Steven Chu will step down as Secretary of Energy at the end of this month, though he "may stay beyond that time so that I can leave the Department in the hands of the new Secretary," he wrote in a farewell letter to Department of Energy (DoE) staff, issued February 1. Regardless, when Chu leaves he will have earned the title of longest serving energy secretary in U.S. history.

Chu also leaves as one of the nation’s most ambitious energy secretaries, having presided over the disbursement of $36 billion under the American Recovery and Reinvestment Act of 2009, more widely known simply as "the stimulus." Chu used that money to back with grants or loan guarantees a wide range of alternative energy technologies in a bid to wean the U.S. from its more than $400 billion-a-year imported oil habit, ranging from electric cars to photovoltaics, as well as the first new nuclear power plant constructed in the country for more than 30 years. "I believe we should be judged not by the money we direct to a particular state or district, company, university or national lab, but by the character of our decisions," Chu wrote.

"Steve helped my administration move America towards real energy independence," President Obama said in a statement. "Over the past four years, we have doubled the use of renewable energy, reduced our dependence on foreign oil, and put our country on a path to win the global race for clean energy jobs."

For Chu, "the sweet spot at DoE has been on the front side: research and development," he told Scientific American last year in an interview about the Obama administration's so-called all-of-the-above energy strategy. (He has also written for Scientific American abut his Nobel Prize-winning work in physics.) "Then, as you go to helping deployment and how to finance deployment, there is an important role for government."

A great part of Chu’s legacy will be his founding of the Advanced Research Projects Agency–Energy, modeled on the longstanding Defense Advanced Research Projects Agency (DARPA). In just four years, ARPA–e has already notched some significant achievements, from better batteries for electric vehicles to better manufacturing technologies for the silicon wafers that comprise solar cells. "While it is too early to tell if we have home runs like ARPA-net [the precursor of today's Internet], there are a number of investments that have certainly rounded second base," Chu wrote, also noting that the ARPA-e spirit has percolated throughout the department more generally, via initiatives like SunShot—a bid to reduce the cost of solar power to $1 per watt.

It was such solar technology that proved most troublesome to Chu's tenure, as some companies funded under the stimulus subsequently failed, notably Solyndra and Abound Solar. And the bankruptcy of A123 Systems may presage a similar fate for some of the advanced battery makers similarly backed. Of course, those failures are a result of the success in making solar power and batteries cheaper and better globally, as well as a typical outcome for high-risk, high-reward efforts like those ARPA-e is focused on. "The test for America's policy makers will be whether they are willing to accept a few failures in exchange for many successes," Chu noted in his letter.

Ultimately, Chu may be best remembered as the scientist who helped spearhead efforts to cap BP's Macondo deep-sea oil well blowout in the Gulf of Mexico. The effort involved hundreds of national lab scientists and employees and successfully stopped the flow of oil. As Chu said in an email enlisting aid that summer, quoting Gregory Peck in Guns of Navarrone: ""Your bystanding days are over! You're in it now, up to your neck! They told me that you're a genius with explosives. Start proving it!" Of course, Chu quickly noted that explosives were not likely to be useful "on this mission… the rest rings true."

On climate change, Chu's record was more mixed: on the one hand supporting clean energy and on the other increasing domestic production of oil and locking in continued reliance on fossil fuels. At the same time, the U.S. saw its best hope for curbing emissions from such fossil fuel use—the CO2-capture-and-storage project at the Mountaineer Power Plant in W. Va.—shuttered. "We need to develop the technologies that enable us to use our fossil fuels in a clean way," Secretary of Energy Steven Chu told Scientific American in 2011. "This is something you don't solve in five years, 10 years. It will take a half century to get our carbon emissions down to where we need to go to protect the climate."

Perhaps Chu's real legacy will be that one of the many research projects he backed during his tenure will prove to be a future breakthrough that helps the U.S. continue to combat climate change. "Just as today's boom in shale gas production was made possible by Department of Energy research from 1978 to 1991, some of the most significant work may not be known for decades," Chu wrote. "What matters is that our country will reap the benefits of what we have started."