It was almost three and a half years ago that I was saying it was (probably) time to say goodbye to Kepler, yet while it was indeed the end of the primary mission, the team found a way to allow the spacecraft to continue gathering data starting the following year. It was a different kind of mission in one important aspect, not being suited for discovering anything but short-period planets anymore, but on the other hand it was decided ever since this new mission was approved that it will be used for other kinds of observations as well.
The solution found by the team actually allowed this secondary mission to last longer than the primary one, stretching the fuel reserves which would have otherwise been exhausted within months to over four years of active use of the spacecraft. Admittedly, the number of planets discovered was far lower than that of the primary mission, at the time I’m writing this the Exoplanet Archive listing 2327 confirmed planets discovered by Kepler and only 354 by K2, with 2426 Kepler Project candidates yet to be confirmed and 473 for K2, but those are discoveries which shouldn’t have been possible, and wouldn’t have been without the team’s unbelievable ingenuity. I guess the potential discovery of a handful of longer-period planets, which would have been possible by continuing the primary mission after the failure of the second reaction wheel, until the fuel would have been exhausted, was sacrificed for this, and of course you can never know that something extraordinary wasn’t among them, but it sure seems to me that the gamble was worth it on the whole.
Unfortunately, even this solution wasn’t going to stretch the fuel reserves forever and back in March it was announced that the mission was nearing its end. Some more life and data could be squeezed out of it, but after observations were first paused in July to ensure there will be enough fuel left to properly point the antenna in order to send the data already gathered and indications of degraded pointing performance after they were resumed eventually led to the spacecraft being put in sleep mode in September, before managing to successfully send the data earlier this month, the fact that Kepler was found to have put itself into sleep mode again due to lack of fuel shortly after it was commanded to resume observations came as no surprise, and neither did yesterday’s announcement marking the end of the mission.
The fact that it was expected doesn’t make it less sad, of course, but there is a silver lining in the fact that Kepler managed to last long enough for TESS to start science operations, the spacecraft in fact being pointed at Observation Sector 4 now, so it can really be said that it passed on the baton. TESS really does seem to me to be aiming for quantity over quality though, moving its “gaze” from one patch of sky to another three times faster than Kepler did even during the K2 mission. There will be a little overlapping, but the discoveries will obviously consist to an overwhelming extent of short-period, likely very short-period, planets. I guess that makes TESS sort of a continuation of K2 with a much shorter attention span, but in no way a replacement for Kepler’s primary mission. Still, considering how much we as a species have been failing at continuing space exploration in recent years, if not even decades, the fact that even such a continuation exists at all is at least a cause for some relief, if not even a reason for hope.