Here's the challenge. Take a bare 100 watt light bulb and
switch it on. Now step back about 300 miles. Once you're in position, arrange
for a friend to slowly pass a pinhead 30 feet in front of the bulb without notice
or warning. Your job? Detect the decrease in light when the pinhead gets
between you and the bulb.
I suspect that's not something you do every day. But NASA's
Kepler
telescope will be doing it every half-hour for the next three years and
more.
Actually, not quite. Kepler will be measuring the brightness
of more than 100,000 "light bulbs."
This new NASA space-borne instrument, which is now
completing its shakedown
cruise, is engaged in the ultimate staring contest. Kepler will
continuously monitor the luminosity of 145,000 stars in the region of constellations
Cygnus and Lyra, looking for dimming of as little as 0.006 percent of a star's
brightness. Unlike other schemes for finding planets around distant stars (so-called
"exoplanets"), Kepler can unearth Earths. That is, it can detect
worlds hundreds of light years away that are comparable in both size and
orbital position to our home planet. Cousins of the Earth – and obvious
candidates for life.
Kepler is perched a ten million miles into space, far beyond
our world's troublesome, restless atmosphere. In truth, it's a 100
megapixel camera, taking a picture (and pretty much the same
picture!) every 30 minutes. During each month of this maniacally repetitive
surveillance, Kepler's most recent photo collection is sent back to terra
firma.
But not all of it. Think about it: 100 million pixels every
half-hour for a month? With about a byte per pixel (usually only brightness
differences are recorded), that's more than a hundred gigabytes of data – a
gush of results that would be painful to transmit from orbit. What to do?
Dealing with that question is part of Jon Jenkins' job
description. Jenkins, a SETI Institute scientist who leads the Kepler Signal
Processing/Detection Algorithm team, reduces the data bandwidth by taking
advantage of the fact that Kepler's photos are mostly of empty space,
punctuated by stars. The stellar targets, even when slightly splattered by
optical imperfections or telescope drift, brighten fewer than 5 percent of the telescope's
pixels. It would be silly to send blank sky (including stars too faint to
measure or background galaxies) back to the lab, over and over.
"It's like condensing a high-school yearbook," Jenkins says.
"I'm only interested in my friends, so I can clip out their faces and leave
most of the pages behind. Saves space."
To account for slightly smeared star images, optical changes
in the telescope or drift caused by pointing error, an average of 32 pixels are
allocated to each star – a patch of the CCD detector called a "stamp."
These are the yearbook faces that are returned to Earth.
The data follow a
tortuous path after being telemetered from Kepler to one of NASA's Deep
Space Network antennas. They're first routed to the Jet Propulsion Lab, and
then sent on to Kepler's Mission Operations Center in Boulder, Colorado. With
processing taking place at every step along this yellow brick road, the
information is forwarded to the Space Telescope Science Institute (think
"Hubble") in Baltimore, where it's re-organized into an easily
readable format. It eventually lands in the offices of Jon and the rest of the
Kepler team at NASA's Ames Research Center in Mountain View, California.
That's a long trek, but fortunately the journey is highly
automated. At Ames, 88 processors pipeline-process the data – combing through it
in a search for planets. Light curves – the astronomers' term for plots of
brightness versus time – are generated for all 145,000 stars. Those that seem
to occasionally dim – the flecks of gold in the mountains of dross – are flagged
for the attention of the scientists. Each month's data will typically have a
few hundred "hits."
Not all hits will score, of course. There are always false
positives requiring further scrutiny. For example, micrometeorites occasionally
slam into Kepler's sunshade, kicking off dust that crosses the field, catches
some sunlight, and shows up in the data as a star whose brightness has suddenly
changed. Other candidates for mis-identification include so-called eclipsing
binaries – double stars that happen to periodically get in front of one
another.
Bottom line: Not every change in brightness is caused by a
planet. So the Kepler team is eager to get its first results analyzed this
summer while telescopes on Earth can verify at least some of its discoveries
(the constellations Cygnus and Lyra are in the nighttime sky now). This "reality
check" will give everyone confidence that – if Kepler turns up smaller,
Earth-like planets – the results will be credible.
Finding those terrestrial worlds will take time – a few
years, at least. But that fact differentiates this mission from most
astronomical exploration. For many forays into the unknown the excitement occurs
with the first few discoveries – the first pulsars, the first quasars, the
first black holes. After that, it's merely a matter of adding to the
collection.
But for Kepler, the excitement waxes, rather than wanes. In
its first month or two, Kepler will uncover large numbers of "hot Jupiters" – bulky
planets in tight orbits. But we know about those. Eventually, however, the
small fry will surface, the rocky worlds that might be the abodes of
extraterrestrial life.
"The excitement is in that glimmer of hope," says
Jenkins. "The capability to find plenty of Earths – if they're there for
us to find."
"It's like snorkeling over a coral reef," he says.
"In the past, we did this with only our naked eyes. The view was blurred.
Now we have goggles. I think we're going to see wondrous things."
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