After 16 years, NASA's Spitzer Space Telescope has come to an end of its mission. The infrared
telescope was decommissioned on January 30th, 2020. But during its lifetime, Spitzer opened
a window onto the old, cold, and dusty Universe. Welcome back to Launch Pad, I’m Christian
Ready, your friendly neighborhood astronomer. While it’s always sad to say goodbye to
such an incredible observatory like Spitzer, it’s also a good time to appreciate the
little space telescope that could literally see what others could not. Spitzer was launched
from Cape Canaveral on August 25 2003. It joined the Compton Gamma Ray Observatory,
the Chandra X-Ray Observatory, and the Hubble Space Telescope in NASA’s Great Observatories
program. Combined, these telescopes would give us our clearest look at the Universe
from gamma rays to the infrared part of the spectrum. Stars are generally hot enough to
radiate most of their energy in the visible part of the spectrum. That makes them pretty
easy to see. But most of the stuff in the universe aren’t stars at all, but dust and
gas. This stuff is much cooler so they radiate most of their energy in the infrared. But
sensing the feeble heat of cold dust requires an even colder detector. It’s the same reason
you need a dark viewing site to see the faintest stars. To that end, Spitzer was launched into
a first-of-its-kind Earth-trailing orbit around the Sun. This kept the spacecraft away from
the thermal environment around the Earth and Moon. It was equipped with a heat shield that
doubled as a solar array. And in a really cool innovation, its opposite half was given
a flat-black paint job to radiate heat away from the spacecraft. Kinda like a wood stove.
Spitzer was equipped with an 85-centimeter telescope and three instruments. These instruments
gave Spitzer sensitivity from the near into the far infrared.
But in order to see in the far infrared, Spitzer had to be kept extremely cold. That’s why
it launched with a cryostat of liquid helium. The cryostat cooled the instruments to temperatures
as low as 1.4 Kelvin (roughly -457 degrees Fahrenheit, or -272 degrees Celsius). As the
liquid helium flowed around the instruments, it gradually boiled off, carrying away heat.
The cryostat was designed to operate for a minimum of 2.5 years. It ended up lasting
5 years and 9 months. That’s 130% longer than the given warranty! In that time Spitzer
executed more than 36,000 hours of science observations. That’s equivalent to more
than 12 years of observations from low earth orbit.
The secret to Spitzer’s efficiency was its unobstructed view of the sky. With no planet
in the way, it could point in any direction as long as it kept its solar arrays toward
the Sun. The Hubble Space Telescope by contrast is in low-Earth orbit, so its view of any
given target can be blocked by Earth for up to half of Hubble's orbit.
Even though its coolant ran out by May 2009, Spitzer still had two working detectors in
its IRAC camera. So Spitzer began its “warm” mission at a balmy -404 degrees Fahrenheit
(-242 Celsius).
Over its 16-year mission, Spitzer revealed more about the cold and dusty universe than
all of the other infrared missions that came before it.
For example, Spitzer discovered the largest ring around Saturn, and it was hiding right
there in plain sight.
Most of Saturn’s rings are close to the planet and are maintained by its inner moons.
But this new ring is much farther and is associated with Saturn’s outer moons, most of which
are captured asteroids. In fact, the ring seems to line up well with Saturn’s largest
outer moon Pheobe.
It’s thought that meteoroid impacts on Pheobe can kick off enough dust to create the ring.
Because these dust particles are dark, they don’t reflect sunlight so much as absorb
it. This causes the dust to heat up, making them very bright in the infrared. Spitzer
captured an edge-on view of a portion of the ring.
The ring explains the strange appearance of Saturn’s moon Iapetus. Iapetus is tidally
locked to Saturn, so one side always faces the planet at all times. Even though its surface
is water ice, one side is covered by a large dark feature that faces into the direction
of its orbit. This led astronomers to wonder if the moon was sweeping up some dark material
as it orbits Saturn. That led to the search for the great dust ring, which Spitzer easily
found. This is the largest ring of Saturn ever discovered and the largest ring in the
solar system. It's one planetary ring to rule them all. Okay, fine, sorry.
Now all of this was possible because dust radiates at far infrared wavelengths. But
mid-infrared wavelengths are just long enough to avoid fine dust particles. This allows
Spitzer to see through the dust that enshrouds young stars forming in their stellar cocoons.
One example is NGC 6334, known as the Cat’s Paw Nebula. It’s a vast region of star formation
shown here in visible light. The red color comes from hydrogen gas energized by hot stars
in the nebula.
But at infrared, the image is completely different. Instead of the hot gas, red now indicates
the cool, darker dust. Notice that we can now even see stars that were otherwise blocked
by the dust. If we include information from the far infrared, the dust that’s being
warmed up by the stars inside now becomes visible. This kind of analysis allows astronomers
to reveal where in the nebula stars are forming. And even then, there are some regions that
are so thick with dust they still appear dark at infrared wavelengths.
Spitzer taught us a lot about extrasolar planets, which is remarkable because extrasolar weren’t
even a thing when the mission was designed. Nevertheless, it produced the first-ever weather
map of a planet beyond our solar system. The planet in question is HD 189733b. Spitzer
measured the infrared light coming from the planet as it circled around its star. The
hot spot on the planet confirms that it's tidally locked to its star. However, the planet's
overall temperature variation is relatively mild. This suggests that winds are spreading
the heat from its day side around to its dark side. Such winds might rage across the surface
at up to 6,000 miles per hour (9600 kph). How cool is that? The first exo-weather report!
But Spitzer’s most dramatic exoplanet result was its look at the TRAPPIST-1 system. After
monitoring the system for 500 hours, Spitzer revealed that TRAPPIST-1 is home to seven,
count ‘em SEVEN Earth-sized planets surrounding the red dwarf star! And if that weren’t
amazing enough, at least 3 of those planets orbit in the star’s “habitable zone”.
Scientists used Spitzer to characterize Brown Dwarfs. These are objects that fall somewhere
between failed stars and highly successful planets. In a 2014 study, astronomers discovered
that brown dwarfs typically have atmospheric storms.
As a brown dwarf rotates, its clouds move in and out Spitzer’s line of sight, causing
changes in its infrared brightness. Astronomers analyzed these brightness variations to determine
how clouds are distributed in brown dwarfs. With the help of some supercomputer modeling,
they showed how large atmospheric waves could combine to form bright spots in the atmosphere
and separate to produce darker regions.
Over the course of a decade, Spitzer devoted a total of 172 days taking images of the disk
of our Galaxy. The project was the Galactic Legacy Mid-Plane Survey Extraordinaire or
GLIMPSE. In 2014, astronomers completed the painstaking work of stitching together each
of the images into the largest infrared panorama of our home Galaxy.
This picture covers only about 3% of the sky, but includes more than half of the galaxy's
stars and the majority of its star formation activity.
The red color shows dusty areas of star formation. Massive stars blast out winds and radiation,
and sculpt giant bubbles and plowing mountains of dust. Elsewhere, clusters of gestating
stars deep in cocoons of gas and dust are revealed.
Spitzer uncovered whole galaxies to reveal their structure. M81 is a relatively nearby
galaxy. Hubble gives us our clearest visible light image, but Spitzer is able to selectively
image the starlight at 3.6 and 4.5 microns. This shows the distribution of stars throughout
the galaxy. But if we view the galaxy at 8 microns, we see the glow of the dust resembling
a galactic skeleton. Combining these two images gives us an infrared map of the galaxy and
if include the 24 micron channel, we see where new stars are forming in the spiral arms.
It was an incredible run for Spitzer, but its unique vantage point away from Earth meant
it wouldn’t be able to be operated indefinitely.
In order to downlink its data and receive new commands, Spitzer must point its high-gain
antenna toward Earth. Early on in the mission, Spitzer could perform this maneuver while
still keeping its heat shield toward the Sun.
But over time, as Spitzer continued to drift farther away from Earth, so it had to tilt
farther away from the Sun to communicate with Earth. Not only that but it had to maintain
this pointing for longer periods as the light travel time between Earth and Spitzer increased.
Throughout its mission, NASA conducted periodic reviews to assess Spitzer. This is a common
practice as NASA has a habit of building spacecraft that last way longer than they’re supposed
to.
Anyway, these reviews often result in mission extensions. Since the James Webb Space Telescope
is an infrared observatory, NASA announced in 2016 that Spitzer would be decommissioned
in 2018, which at the time was when JWST was supposed to launch.
However, when JWST’s launch was postponed to 2021, NASA announced a final extension
of the Spitzer mission, called Spitzer Beyond.
The name was appropriate. Not only was Spitzer operating well beyond its original lifetime,
but it would have to operate Spitzer way beyond its design limits.
During normal science operations, Spitzer could aim up to 30 degrees away from perpendicular
to the Sun. Any further and Spitzer’s heat shield would no longer protect the spacecraft.
In fact, Spitzer had onboard fault protection software to prevent the telescope from pointing
beyond this limit.
During the Beyond mission, engineers would disable the fault protection to allow Spitzer
to point way past this limit during data downlink. This was incredibly risky, because solar array
couldn’t provide power and the spacecraft so it would need to rely on batteries. But
the exposure to sunlight would raise the temperature of the high gain antenna and struts. That
heat would work its way up to the rest of the spacecraft.
Still, the maneuver worked. So it’s natural to wonder why not just continue to use Spitzer
until it breaks? The answer ultimately comes down to funding. NASA’s budget is roughly
one half of one percent of the federal budget and there are new missions coming. Not the
least of which are the James Webb Space Telescope, which, again, will hopefully launch in 2021
(although we’ll see) and the Wide-Field Infrared Survey Telescope or WFIRST which
will hopefully launch later this decade.
JWST in particular is much larger and more sensitive than Spitzer and now that it looks
like it's going to launch within our lifetimes, the decision was made to bring the Spitzer
mission to a responsible end.
On January 30th, spacecraft operators issued their final command to put Spitzer into “safe
mode”. This mode powers down all non-essential systems and orients the solar panels toward
the Sun.
Safe mode keeps the spacecraft in a powered hibernation state while waiting for a recovery
command from Earth. Only in this case, no such command will ever be sent. Spitzer will
remain in its Sun coning attitude, sleeping in its orbit around the Sun.
It will continue to drift farther away from Earth, until it reaches a maximum distance.
After which, Earth will begin to catch up and eventually lap Spitzer in its orbit around
the Sun. This will occur sometime around the year 2056, 53 years after it launched.
This will send Spitzer into a slightly closer orbit to the Sun and speed up, until it comes
around and laps past Earth once again. Only this time, it will be boosted into a more
distant orbit around the Sun. It’s neat to think that Sptizer’s always going to
be out there, somewhere and who knows, maybe in a few hundred years our descendants will
go out there, pick it up and put it in a museum. It would be a fitting tribute to our early
glimpses into the cold, dusty universe.
So what’s your favorite Spitzer result? Let me know in the comments below and I'll
be sure to check them out. And, I’d like to thank my Patreon supporters for helping
to keep this channel going, and I’d like to welcome my newest patrons, Dave Mausner,
Andy Law, and a special welcome my newest Cosmological supporter, Michael Dowling along
with, once again, to Steven J. Morgan. If you’d like to help support Launch Pad for
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Until next time, stay curious, my friends.