Since its 2009 launch, NASA's Kepler space telescope has chalked up an impressive list of firsts and logged a tidy tally of newly discovered exoplanets (planets outside our solar system): 132 confirmed, plus another 2,740 unconfirmed "candidates."
Perhaps most impressive, the craft has helped make a household notion of the idea that there may in fact be oodles of Earth-like, potentially life-supporting planets tucked among the many stars of the Milky Way.
With NASA announcing this week that an equipment malfunction might mean an end to Kepler's mission, we thought we'd pay homage to the craft and take a look back at its life and work.
The image above is an artist's rendition of Kepler on the job, gazing intently into the cosmos. Click through the rest of the slideshow to refresh your memory of the mission, watch the craft come into being, and check out some of Kepler's mind-expanding, and imagination-fueling, discoveries.
NASA has described the Kepler mission as "a search for habitable planets," that is, Earth-size planets that orbit their star in the "habitable zone," a temperate realm hospitable to H20, and thus, possibly, to the carbon-based life we're familiar with.
"The habitable zone is where we think water will be," Kepler principal investigator Bill Borucki has explained. "If you can find liquid water on the surface, we think we may very well find life there. So that zone is not too close to the star, because it's too hot and water boils, and not too far away where the water is condensed...a planet covered with glaciers. It's the Goldilocks zone -- not too hot, not too cold, just right for life."
The planets also need to be Earth-size. If they're too small, they don't have enough gravity to hold air molecules and create a life-friendly atmosphere. If they're too big, they hold hydrogen and helium and turn into gas giants like Jupiter and Saturn.
Here we see Borucki discussing the plans for the Kepler mission during a meeting at the SETI Institute in Mountain View, Calif., two years before the craft's launch. At that time he said, "We are trying to find man's place in the universe. The first step in doing that is finding Earth-like planets."
The way Kepler has discovered all those "new" planets and their characteristics is by staring at stars.
When a planet orbits and passes in front of its star (in what's known as a "transit"), it naturally blocks some of the light being emitted by that star. The brightness of that star, then, drops. And, under certain conditions, Kepler's instruments can register that drop. As NASA puts it:
"By measuring the depth of the dip in brightness and knowing the size of the star, scientists can determine the size or radius of the planet. The orbital period of the planet can be determined by measuring the elapsed time between transits. Once the orbital period is known, [Johannes] Kepler's Third Law of Planetary Motion can be applied to determine the average distance of the planet from its star." And this, along with the probable temperature of the star, can be used to determine the likely temperature on the planet.
Earth-based instruments have used a similar technique -- involving a planet's gravitational pull on its star, as opposed to changes in the star's brightness -- to spot new planets. In fact, in 2010, astronomers working with a spectrometer and this "wobble method" at Hawaii's Keck Observatory announced they'd discovered the first real example of a potentially life-friendly exoplanet.
But the brightness-based "transit method" of planet-finding provides information the wobble method doesn't -- perhaps most importantly, a planet's size. And Earthbound tools can't use the transit method; Earth's orbit and the changing night sky prevent constant monitoring of the same stars, and atmospheric conditions interfere. Since it sits comfortably out in space, Kepler avoids these issues (and it has special characteristics that a space telescope like Hubble doesn't). Its data can be combined with info gleaned by Earthbound and other instruments to create profiles of planets.
So what is Kepler? In simple terms, it's a giant light meter, made up of a telescope, a "camera," and various electronics, that stands on a spacecraft base while nestled in a wraparound solar array (which powers the setup).
Here's a model of Kepler, from the 2007 SETI meeting mentioned in slide number two. Note the foil-wrapped light meter (or "photometer"), the putty-colored spacecraft base, and the wraparound solar array.
And here's a more detailed artist's rendering, sans foil. Note the two black, spool-like structures at the far left, sticking out of the side of the craft's base, below the solar array -- they look a bit like auto rims without tires. Those are two of the four "reaction wheels" on Kepler.
In order for the craft to reliably establish the existence of a planet, it's had to track that potential planet's transit across a star several times, not just once. And that's meant Kepler has had to maintain a precise field of view over time. (It would, of course, take an Earth-size planet in an Earth-like position about a year to circle its star a single time.)
The reaction wheels have been keeping Kepler focused on the stars it's been monitoring. At least, they had been doing that. Read on...
A closer look at two of Kepler's four reaction wheels, during the craft's assembly at Ball Aerospace & Technologies. The reaction wheels are, as NASA has said, "special electric motors mounted on the spacecraft that act like specialized gyroscopes. Changes in the motor spin rates result in changes in the spacecraft orientation in different directions without resorting to firing rockets or jets." The wheels were designed to keep Kepler's light meter constantly pointed at the same stars:
"The motor spin rates are controlled electronically by computer and are essential for altering spacecraft orientation by very small amounts, as needed for keeping the Kepler telescope pointed precisely at its designated sky target area." They've also been rolling Kepler 90 degrees every three months to keep the solar panels pointed at the sun.
...it looks like one too many of the reaction wheels may be dead or dying. Kepler needs only three wheels in order to stay properly positioned, and NASA provided four just in case. But one failed earlier, so now we're down to two. Hence, Kepler's gaze is drifting.
NASA isn't ready to call the mission over just yet; Earthbound technicians are trying to jump-start the misbehaving wheel (at 42.4 million miles from Earth, Kepler is too far away for a Hubble-like, astronaut repair job).
But in any case, the errant wheel lasted about eight and a half months beyond the originally planned three-and-a-half-year duration of the Kepler mission. So with its payload of other sophisticated equipment, Kepler has accomplished quite a lot.
Here's what is perhaps the centerpiece of Kepler's collection of gear: the focal plane assembly, also known as the biggest camera NASA has ever flown in space. The 21 purplish-blue squares you see here are each made up of two rectangular, 2,200x1,024-pixel "charged coupled devices," or CCDs, which have been measuring the light from Kepler's targeted stars.
This camera, rated at a whopping 95 megapixels, hasn't been taking the sort of pictures you're used to, however. It's been collecting data on brightness and sending it to an onboard computer, which in turn has been beaming the data to Earth once a month.
Remember that pattern of squares -- you'll see it again very soon.
Voila. This is the view that's been enjoyed by the focal plane assembly for more than four years now: "an expansive star-rich patch of sky in the constellations Cygnus and Lyra," as NASA describes it. The view encompasses more than 100,000 stars. Kepler was designed to observe so many because only a small percentage of the stars might actually show a planet's transit in front of them. That's because in order for a transit to be visible, a star's planetary system has to be perfectly aligned with our line of sight.
Create a fist with one hand and call that a star. Then create a planet with the tip of your other index finger and orbit it around your fist at different distances and angles. You'll begin to understand the alignment issue. NASA says that "for Earth-size planets around sunlike stars, the chances of randomly oriented orbital planes being in the correct orientation for Kepler to see a transit is about 0.5 percent." Remember those low odds -- they'll be used to make a rather breathtaking point in an upcoming caption.
(By the way, the detailed areas called out in this image show a cluster of stars, called NGC 6791, and a star with a known planet called TrES-2 [circled in blue].)
Here, we've zoomed out a little, to show the region of the Milky Way that's home to the Cygnus and Lyra constellations. Some of the stars Kepler has been staring at are as many as 3,000 light-years away.
Now let's take a very quick trip back through time to watch Kepler grow into a fully formed adult, ready to leave the nest.
Here's the focal plane assembly that we saw earlier being prepared for mounting inside of Kepler's telescope.
And here, it's being guided into the telescope's lower housing.
This diagram shows the focal plane assembly's eventual placement inside the telescope, between the mirror, at the bottom, and the Schmidt corrector lens, which corrects for the mirror's curvature, at the top. The image of the stars is, of course, bounced off the superhigh-tech mirror onto the focal plane assembly and its equally high-tech CCDs.
Together, all this gear forms Kepler's giant light meter, or photometer.
Here's the back side of the primary mirror assembly. The honeycomb structure keeps the mirror very light -- it's just one-seventh the weight of a solid mirror with the same thickness and diameter.
The lower housing is guided onto the primary mirror assembly. (You can see the head of one of the technicians reflected in the curved mirror.)
The assembled photometer. (That's some light meter, eh?)
And here's the gold-wrapped photometer being lowered onto its spacecraft base.
Last but not least, the solar array was added. (And the little white elves finally got to take a sandwich break.)
So what do you do with millions of dollars' worth of custom-built, highly sensitive instrumentation? You set it atop a huge amount of highly flammable liquid and light a match.
On March 6, 2009, Kepler leaped toward the stars on top of a Delta II rocket, on the way to making its historic discoveries...
On January 4, 2010, NASA announced Kepler's first modest discovery: five exoplanets -- "hot jupiters," with high masses, extreme temperatures, and large sizes (from about the size of Neptune to larger than Jupiter -- both of which are far larger than the planet we call home). So, nothing habitable. But since then, the industrious floating photometer has gone on to discover more than one orb in the habitable zone, along with a few tantalizing space oddities.
The beautiful blue-green ball you see here, in a NASA artist's rendition of course, is Kepler-22b, the first planet Kepler confirmed (on December 5, 2011) as orbiting in a star's habitable zone.
The planet grabbed headlines as a potential doppelganger for Earth (despite it being two and a half times larger). But scientists aren't sure if it has a predominantly rocky, gaseous, or liquid composition. Still, Douglas Hudgins, Kepler program scientist at NASA Headquarters in Washington, said at the time of the discovery, "This is a major milestone on the road to finding Earth's twin."
And it presumably made a lot of people sit up and pay attention.
About three months prior to the discovery of Kepler 22-b, NASA announced, on August 26, 2010, Kepler's discovery of the first confirmed planetary system with more than one planet crossing in front of the same star.
Here, we see the star, Kepler-9, being circled by its two planets, Kepler-9b, on the right, and Kepler 9c. Both planets are close in size to Saturn. Another super-Earth-size planet was later spotted in the same system. And still later, on February 2, 2011, Kepler confirmed a system with six planets circling their star, Kepler-11. NASA has called this Kepler-11 system "the fullest, most compact planetary system yet discovered beyond our own."
One of the oddities spied by Kepler is this possible "evaporating planet," discovered on May 18, 2012. Analyzing data beamed back by Kepler, researchers identified a strange light pattern coming from a star called KIC 12557548. This led them, as NASA puts it, to:
"hypothesize that the star-facing side of the potentially rocky inferno is an ocean of seething magma. The surface melts and evaporates at such high temperatures that the energy from the resulting wind is enough to allow dust and gas to escape into space. This dusty effluence trails behind the doomed companion as it disintegrates around the star."
The doomed companion has not yet been confirmed as a planet, however.
NASA announced the discovery of this system, Kepler-47, on August 28, 2012. Here, we see it compared to part of our own solar system. There's an intriguing little detail in this diagram. Can you spot it? Read on...
If you spotted the Kepler-47 system's two suns, consider yourself an honorary member of Kepler's focal plane assembly. Kepler-47 was the first instance the space telescope found of multiple transiting planets orbiting a pair of stars.
Earlier, on September 15, 2011, Kepler had spotted its first confirmed single planet orbiting two stars: Kepler-16b. And on January 11, 2012, it discovered two more double-sun planets: Kepler-34b and Kepler-35b. (We'll be quizzing you on these planet names, so we hope you're taking notes.)
But if two stars aren't enough for you, how about four? On October 15, 2012, a joint effort between scientists and amateur astronomers with the Planet Hunters project tapped data from Kepler to discover PH1, the first known planet orbiting a double-star that itself is orbited by a distant pair of stars.
But let's not get greedy. In the above image, we see Kepler-47c in the foreground and Kepler 47b in the distance, with their two suns glowing in the middle. The foreground planet is a gaseous giant, inhospitable to life, but just for the sake of discussion, let's ask the following question:
If future generations of humans were to somehow colonize Kepler-47c, would they go for an evening stroll and see...
...this? And if they did, would they recall their history books -- er, data sets -- and fondly remember Kepler?
(This, of course, is a memorable scene from 1977's "Star Wars," depicting Luke Skywalker on his home planet of Tatooine.)
Evaporating planets and twins of Tatooine are all well and good, but what of Kepler's stated mission, to find Earth-size planets in the habitable zone? Are we there yet?
Kepler's most recent discovery, announced last month, is of the smallest habitable-zone planets found to date, planets that are beginning to approach the size of our own.
Above is an artist's take on the smallest such planet yet, Kepler-62f.
And here's a size comparison of habitable-zone planets discovered by Kepler to date, alongside Earth. From left to right: Kepler-22b, Kepler-69c, Kepler-62e, Kepler-62f, and Earth. (They're all artist's renderings, except for Earth.)
And here's the Kepler-62 system alongside a part of our solar system. At a glance it looks pretty similar, doesn't it? (Of course, there are differences. For one thing, the Kepler-62 system's "sun" is two-thirds the size of our sun and only one-fifth as bright.)
Clearly, Kepler has yet to find a dead ringer for Earth. Still, as John Grunsfeld, associate administrator of the Science Mission Directorate at NASA Headquarters in Washington, was quoted as saying, in the space agency's announcement about the Kepler 62 system:
"The discovery of these rocky planets in the habitable zone brings us a bit closer to finding a place like home. It is only a matter of time before we know if the galaxy is home to a multitude of planets like Earth, or if we are a rarity."
Unfortunately, unless NASA's technicians can get Kepler's troubled reaction wheel spinning again, time may have run out for Kepler itself. But check out the last two slides...
Remember this one? Let's zoom out again...
OK, here we've zoomed out a lot, to show the entire Milky Way, along with the area Kepler has been looking at and through.
Remember that low probability we mentioned back at slide nine? Of Kepler spotting the transit of a planet across a given star? You'll recall that a transit can be seen only with the proper orientation of a planet's orbit to our line of sight, and that the probability of Kepler spying a transit among its 100,000 stars has been about 0.5 percent.
NASA says that "statistically, we can infer that every planet Kepler detects represents hundreds more planets that are out there but not detectable due to inopportune orbital orientation."
As mentioned before, Kepler has spotted 132 confirmed planets, plus 2,740 potential planets. And it's been looking at a relatively miniscule patch of the galaxy. How many hundreds, or thousands, or millions of Earth-like planets might there be?
Or here's another way of thinking about it. Kepler has discovered a fascinating variety of planetary systems, which suggest further, perhaps infinite, varieties. Given those differences, how many solar systems exactly like ours, or even all that similar to ours, might there not be?
This is perhaps Kepler's main achievement: the tweak it's given to our perception of the universe and, as principal investigator Borucki put it, our "place in it." Maybe life is far more abundant than we ever imagined, and thus, perhaps, that much more amazing.
Or maybe it's rarer, more unique, than we might've thought -- and that much more precious.