Years and lots of perseverance



Yüklə 48,4 Kb.
Pdf görüntüsü
tarix02.01.2018
ölçüsü48,4 Kb.
#19334


Title

By 


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.

hc

etl


Ca-

L

PJ



/s

me

A/



A

S

A



 N:ti

de

r



 Ce

ag

mI



story

 | ASK MAGAZINE | 5

story

 | ASK MAGAZINE | 5




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. 

ec

a



ps

or

e



l Al

a

d B



n

A a


S

A

: Nti



de

r

o Ct



o

h

P



6 | ASK MAGAZINE


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.

ma

e



r Tel

pe

K/



A

S

A



: Nti

de

r



o Ct

o

h



P

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.

A

A

S



S

K

K



 

 

M



M

A

A



G

G

A



A

Z

Z



I

I

N



N



| 7

| 7



“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.

n

oi

s



is

 Mr


e

ple


K/

A

S



A

 N:ti


de

 Cr


ot

ho

P



8 | ASK MAGAZINE


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.” 

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.

ASK MAGAZINE | 9

 h

ce

tl



Ca-

L

PJ



/s

me

A/



A

S

A



 N:ti

de

r



 Ce

ag

mI



Yüklə 48,4 Kb.

Dostları ilə paylaş:




Verilənlər bazası müəlliflik hüququ ilə müdafiə olunur ©genderi.org 2024
rəhbərliyinə müraciət

    Ana səhifə