corresponded with Galileo.
Tycho's observations included some very accurate measurements of the position of the planet Mars, which did not agree with either Ptolemy or Copernicus. When Tycho died, Kepler got hold of those observations and tried to puzzle them out. In 1609, the same magic year when Galileo first turned his telescope towards the heavens, Kepler caught a glimpse of what he thought might be the answer. That was when he published his first two laws of planetary motion:
- Planets move along ellipses, with the Sun at one focus.
- The line from the Sun to the planet
covers equal areas in equal times.
Each of these statement requires some explanation.
Ellipses
The ellipse, the shape of a flattened circle, was well known to the ancient Greeks. It belonged to the family of "conic sections," of curves produced by the intersections of a plane and a cone.
As the drawing above on the left shows, when that plane is...
--perpendicular to the axis of the cone, the result is a circle.
--moderately inclined, an ellipse.
--inclined so much that it is parallel to one side of the cone, a parabola.
--inclined even more, a hyperbola.
All these intersections are easily produced by a flashlight in a moderately dark room (drawing below). The flashlight creates a cone of light and when that cone hits a wall, the shape produced is a conic section--the intersection of the cone of light with the flat wall.
The axis of the flashlight is also the axis of the cone of light. Aim the beam perpendicular to the wall to get a circle of light. Slant the beam: an ellipse. Slant further, to where the closing point of the ellipse is very, very far: a parabola. Slant even more, to where the two edges of the patch of light not only fail to meet again, but seem to head in completely different directions: a hyperbola.
The Third Law
After Tycho's death, Kepler became the court astronomer, although the superstitious emperor was more interested in astrology than in the structure of the solar system. In 1619 Kepler published his third law: the square of the orbital period T is proportional to the cube of the mean distance a from the Sun (half the sum of greatest and smallest distances). In formula form
T2= k a3
with k some constant number, the same for all planets. Suppose we measure orbital periods in years and all distances in "astronomical units" or AUs, with 1 AU the mean distance between the Earth and the Sun. Then if a = 1 AU, T is one year, and k with these units just equals 1, i.e. T2= a3. Applying now the formula to any other planet, if T is known from the observations of many years, the planet's a, its mean distance from the Sun, is readily derived.
Finding the value of 1 AU in miles or kilometers, that is, finding the actual scale of the solar system, is not easy. This subject is discussed in the next section. Our best values nowadays are the ones provided by space-age tools, by radar-ranging of Venus and by planetary space probes; to a good approximation, 1 AU = 150 000 000 km.
Not only were Kepler's laws confirmed and explained by later scientists, but they apply to any orbital system of two bodies--even artificial satellites in orbit around the Earth. The constant k' for artificial satellites differs from k obtained for planets (but is the same for any satellite). By Kepler's formula
T = SQRT (k' a3)
where SQRT stands for "square root of" (the world-wide web does not offer more specific symbols). If T is measured in seconds and a in Earth radii (1 RE = 6371 km = 3960 miles)
T = 5063 SQRT (a3)
Kepler's later years were not too happy. His patron, Emperor Rudolf, died in 1612, and although Kepler retained his post as court mathematician and continued to produce important work, his life was increasingly disrupted by war. That was the 30 years' war, a bitter religious battle which pitted Protestants against Catholics; it began in Prague in 1618 and engulfed all of Kepler's part of Europe.
Featured Article
Today, cosmologists believe that 13.7 billion years ago, there was nothing. No universe – no galaxies, no stars, no planets, no light, no space, and no time, nothing at all. Suddenly, for reasons that are still not understood, an astronomical explosion took place, and space and time began.
Part of the energy of this incredibly hot explosion was converted to primitive matter of quarks and electrons. The quarks quickly combined and within microseconds formed the first protons and neutrons in the universe, the elementary particles that form all of the atoms in the universe. As the new super-hot universe continued to expand, it also continued to cool, and within a few hundred seconds, it cooled from trillions of degrees to millions of degrees of temperature, which enabled some of the protons and neutrons to fuse and form the first light elements like helium, beryllium, and lithium. The fast expansion of the universe continued, and within 300,000 years, it cooled to a few thousand degrees, which allowed the electrons to combine with the protons to form the first neutral hydrogen atoms. Recently, using powerful space telescopes, astronomers have been able to map how the universe looked at that early epoch.
Within less than a billion years after the beginning, due to complex gravitational instabilities, matter separated into pieces that formed billions of galaxies. Stars were formed by the gravitational collapse of interstellar clouds that populate all galaxies. At first, when interstellar matter was abundant, the rate of star formation was higher than at the present time. Trillions and trillions of stars formed in these galaxies, many with planets surrounding them. Earth’s star, the Sun, was formed some five billion years ago in the Milky Way galaxy. Today, astronomers estimate that about one new star is born each year in our galaxy.
The Milky Way galaxy is peppered with interstellar clouds. Some are predominantly composed of cold neutral hydrogen, others are comprised of molecular species, and still others are made of a hot plasma (ionized gas) that is powered by young massive stars. Ultraviolet radiation from the stars ionizes and heats these clouds to temperatures of about 10,000 degrees. The free-moving electrons in these clouds recombine with the protons, or other heavier nuclei, and in the process, emit radiation at specific wavelengths characteristic of the atomic structures of the various elements. With radio waves, radio astronomers can detect such radiation as spectral lines, and from observed features, can deduce the physical characteristics of the interstellar hot gases, such as temperatures, densities, turbulent velocities of the gases, and cloud velocities in the line of sight.
Using the Arecibo giant radio telescope located in Puerto Rico (Figure 1), we have been investigating interstellar clouds in order to understand their physical nature and to test atomic theory. Figure 2 shows a spectrum of an ionized cloud (Figure 3 cloud named S88) in the interstellar space. The telescope was used at a wavelength of 6 cm (5,000 MHz) and obtained the spectral signatures of the various line emissions from this object. The strong hydrogen emission lines, and those of helium and carbon, as well as weak signals from heavier elements can be seen. The superb sensitivity of the Arecibo telescope makes possible the clear and unambiguous detection of such spectra to study the dynamics and chemical evolution of interstellar clouds and hence, to understand the formation and evolution of the Milky Way galaxy.
After living a long life of a few billion years, producing energy from nuclear reactions in their cores, stars explode their outer mantles, and their cores collapse into fascinating super-dense hot dwarf stars, neutron stars, or black holes, depending on their initial mass. The ejected material flies out and back into interstellar space, and eventually mixes with other diffuse clouds to recollapse and form second- or third-generation stars. The ejected envelopes of dying stars can serve as testbeds for researchers to study low-density, hot plasma and to check the predictions of atomic theories.
Using the Arecibo radio telescope we have made hundreds of observations of the material ejected from exploding stars. We have observed the initial cooler ejected material composed, in part, of molecules such as OH that emit maser spectral lines at a wavelength around 18 cm (1612 MHz). We have also observed the radio thermal emission from hotter material surrounding the dying stars and have detected the weak signals of the predicted recombination lines. These observations have been combined with optical images taken with the Hubble Space Telescope to understand these objects better. Figure 4 shows a dying star with its surrounding expanding nebula called the Ring Nebula (NGC 6720), and Figure 5 shows the Dumbbell Nebula (NGC 6853), both of which have been observed with the Hubble telescope and studied with the Arecibo radio telescope.
We plan to use the exceptional sensitivity of the Arecibo radio telescope to survey the plane of the galaxy and to map all the ionized radio-emitting interstellar clouds. We shall then understand our galaxy better.
Observatories
The Observatory
From a remote outpost on the summit of Hawaii's dormant Mauna Kea volcano, astronomers at the W. M. Keck Observatory probe the deepest regions of the Universe with unprecedented power and precision.
Their instruments are the twin Keck Telescopes, the world's largest optical and infrared telescopes. Each stands eight stories tall and weighs 300 tons, yet operates with nanometer precision. At the heart of each Keck Telescope is a revolutionary primary mirror. Ten meters in diameter, the mirror is composed of 36 hexagonal segments that work in concert as a single piece of reflective glass.
Made possible through grants totaling more than $140 million from the W. M. Keck Foundation, the Observatory is operated by the California Association for Research in Astronomy (CARA), whose Board of Directors includes representatives from the California Institute of Technology and the University of California. In 1996, the National Aeronautics and Space Administration (NASA) joined as a partner in the Observatory. The Keck I telescope began science observations in May 1993; Keck II began in October 1996.
Keck's capabilities make full use of Mauna Kea's research potential. Surrounded by thousands of miles of relatively thermally stable ocean, the 13,796-foot Mauna Kea summit has no nearby mountain ranges to roil the upper atmosphere or throw light-reflecting dust into the air. Few city lights pollute its extremely dark skies. For most of the year, the atmosphere above Mauna Kea is clear, calm and dry.
Telescopes
An altitude-azimuth design gives each 10-meter Keck telescope the optimal balance of mass and strength. Extensive computer analysis determined the greatest strength and stiffness for the least amount of steel- about 270 tons per telescope. This is critically important, and not only for economic reasons. A large telescope must remain resistant to the deforming forces of gravity as it tracks objects moving across the night sky.
Chilling the interior of the insulated dome during the day controls temperature variations that could induce deformation of the telescope's steel and mirrors. This is a big task: The volume of each dome is more than 700,000 cubic feet. Giant air conditioners run constantly during the day, keeping the dome temperature at or below freezing.
Astronomers use the telescopes in shifts of one to five nights. Time allocation committees pre-approve all observing. Assistants operate the telescopes at the summit while astronomers gather data via remote observing from observatory headquarters in Waimea. The W. M. Keck Observatory was the first facility on Mauna Kea to use remote observing.
The Mirror
A telescope tracks objects, sometimes for hours, across the sky as the Earth turns. This constant but subtle movement results in slight deformations of the telescope structure despite all engineered precautions. Without active, computer-controlled correction of the primary mirror, scientific observations would be impossible.
New techniques for manufacturing, polishing, and testing the mirror segments had to be invented, including "stressed mirror" polishing. Each segment's surface is so smooth that if it were the width of Earth, imperfections would be only three feet high.
On the telescope, each segment's figure is kept stable by a system of extremely rigid support structures and adjustable warping harnesses. During observing, a computer-controlled system of sensors and actuators adjusts the position of each segment - relative to its neighbors - to an accuracy of four nanometers, about the size of a few molecules, or about 25,000 times thinner than a human hair. This twice-per-second adjustment effectively counters the tug of gravity.
Ever since their invention nearly 400 years ago, Earth-based telescopes have suffered from image blurring caused by the turbulent atmosphere above them. This is true of even the world's best observatory sites like Mauna Kea, though to a considerably lesser extent than elsewhere. In recent years, advances in optical and computing technology have made it possible to greatly reduce this blurring through the use of "adaptive optics" (AO). At the heart of the AO system is a 6" diameter deformable mirror that changes its shape up to 670 times per second to cancel out atmospheric distortion, resulting in images 10 times sharper than before. The successful installation of AO systems on both Keck telescopes has made it possible for Keck astronomers to study objects in far greater detail than ever before.
Research
Today, the Keck telescopes are used to seek answers to ancient questions: How did the universe evolve since creation to its present state? How, and when, did galaxies form? What is the rate of star formation in galaxies far away, and far back in time? How much does the expansion rate of the universe vary over history? How do solar systems form? Where is the missing mass of the universe? What is the ultimate fate of the universe? In just the past few years, astronomers at the W. M. Keck Observatory have made tremendous progress in answering these and other questions. Among numerous research projects, Keck astronomers are using gravitational lenses to discover galaxies at the edge of the universe; using supernovaes to determine the expansion rate of the universe; searching for atomic gases in the immense regions of space between galaxies; helping to solve the riddle of gamma ray bursts, and discovering planets around other stars.
The next phase of the W. M. Keck Observatory is underway as teams of scientists and engineers continue work on improving the Keck Interferometer. The Keck-Keck Interferometer combines the light of both Keck telescopes to obtain a tenfold increase in resolution. It is a significant cornerstone of NASA�s "Origins" program, which ultimately seeks to identify and characterize planets around Sun-like stars. The interferometer will also help astronomers detect giant gas planets, measure and characterize planet-forming dust around stars, and obtain extremely high-resolution images of protoplanetary disks. It has already produced significant results, including observation of a supermassive black hole in the center of a galaxy (NGC 4151) more than 40 million light years away.
An altitude-azimuth design gives each 10-meter Keck telescope the optimal balance of mass and strength. Extensive computer analysis determined the greatest strength andstiffness for the least amount of steel- about 270 tons per telescope. This is critically important, and not only for economic reasons. A large telescope must remain resistant to the deforming forces of gravity as it tracks objects moving across the night sky.
Headquarters
The Observatory Headquarters is an architecture-award-winning facility located on a 7-acre campus in Kamuela on the Big Island of Hawaii. The land for our headquarters was donated by Parker Ranch.
Named by the 2000 Robb Report as one of the 10 most desirable places to live in the country, Kamuela is a charming town of 6,000; it sits 2,500 feet above sea level and offers a peaceful, rural lifestyle, a benign climate, excellent schools, and freedom from the frenetic pace so characteristic of much of the mainland United States.
About 125 people work full-time at Keck, of which two-thirds were hired from Hawaii. With an annual operating budget of $11 million, the Observatory is one of the town's largest employers.
Instrumentation
VISIBLE BAND (0.3-1.0 Micron)
DEIMOS - The Deep Extragalactic Imaging Multi-Object Spectrograph is the most advanced optical spectrograph in the world, capable of gathering spectra from 130 galaxies or more in a single exposure. In 'Mega Mask' mode, DEIMOS can take spectra of more than 1,200 objects at once, using a special narrow-band filter.
ESI - The Echellette Spectrograph and Imager captures high-resolution spectra of very faint galaxies and quasars ranging from the blue to the infrared in a single exposure. It is a multimode instrument that allows users to switch among three modes during a night. It has produced some of the best non-AO images at the Observatory.
HIRES - The largest and most mechanically complex of the Keck's main instruments, the High Resolution Echelle Spectrometer breaks up incoming starlight into its component colors to measure the precise intensity of each of thousands of color channels. Its spectral capabilities have resulted in many breakthrough discoveries, such as the detection of planets outside our solar system and direct evidence for a model of the Big Bang theory.
LRIS - The Low Resolution Imaging Spectrograph is a faint-light instrument capable of taking spectra and images of the most distant known objects in the universe. The instrument is equipped with a red arm and a blue arm to explore stellar populations of distant galaxies, active galactic nuclei, galactic clusters, and quasars.
NEAR-INFRARED (1-5 Micron)
ADAPTIVE OPTICS - Adaptive optics senses and compensates for the atmospheric distortions of incoming starlight up to 670 times per second. This results in an improvement in image quality on fairly bright astronomical targets by a factor 10 to 20.
LASER GUIDE STAR ADAPTIVE OPTICS - The Keck Laser Guide Star expands the range of available targets for study with the Keck II adaptive optics system. It uses a 15-watt sodium-dye laser to excite sodium atoms that naturally exist in the atmosphere 90 km (55 miles) above the Earth's surface. The laser creates an "artificial star" that allows the Keck adaptive optics system to observe 70-80 percent of the targets in the northern sky, compared to the 1 percent accessible without the laser.
NIRC - The Near Infrared Camera for the Keck I telescope is so sensitive it could detect the equivalent of a single candle flame on the Moon. This sensitivity makes it ideal for ultra-deep studies of galactic formation and evolution, the search for proto-galaxies and images of quasar environments. It has provided ground-breaking studies of the Galactic center, and is also used to study protoplanetary disks, and high-mass star-forming regions.
NIRC-2/AO - The second generation Near Infrared Camera works with the Keck Adaptive Optics system to produce the highest-resolution ground-based images and spectroscopy in the 1-5 micron range. Typical programs include mapping surface features on solar system bodies, searching for planets around other stars, and analyzing the morphology of remote galaxies.
NIRSPEC - The Near Infrared Spectrometer studies very high redshift radio galaxies, the motions and types of stars located near the Galactic Center, the nature of brown dwarfs, the nuclear regions of dusty starburst galaxies, active galactic nuclei, interstellar chemistry, stellar physics, and solar-system science.
OSIRIS - The OH-Suppressing Infrared Imaging Spectrograph is a near-infrared spectrograph for use with the Keck II adaptive optics system. OSIRIS takes spectra in a small field of view to provide a series of images at different wavelength. The instrument allows astronomers to ignore wavelengths where the Earth's atmosphere shines brightly due to emission from OH (hydroxl) molecules, thus allowing the detection of objects 10 times fainter than previously available.
MID-INFRARED (5-27 Micron)
KECK INTERFEROMETER - The Keck-Keck Interferometer combines light from the two Keck telescopes to measure the diameters of stars, disks orbiting nearby stars, and the orbital characteristics of binary systems. It also directly detects and characterizes hot giant planets. The interferometer can reach high angular resolutions to a small fraction of an arcsecond, providing the effective resolution of a telescope 85-meters in diameter.
© 2005 - 2008 W. M. Keck Observatory
Country Profile
Importance of Planet Detection
Kepler is NASA's first mission capable of finding Earth-size and smaller planets around other stars.
The centuries-old quest for other worlds like our Earth has been rejuvenated by the intense excitement and popular interest surrounding the discovery of giant planets like Jupiter orbiting stars beyond our solar system. With the exception of the pulsar planets, all of the extrasolar planets detected so far are gas giants, approximately 150 as of 2005. The challenge now is to find terrestrial planets (habitable planets like Earth), which are 30 to 600 times less massive than Jupiter.
The Kepler Mission, a NASA Discovery mission, is specifically designed to survey our region of the Milky Way galaxy to detect and characterize hundreds of Earth-size and smaller planets in or near the habitable zone. The habitable zone encompasses the distances from a star where liquid water can exist on a planet's surface.
Results from this mission will allow us to place our solar system within the continuum of planetary systems in the Galaxy.
Scientific Objective
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The scientific objective of the Kepler Mission is to explore the structure and diversity of planetary systems. This is achieved by surveying a large sample of stars to:
- Determine the percentage of terrestrial and larger planets there are in or near the habitable zone of a wide variety of stars;
- Determine the distribution of sizes and shapes of the orbits of these planets;
- Estimate how many planets there are in multiple-star systems;
- Determine the variety of orbit sizes and planet reflectivities, sizes, masses and densities of short-period giant planets;
- Identify additional members of each discovered planetary system using other techniques; and
- Determine the properties of those stars that harbor planetary systems.
The Kepler Mission also supports the objectives of future NASA Origins theme missions Space Interferometry Mission (SIM) and Terrestrial Planet Finder (TPF),
- By identifying the common stellar characteristics of host stars for future planet searches,
- By defining the volume of space needed for the search and
- By allowing SIM to target systems already known to have terrestrial planets.
The Transit Method of Detecting Extrasolar Planets:
When a planet crosses in front of its star as viewed by an observer, the event is call a transit. Transits by terrestrial planets produce a small change in a star's brightness of about 1/10,000 (100 parts per million, ppm), lasting for 2 to 16 hours. This change must be absolutely periodic if it is caused by a planet. In addition, all transits produced by the same planet must be of the same change in brightness and last the same amount of time, thus providing a highly repeatable signal and robust detection method.
Once detected, the planet's orbital size can be calculated from the period (how long it takes the planet to orbit once around the star) and the mass of the star using Kepler's Third Law of planetary motion. The size of the planet is found from the depth of the transit (how much the brightness of the star drops) and the size of the star. From the orbital size and the temperature of the star, the planet's characteristic temperature can be calculated. From this the question of whether or not the planet is habitable (not necessarily inhabited) can be answered.
Design
For a planet to transit, as seen from our solar system, the orbit must be lined up edgewise to us. The probability for an orbit to be properly aligned is equal to the diameter of the star divided by the diameter of the orbit. This is 0.5% for a planet in an Earth-like orbit about a solar-like star. (For the giant planets discovered in four-day orbits, the alignment probability is more like 10%.) In order to detect many planets one can not just look at a few stars for transits or even a few hundred. One must look at thousands of stars, even if Earth-like planets are common. If they are rare, then one needs to look at many thousands to find even a few. Kepler looks at 100,000 stars so that if Earths are rare, a null or near null result would still be significant. If Earth-size planets are common then Kepler should detect hundreds of them.
Considering that we want to find planets in the habitable zone, the time between transits is about one year. To reliably detect a sequence one needs four transits. Hence, the mission duration needs to be at least three and one half years.
The Kepler instrument is a specially designed 0.95-meter diameter telescope called a photometer or light meter. It has a very large field of view for an astronomical telescope —105 square degrees— or about the area of both your hands held at arm's length, in order to observe the necessary large number of stars. It stares at the same star field for the entire mission and continuously and simultaneously monitors the brightnesses of more than 100,000 stars for the life of the mission—3.5 years.
The diameter of the telescope needs to be large enough to reduce the noise from photon counting statistics, so that it can measure the small change in brightness of an Earth-like transit. The design of the entire system is such that the combine differential photometric precision over a 6.5 hour integration is less than 20 ppm (one-sigma) for a 12th magnitude solar-like star including an assumed stellar variability of 10 ppm. This is a conservative, worse-case assumption of a grazing transit. A central transit of the Earth crossing the Sun lasts 13 hours. And about 75% of the stars older than 1 Gyr are less variable then the Sun on the time scale of a transit.
The photometer must be spacebased to obtain the photometric precision needed to reliably see an Earth-like transit and to avoid interruptions caused by day-night cycles, seasonal cycles and atmospheric perturbations, such as, extinction associated with ground-based observing.
Extending the mission beyond three and one half years provides for:
- Improving the signal to noise by combining more transits to permit detection of smaller planets
- Finding planets in orbits with larger periods
- Finding planets around stars that are noisier either due to being fainter or having more variability
Expected Results
Based on the mission described above, including conservative assumptions about detection criteria, stellar variability, taking into account only orbits with 4 transits in 3.5 years, etc., and assuming that planets are common around other stars like our Sun, then we expect to detect:
From transits of terrestrial planets in one year orbits
- About 50 planets if most are the same size as Earth (R~1.0 Re) and none larger,
- About 185 planets if most have a size of R~1.3 Re,
- About 640 planets if most have a size of R~2.2 Re,
- About 12% with two or more planets per system.
These numbers come out substantially higher, when one takes into consideration all orbits from a few days to more than one year.
From modulation of the reflected light from giant inner planets
- About 870 planets with periods less than one week.
From transits of giant planets
- About 135 inner-orbit planet detections,
- Densities for 35 inner-orbit planets, and
- About 30 outer-orbit planet detections.
Detection of the short-period giant planets should occur within the first several months of the mission.
The sample size of stars for this mission is large enough to capture the richness of the unexpected. Should no detection be made, a null result would still be very significant.
Mission Characteristics
Continuously point at a single star field in Cygnus-Lyra region except during Ka-band downlink.
Roll the spacecraft 90 degrees about the line-of-sight every 3 months to maintain the sun on the solar arrays and the radiator pointed to deep space.
Monitor 100,000 main-sequence stars for planets
Mission lifetime of 3.5 years extendible to at least 6 years
D2925-10L (Delta II) launch into an Earth-trailing heliocentric orbit
Scientific Operations Center and Project management (operations) at Ames Research Center
Project management (development) at Jet Propulsion Laboratory
Flight segment design and fabrication at Ball Aerospace & Technologies Corp.
Mission Operations Center at Laboratory for Atmospheric and Space Physics (LASP)—University of Colorado
Data Management Center at Space Telescope Science Institute
Deep Space Network for telemetry
Routine contact
X-band contact twice a week for commanding, health and status
Ka-band contact once a month for science data downlink
Article courtesy of NASA Ames Research Center Kepler Mission