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Intro
Navigating to alien planets similar to our own is a universal theme of science fiction. But how do our
space heroes know where to find those planets? And how do they know they won’t suffocate as soon
as they beam down to the surface? Discovering these Earth-like planets has taken a step out of the
science fiction realm with NASA’s Kepler mission, which seeks to find planets within the Goldilocks
zone of other stars: not too close (and hot), not too far (and freezing), but just right for potentially
supporting life. While Kepler is only the first step on a long road of future missions that will tell us
more about these extrasolar planets, or exoplanets, its own journey to launch took more than twenty
years and lots of perseverance.
Kepler:
The Long Road
to Other Worlds
BY KERRY ELLIS
Kepler-20e is the first planet
smaller than Earth discovered to
orbit a star other than the sun.
A year on Kepler-20e lasts only
six days, as it is much closer
to its host star than Earth is to
the sun. The temperature at the
surface of the planet, around
1,400ºF, is much too hot to
support life as we know it.
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story
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story
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Looking for planets hundreds of light-years away is tricky. The
This particular orbit between Earth and the sun is relatively
stars are very big and bright, the planets very small and faint. stable due to the balancing gravitational pulls of Earth and the
Locating them requires staring at stars for a long time in hopes sun. Since it isn’t perfectly stable, though, missions in this orbit
of everything aligning just right so we can witness a planet’s require rocket engines and fuel to make slight adjustments—
transit—that is, its passage in front of its star, which obscures both of which can get expensive. Reviewers again rejected the
a tiny fraction of the star’s light. Measuring that dip in light is proposal, this time because they estimated the mission cost to
how the Kepler mission determines a planet’s size.
exceed the Discovery cost cap.
The idea of using transits to detect extrasolar planets was
The team proposed again in 1996. “To reduce costs, the
first published in 1971 by computer scientist Frank Rosenblatt. project manager changed the orbit to heliocentric to eliminate
Kepler’s principal investigator, William Borucki, expanded on the rocket motors and fuel, and then cost out the design using
that idea in 1984 with Audrey Summers, proposing that transits three different methods. This time the reviewers didn’t dispute
could be detected using high-precision photometry. The next the estimate,” Borucki explained. “Also at this time, team
sixteen years were spent proving to others—and to NASA— members like Carl Sagan, Jill Tarter, and Dave Koch strong-
that this idea could work.
armed me into changing the name from FRESIP to Kepler,” he
recalled with a laugh.
Proving Space Science on the Ground
The previous year, the team tested charge-coupled device
To understand how precise “high-precision” needed to be for (CCD) detectors at Lick Observatory, and Borucki and his
Kepler, think of Earth-size planets transiting stars similar to our colleagues published results in 1995 that confirmed CCDs—
sun, but light-years away. Such a transit would cause a dip in combined with a mathematical correction of systematic errors—
the star’s visible light by only 84 parts per million (ppm). In had the 10-ppm precision needed to detect Earth-size planets.
other words, Kepler’s detectors would have to reliably measure
But Kepler was rejected again because no one believed that
changes of 0.01 percent.
high-precision photometry could be automated for thousands
Borucki and his team discussed the development of a high- of stars. “People did photometry one star at a time. The data
precision photometer during a workshop in 1987, sponsored by analysis wasn’t done in automated fashion, either. You did it by
Ames Research Center and the National Institute of Standards hand,” explained Borucki. “The reviewers rejected it and said,
and Technology, and then built and tested several prototypes.
‘Go build an observatory and show us it can be done.’ So we did.”
When NASA created the Discovery Program in 1992,
They built an automated photometer at Lick Observatory
the team proposed their concept as FRESIP, the Frequency of and radio linked the data back to Ames, where computer
Earth-Size Inner Planets. While the science was highly rated, programs handled the analysis. The team published their
the proposal was rejected because the technology needed to results and prepared for the next Discovery announcement of
achieve it wasn’t believed to exist. When the first Discovery opportunity in 1998.
announcement of opportunity arose in 1994, the team again
“This time they accepted our science, detector capability,
proposed FRESIP, this time as a full mission in a Lagrange orbit. and automated photometry, but rejected the proposal because
Kepler’s focal plane
consists of an array of
forty-two charge-coupled
devices (CCDs). Each CCD
is 2.8 cm by 3.0 cm with
1,024 by 1,100 pixels. The
entire focal plane contains
95 megapixels.
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ThE SCIENCE MERIT fUNCTIoN ThAT BILL dEVELoPEd wAS A BRIdGE BETwEEN ThE
SCIENCE ANd ENGINEERING ThAT wE USEd IN doING ThESE KINd of TRAdE STUdIES …
we did not prove we could get the required precision in the
presence of on-orbit noise, such as pointing jitter and stellar
variability. We had to prove in a lab that we could detect Earth-
size transits in the presence of the expected noise,” said Borucki.
The team couldn’t prove it using ground-based telescope
observations of stars because the atmosphere itself introduces
too much noise. Instead, they developed a test facility to
simulate stars and transits in the presence of pointing jitter.
A thin metal plate with holes representing stars was illuminated
from below, and a prototype photometer viewed the light
from the artificial stars while it was vibrated to simulate
spacecraft jitter.
The plate had many laser-drilled holes with a range of sizes
to simulate the appropriate range of brightness in stars. To study
the effects of saturation (very bright stars) and close-together
stars, some holes were drilled large enough to cause pixel
saturation and some close enough to nearly overlap the images.
“To prove we could reliably detect a brightness change
of 84 ppm, we needed a method to reduce the light by that
amount. If a piece of glass is slid over a hole, the glass will reduce
the flux by 8 percent—about one thousand times too much,”
Borucki explained. “Adding antireflection coatings helped by a
factor of sixteen, but the reduction was still sixty times too large.
How do you make the light change by 0.01 percent?
“There really wasn’t anything that could do the job for
us, so we had to invent something,” said Borucki. “Dave Koch
realized that if you put a fine wire across an aperture—one
of the drilled holes—it would block a small amount of light.
When a tiny current is run through the wire, it expands and
blocks slightly more light. Very clever. But it didn’t work.”
With a current, the wire not only expanded, it also curved.
As it curved, it moved away from the center of a hole, thereby
allowing more light to come through, not less.
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This star plate is an important Kepler relic. It was used in the first laboratory
experiments to determine whether charge-coupled devices could produce very
precise differential photometry.
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“So Dave had square holes drilled,” said Borucki. “With a mission,” explained Duren, “and this became a key tool for us
square hole, when the wire moves off center, it doesn’t change the in the years that followed.”
amount of light. To keep the wire from bending, we flattened
The science merit function helped the team determine the
it.” The results demonstrated that transits could be detected at best course of action when making design trade-offs or descope
the precision needed even in the presence of on-orbit noise.
decisions. One trade-off involved the telecommunications
After revising, testing, publishing, and proposing for nearly systems. Kepler’s orbit is necessary to provide the stability
twenty years, Kepler was finally approved as a Discovery mission needed to stare continuously at the same patch of sky, but it
in 2001.
puts the observatory far enough away from Earth that its
telecommunications systems need to be very robust. The
Engineering Challenges
original plan included a high-gain antenna that would deploy
After Kepler officially became a NASA mission, Riley Duren on a boom and point toward Earth, transmitting data without
from the Jet Propulsion Laboratory joined the team as project interrupting observations. When costs needed to be cut later
systems engineer, and later became chief engineer. To help on, descoping the antenna offered a way to save millions. But
ensure a smooth progression, Duren and Borucki set out to this would mean turning the entire spacecraft to downlink data,
create a common understanding of the scientific and engineering interrupting observations.
trade-offs.
“Because we’re looking for transits that could happen
“One of the things I started early with Bill and continued any time, it wasn’t feasible to rotate the spacecraft to downlink
throughout the project was to make sure that I was in sync with every day. It would have had a huge impact on the science,”
him every step of the way, because, after all, the reason we’re Duren explained. So the team had to determine how frequently
building the mission is to meet the objectives of the science it could be done, how much science observation time could
team,” said Duren. “It was important to develop an appreciation be lost, and how long it would take to put Kepler back into
for the science given the many complex factors affecting Kepler its correct orientation. “We concluded we could afford to
mission performance, so early on I made a point of going to do that about once a month,” said Duren. Since the data
every science team meeting that Bill organized so I could hear would be held on the spacecraft longer, the recorder that
and learn from the science team.”
stored the data had to be improved, which would increase its
The result was something they called the science merit cost even as the mission decreased cost by eliminating the high-
function: a model of the science sensitivity of mission features— gain antenna.
the effects on the science of various capabilities and choices.
“The science merit function that Bill developed was a
Science sensitivities for Kepler included mission duration, how bridge between the science and engineering that we used in
many stars would be observed, the precision of the photometer’s doing these kind of trade studies,” said Duren. “In my opinion,
light measurements, and how many breaks for data downlinks the Kepler mission was pretty unique in having such a thing.
could be afforded. “Bill created a model that allowed us to And that’s a lesson learned that I’ve tried to apply to other
communicate very quickly the sensitivity of the science to the missions in recent years.”
A single Kepler science module with two CCDs
and a single field-flattening lens mounted onto
an Invar carrier. Each of the twenty-one CCD
science modules are covered with lenses of
sapphire. The lenses flatten the field of view to a
flat plane for best focus.
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This image from Kepler shows the
telescope’s full field of view—an expansive
star-rich patch of sky in the constellations
Cygnus and Lyra stretching across
100 square degrees, or the equivalent of
two side-by-side dips of the Big Dipper.
The tool came in handy as Kepler navigated through other
Extended Mission
engineering challenges, ensuring the mission could look at Kepler launched successfully in 2009. After taking several images
enough stars simultaneously for long periods of time, all the with its “lens cap” on to calculate the exact noise in the system,
while accommodating the natural noise that comes from long the observatory began its long stare at the Cygnus-Lyra region
exposures, spacecraft jitter in orbit, and instrumentation. This of the Milky Way. By June 2012, it had confirmed the existence
meant Kepler had to have a wide enough field of view, low-noise of seventy-four planets and identified more than two thousand
detectors, a large aperture to gather enough light, and very stable planet candidates for further observation. And earlier in the year,
pointing. Each presented its own challenges.
NASA approved it for an extended mission—to 2016.
Kepler’s field of view is nearly 35,000 times larger than
“The Kepler science results are essentially a galactic census
Hubble’s. It’s like a very large wide-angle lens on a camera and of the Milky Way. And it represents the first family portrait, if
requires a large number of detectors to see all the stars in that you will, of what solar systems look like,” said Duren.
field of view.
Kepler’s results will be important in guiding the next
Ball Aerospace built an instrument that could accommodate generation of exoplanet missions. Borucki explained, “We
about 95 million pixels—essentially a 95-megapixel camera. all know this mission will tell us the frequency of Earth-size
“It’s quite a bit bigger than any camera you’d want to carry planets in the habitable zone, but what we want to know is the
around under your arm,” Duren said. “The focal plane and atmospheres of these planets. Kepler is providing the information
electronics for this camera were custom built to meet Kepler’s needed to design those future missions.”
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unique science objectives. The entire camera assembly resides
inside the Kepler telescope, so a major factor was managing the
power and heat generated by the electronics to keep the CCD
detectors and optics cold.”
What might be surprising is that for all that precision,
Kepler’s star images are not sharp. “Most telescopes are designed
to provide the sharpest possible focus for crisp images, but doing
that for Kepler would have made it very sensitive to pointing
jitter and to pixel saturation,” explained Duren. “That would
be a problem even with our precision pointing control. But of
course there’s a trade-off: if you make the star images too large
[less sharp], each star image would cover such a large area of the
sky that light from other stars would be mixed into the target
star signal, which could cause confusion and additional noise. It
was a careful balancing act.”
And it’s been working beautifully.
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