In March 2017, at a small summit in Washington, DC, two Harvard professors, David Keith and Frank Keutsch, laid out plans to conduct what would have been the first solar geoengineering experiment in the stratosphere.

The basic concept behind solar geoengineering is that by spraying certain particles high above the planet, humans could reflect some amount of sunlight back into space as a means of counteracting climate change. But critics have argued that an intervention that could tweak the entire planet’s climate system is too dangerous to study in the real world.

The single, small balloon experiment came to represent all of these fears—and, in the end, it was more than the researchers were prepared to take on. Last month, a decade after the project was first proposed, Harvard officially announced the project’s termination. So what went wrong? And what does that failure say about the latitude that researchers have to explore such a controversial subject? Read the full story.

—James Temple

Why the lifetime of nuclear plants is getting longer

The average age of reactors in nuclear power plants around the world is creeping up. In the US, which has more operating reactors than any other country, the average reactor is 42 years old. Nearly 90% of reactors in Europe have been around for 30 years or more. 

Older reactors, especially smaller ones, have been shut down in droves due to economic pressures, particularly in areas with other inexpensive sources of electricity, like cheap natural gas. But there could still be a lot of life left in older nuclear reactors.