Credit: NASA/JPL-Caltech
Now approaching its 10th anniversary, NASA's Spitzer Space Telescope
has evolved into a premier observatory for an endeavor not envisioned in
its original design: the study of worlds around other stars, called
exoplanets. While the engineers and scientists who built Spitzer did not
have this goal in mind, their visionary work made this unexpected
capability possible. Thanks to the extraordinary stability of its design
and a series of subsequent engineering reworks, the space telescope now
has observational powers far beyond its original limits and
expectations.
"When Spitzer launched back in 2003, the idea that
we would use it to study exoplanets was so crazy that no one considered
it," said Sean Carey of NASA's Spitzer Science Center at the California
Institute of Technology in Pasadena. "But now the exoplanet science work
has become a cornerstone of what we do with the telescope."
Spitzer
views the universe in the infrared light that is a bit less energetic
than the light our eyes can see. Infrared light can easily pass through
stray cosmic gas and dust, allowing researchers to peer into dusty
stellar nurseries, the centers of galaxies, and newly forming planetary
systems.
This infrared vision of Spitzer's also translates into
exoplanet snooping. When an exoplanet crosses or "transits" in front of
its star, it blocks out a tiny fraction of the starlight. These
mini-eclipses as glimpsed by Spitzer reveal the size of an alien world.
Exoplanets
emit infrared light as well, which Spitzer can capture to learn about
their atmospheric compositions. As an exoplanet orbits its sun, showing
different regions of its surface to Spitzer's cameras, changes in
overall infrared brightness can speak to the planet's climate. A
decrease in brightness as the exoplanet then goes behind its star can
also provide a measurement of the world's temperature.
While the
study of the formation of stars and the dusty environments from which
planets form had always been a cornerstone of Spitzer's science program,
its exoplanet work only became possible by reaching an unprecedented
level of sensitivity, beyond its original design specifications.
Researchers
had actually finalized the telescope's design in 1996 before any
transiting exoplanets had even been discovered. The high degree of
precision in measuring brightness changes needed for observing
transiting exoplanets was not considered feasible in infrared because no
previous infrared instrument had offered anything close to what was
needed.
Nevertheless, Spitzer was built to have excellent control
over unwanted temperature variations and a better star-targeting
pointing system than thought necessary to perform its duties. Both of
these foresighted design elements have since paid dividends in obtaining
the extreme precision required for studying transiting exoplanets.
The
fact that Spitzer can still do any science work at all still can be
credited to some early-in-the-game, innovative thinking. Spitzer was
initially loaded with enough coolant to keep its three
temperature-sensitive science instruments running for at least
two-and-a-half years. This "cryo" mission ended up lasting more than
five-and-a-half-years before exhausting the coolant.
But Spitzer's
engineers had a built-in backup plan. A passive cooling system has kept
one set of infrared cameras humming along at a super-low operational
temperature of minus 407 degrees Fahrenheit (minus 244 Celsius, or 29
degrees above absolute zero). The infrared cameras have continued
operating at full sensitivity, letting Spitzer persevere in a "warm"
extended mission, so to speak, though still extremely cold by Earthly
standards.
To stay so cool, Spitzer is painted black on the side
that faces away from the sun, which enables the telescope to radiate
away a maximum amount of heat into space. On the sun-facing side,
Spitzer has a shiny coating that reflects as much of the heat from the
sun and solar panels as possible. It is the first infrared telescope to
use this innovative design and has set the standard for subsequent
missions.
Fully transitioning Spitzer into an exoplanet spy
required some clever modifications in-flight as well, long after it flew
beyond the reach of human hands into an Earth-trailing orbit. Despite
the telescope's excellent stability, a small "wobbling" remained as it
pointed at target stars. The cameras also exhibited small brightness
fluctuations when a star moved slightly across an individual pixel of
the camera. The wobble, coupled with the small variation in the cameras,
produced a periodic brightening and dimming of light from a star,
making the delicate task of measuring exoplanet transits that much more
difficult.
To tackle these issues, engineers first began looking
into a source for the wobble. They noticed that the telescope's
trembling followed an hourly cycle. This cycle, it turned out, coincided
with that of a heater, which kicks on periodically to keep a battery
aboard Spitzer at a certain temperature. The heater caused a strut
between the star trackers and telescope to flex a bit, making the
position of the telescope wobble compared to the stars being tracked.
Ultimately,
in October 2010, the engineers figured out that the heater did not need
to be cycled through its full hour and temperature range -- 30 minutes
and about 50 percent of the heat would do. This tweak served to cut the
telescope's wobble in half.
Spitzer's engineers and scientists
were still not satisfied, however. In September 2011, they succeeded in
repurposing Spitzer's Pointing Control Reference Sensor "Peak-Up"
camera. This camera was used during the original cryo mission to put
gathered infrared light precisely into a spectrometer and to perform
routine calibrations of the telescope's star-trackers, which help point
the observatory. The telescope naturally wobbles back and forth a bit as
it stares at a particular target star or object. Given this unavoidable
jitter, being able to control where light goes within the infrared
camera is critical for obtaining precise measurements. The engineers
applied the Peak-Up to the infrared camera observations, thus allowing
astronomers to place stars precisely on the center of a camera pixel.
Since
repurposing the Peak-Up Camera, astronomers have taken this process
even further, by carefully "mapping" the quirks of a single pixel within
the camera. They have essentially found a "sweet spot" that returns the
most stable observations. About 90 percent of Spitzer's exoplanet
observations are finely targeted to a sub-pixel level, down to a
particular quarter of a pixel. "We can use the Peak-Up camera to
position ourselves very precisely on the camera and put light right on
the best part of a pixel," said Carey. "So you put the light on the
sweet spot and just let Spitzer stare."
These three
accomplishments -- the modified heater cycling, repurposed Peak-Up
camera and the in-depth characterization of individual pixels in the
camera -- have more than doubled Spitzer's stability and targeting,
giving the telescope exquisite sensitivity when it comes to taking
exoplanet measurements.
"Because of these engineering
modifications, Spitzer has been transformed into an exoplanet-studying
telescope," said Carey. "We expect plenty of great exoplanetary science
to come from Spitzer in the future."
For more information on exoplanets, visit http://planetquest.jpl.nasa.gov.
NASA's
Jet Propulsion Laboratory in Pasadena, Calif., manages the Spitzer
Space Telescope mission for NASA's Science Mission Directorate in
Washington. Science operations are conducted at the Spitzer Science
Center at the California Institute of Technology in Pasadena. Data are
archived at the Infrared Science Archive housed at the Infrared
Processing and Analysis Center at Caltech. Caltech manages JPL for NASA.
