It wasn’t until the Kepler began collecting data in 2009 that we knew just how many planetary neighbors we have. Since then, the telescope has found over 4,000 planetary candidates within 3,000 light years from us, almost 1,000 of which have been confirmed by the observations of other telescopes.

This graphic, from 2013, represents the relative size of the 2,740 stars found by Kepler to have planetary candidates. The black dots in front of each star represent the size of each planetary candidate. A hi-res pdf version is available here.

It also helped discover that multi-planet solar systems — like our own — are also more abundant than scientists had imagined. Many of these planet candidates are even orbiting stars similar to ours, and some of those are even near the vaunted "goldilocks zone" — the distance from a star where a planet is cool enough to allow water to form but not so cold that it freezes.

Kepler was changing how we viewed our place in the galaxy

For over three years Kepler painted this increasingly diverse picture of the nearby galaxy. The section of the Milky Way that it studied — referred to as the Kepler Field — contains around 100,000 stars, some as far as 3,000 light years away and some as close as 600. Finding so many planets so relatively close was changing how we viewed our place in the galaxy, and the results kept pouring in. The mission was succeeding beyond most everyone’s expectations.

Then, in July of 2012 one of the telescope’s four gyroscopic reaction wheels failed. That’s a problem because the wheels — which are spun at varying speeds to make precise targeting movements — are crucial for aiming. Since Kepler only needed three to operate, this wasn’t an immediate problem; in fact it was a strange, temporary blessing. "It turned out that using the three wheels gave us very, very slightly improved pointing than when we were using four," says Dr. Steve Howell, a project scientist for Kepler.

But the long-term implications were grim. By studying the amount of friction the wheels were encountering at different speeds, the team saw a problem. The typically smooth data curves representing the wheel’s performance were distorted by both an increase in friction and an increase in the amount of power needed to overcome that friction. Worse, the same anomalies were also showing up in the data from a second reaction wheel — meaning Kepler was facing another mechanical failure.

A visual representation from 2013 of all the multiple-planet candidate systems discovered by Kepler in orbit around their stars. Hot to cool colors represent big planets to smaller planets, relative to the other planets in the system. Video is available here.

Howell said the team knew that meant they were dealing with a potentially mission-threatening problem. "When the first reaction wheel failed, a few of us here and a few people at Ball Aerospace (who built Kepler) already started in the back of our minds kind of thinking ‘What if we lose another reaction wheel?’ You don't want to just throw away a relatively good, expensive telescope that can still do great science."

"What if we lose another reaction wheel?"

The teams called on scientists and engineers in the community at large to help come up with a solution. Before they arrived at one, the second wheel failed in May of 2013. And while Kepler loosely follows the Earth’s orbit around the Sun, it does so at 45 million miles away from us — nowhere close enough to attempt a repair.

The Kepler team, about 40 people strong, had already completed the main mission objectives, so they declared in August along with NASA that they were ending their attempts to fully recover the telescope. But behind the scenes, everyone got to work on an idea that allowed the mission to survive by evolution. The solicitation had stirred up an ingenious solution that the team at Ball Aerospace ran with: use the pressure exerted on the telescope’s solar panels by the Sun as a stand-in for the third wheel.

In space, even a tiny force can move an object as big as a telescope. Something negligible on Earth, like the small amount of pressure that the Sun’s light exerts on things it touches, is still a force that has to be calculated and corrected for in zero-gravity. That’s why the wheels were there: to create forces that worked in opposition to allow for precise adjustments. But that seemed impossible after the second wheel’s failure. Luckily, the remaining wheels were aligned in such a way that pressure from the Sun could virtually replace the crucial third wheel. That means that the scientists can still use the other two wheels to maneuver, a development so lucky that Howell called it "almost magic."