An Obituary for Kepler

Post by Nic Quattromani:

I’ve got some tragic news to share today: NASA’s Kepler space telescope, formerly our premier planet-hunter floating in the sky, has ceased operations. This was not due to any technical failure aboard the craft. Rather, it simply ran out of fuel, rendering it unable to conduct stationkeeping or even orient itself properly, and so NASA has pulled the plug, albeit much less dramatically than JPL did when Cassini faced the same problem last year.

Kepler launched in 2009 atop a Delta II, entering a heliocentric orbit trailing well behind Earth. From there it spent around four years studying just 150,000 stars in one small patch of sky. Its sole scientific instrument, a photometer, monitored each star for changes in brightness caused by planets passing in front of it—this is called the direct transit method of planet-hunting. The spacecraft ultimately catalogued 2,327 exoplanets during this period, which were later confirmed by other sources.

This is an artist’s conception of Kepler, sadly not a photograph taken by another spacecraft orbiting next to it. Image is in the public domain.

By 2013 Kepler had lost functionality in two of its four reaction wheels. This made precise stationkeeping impossible, but NASA’s engineers, being NASA’s engineers, did not simply throw up their hands and announce that its days of planet-hunting were over. Rather, they devised a neat technological workaround. Using the pressure of the solar wind, delicately balancing radiation pressure with the force of the remaining reaction wheels, they stabilized the craft and inaugurated the second phase of its mission, dubbed K2, which lasted from 2014 to October 30 of this year. During that time Kepler discovered 354 new planets, while also studying supernovas, nascent stars, and asteroids.

Official, public-domain infographic, showing how they turned Kepler into an extremely low-thrust solar sail.

It is a testament to the sheer ridiculous vastness of the universe that Kepler found so many new worlds. Think about it: for a planet to obscure a star from Earth’s point of view and register on Kepler’s sensors, its orbital plane must be exactly parallel to an axis directly through the star, and that orbital plane is under no obligation to point in any particular direction. For every planet we discover using the transit method there must be countless others with off-axis orbits we can’t detect. To draw 2,327 planets from 150,000 stars must mean that planets are ridiculously abundant, like so many grains of sand on a cosmic beach, or so many bad romance novels in a cosmic bookstore.

Among the thousands of planets it pointed out to us, Kepler spotted between two and twelve Earth-sized ones located in their stars’ habitable zones. Extrapolating from this data, and taking into account the limitations I mentioned above, researchers on the Kepler team determined that 20 to 50 percent of stars in the sky might have small, rocky planets capable of supporting liquid water.

That is a large, large percentage. And here’s another eye-popping number: Kepler’s successor, the Transiting Exoplanet Survey Satellite (TESS) which launched in 2018, is expected to discover a further 20,000 worlds. How many of those might be planets with complex geography and weather, like Earth or Mars? How many could support life? How many support hostile and expansionistic alien empires, which at this very moment are aiming missiles at us?

Brave and vigilant TESS was the first to see the attack coming. Image is in the public domain.

Hopefully the answers are very many, many, and zero. But that’s the thing about space exploration—you never know until you roll up your sleeves, go out there, and look. Kepler inaugurated a new era in exoplanet science, putting within our knowledge the contents of solar systems light-years away, and its follow-up missions, TESS and others, will build upon that foundation with breakthrough discoveries of their own.


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