Engineers devised a clever trick to compensate for the floating telescope’s failed reactor wheels.
NASA's Kepler Space Telescope is arguably the best planet hunter in the cosmos. And after recovering from a severe injury, the telescope is back in the game.
Launched in 2009, the telescope was tasked with finding nearby Earth-like exoplanets in the habitable zone—the area around a star that could support liquid water. Kepler spent four years scanning a small patch of sky, identifying thousands of potential exoplanets.
Kepler began its mission looking at one patch of sky, trying to determine how common rocky exoplanets were. During its first four years, over 4,000 exoplanet candidates were detected, all within 500 to 1,000 light-years from Earth. Nearly half of those are designated to be Earth-sized or Super Earths, which tells scientists there could be nearly a billion Earth-like planets in our galaxy alone. This definitely increases the chances of finding habitable planets and maybe even life.
To find exoplanets, the telescope searches for tiny dips in starlight produced as a planet crosses in front of its host star—an event known as a transit. Detecting transits takes incredible precision, and Kepler performed beautifully for four years, finding as much as half of all the known exoplanets.
Then a hardware malfunction occurred, nearly crippling the telescope. Kepler relies on four reaction wheels to keep on target. When two of them failed in 2013, it looked like Kepler was down for the count, lacking the ability to point with any precision. However, thanks to some crafty engineering, the telescope was given a second chance at life.
Engineers at Ball Aerospace—who designed and built the space observatory—devised a clever trick to harness the energy of the Sun in order to help steady the telescope. The science team then submitted a new proposal to NASA, and the K2 mission was born.
"The K2 mission takes the same telescope," said Steve Howell, Kepler Project Scientist. "But uses the light of the Sun as a force to push on the solar panels, and then using the two good reaction wheels to push against that force, we are able to point the telescope."
Earth-like exoplanets discovered by Kepler as compared to Earth, seen on the far right. Image: NASA
With the K2 mission, we can now look at points all along the ecliptic (plane of the Solar System). Instead of staring at one field for four years, the telescope is staring at different points along the ecliptic for about 80 days each.
"We just keep going around the ecliptic, staring at different fields of view. This has allowed us to greatly expand the science that comes out of the telescope. We're still doing exoplanets," Howell said. "We're studying what's called high-value exoplanets: ones that are orbiting very close, very bright stars. So we can also study in detail from the ground and characterize the stars and planets and search for atmospheric composition especially in rocky habitable-zone planets. Which you can't do with Kepler planets essentially because they are all too far away."
Due to their close proximity to Earth, the worlds discovered as part of the K2 mission will be the perfect subjects for future space-based observatories like the James Webb Space Telescope (JWST) to study in greater detail. Once we can unlock the secrets of exoplanet atmospheres, we can definitively determine whether or not they are habitable.
"We've discovered over 250 candidates in K2 with 110 already proven to be planets. Six of those are already targets that are interesting enough rocky planets to be looked at by Hubble and eventually JWST to get atmospheric measurements of the planet. Will we see signs of life? What will we see? Pollution? Chlorophyll? It's exciting," Howell said.
But that's not all. With the K2 mission, Kepler has now become a very valuable tool for astrophysicists. "We're doing other great science too," Howell said. "Looking at star clusters like the Pleiades, globular clusters, exoplanets, supernovae, planets in our solar system, comets, active galaxies and more. Just tons and tons of science."
Howell explained that the K2 fields of view along the ecliptic don't overlap, so the scientists are seeing new things all the time. Eventually, if funding permits, the K2 mission will observe every part of the cosmos along the ecliptic.
Early on in the K2 mission, the telescope got a peek at Comet Siding Spring as it zipped passed Mars. The data collected by Kepler was combined with data from various spacecraft orbiting the Red Planet, and observatories here on Earth. As a result, we have the first ever 3D view of a comet's tail.
"As comets pass through the Solar System, they interact with the solar wind, which strips away bits of the tail. With Kepler and K2 we can observe the comet over an 80-day period," explains Howell. "You can watch the tail structure evolve and see what happens to it as it interacts with the solar wind."
One of the benefits of K2 is the constantly changing field of view. This is a big advantage in observing supernovae—the explosive death of a star. Type IA supernovae are thought to be all the same, in fact they are thought of as standard candles or cosmic mile markers, used to measure distances in the Universe. However, astronomers still aren't exactly sure what causes them. To determine what blew up, you need to observe the supernova in the first few hours after its death. "You generally can't observe a supernova in the first few hours from the ground, because you don't know it's a supernova yet," Howell explained. "It's not bright enough for you to observe."
The vast majority (about 99.99 percent) of supernovae are observed at least a day after exploding because they haven't reached peak brightness yet. As such, scientists have concluded they all look the same. But do they really? This is one of the questions astrophysicists are hoping K2 will answer.
By looking at thousands and thousands of galaxies in each K2 field, all scientists have to do is wait for one to explode. "In the meantime we are doing wonderful science, but if you do spot one then you can measure it in the first few hours to the first few days. And you can tell exactly what thing blew up by matching it to theoretical models," Howell said.
So far, Kepler has witnessed three Type IA supernovae during the K2 mission. All three are a result of two white dwarfs—the dying embers of a Sun-like star—merging together. "We've eliminated it being a white dwarf and another 'real" star,'" he said. Howell expects that over the course of the entire K2 mission, at least 80 supernovae will be observed. The data collected will definitively tell scientists if each Type IA supernova observed has the same luminosity, and ultimately if they are the standard candles we've come to rely on.
"So many cool things, it's very exciting science. If you don't know about K2, you should," said Howell.