The president weighs in

Following a series of contentious congressional hearings, President Obama traveled to Cape Canaveral in April 2010 in an attempt to placate critics. First, he visited SpaceX's launch pad and met with CEO Elon Musk. Then, he took the stage at Kennedy Space Center's Operations and Checkout building, where a familiar, gumdrop-shaped capsule loomed over his left shoulder.

It was the Orion crew vehicle—axed with Constellation just two months earlier. Obama said the spacecraft would "be part of the technological foundation" for future deep space exploration missions, and also see double-duty as an ISS lifeboat.

Next, Obama gave his $3 billion, heavy lift rocket engine design effort a deadline: 2015. After that, NASA would start to build the rocket itself.

Thirdly, he gave NASA the deep space destinations it is still striving to reach today, one-upping his presidential predecessor with locations harder to reach than the moon.

"By 2025, we expect new spacecraft designed for long journeys to allow us to begin the first-ever crewed missions beyond the moon into deep space," he said. "We'll start by sending astronauts to an asteroid for the first time in history. By the mid-2030s, I believe we can send humans to orbit Mars and return them safely to Earth. And a landing on Mars will follow. And I expect to be around to see it."

Orion and a heavy lift rocket were back in play. Together, they would be used to send humans to an asteroid and Mars.

Your move, Congress

Like most legislative processes, crafting space policy is like playing a game of chess. There are moves and countermoves.

Because the White House proposes the federal budget, it always gets the opening move. Congress counters by passing bills that authorize programs and allocate funding. The President gets a final say with the ability to either sign or veto those bills.

After Obama's April policy speech, Congress began work on what would eventually become the NASA Authorization Act of 2010. As the name implies, the Act authorizes the agency's programs—which are proposed by the White House—while also setting spending limits.

The bill went along with many of Obama's proposals, including outsourcing ISS transportation to private companies.

There was, however, one major change: NASA would not spend five years researching heavy lift engine technology. Instead, it would begin building the rocket right away, using its current workforce and existing shuttle and Constellation technologies. When feasible, the agency was also directed to use or modify existing Constellation contracts, and both the rocket and Orion should be ready for test flights by the end of 2016.

The rocket was given a name: the Space Launch System, a nod to the Space Transportation System, the formal name of the space shuttle.

The Authorization Act is notorious for not only having created SLS, but for dictating design elements right down to payload capabilities: at least 70 tons for the initial version, and 130 after future upgrades.

The constraints left little wiggle room for a design resembling anything other than Ares V, which had been canceled with the Constellation program. Former NASA deputy administrator Lori Garver, who has openly criticized SLS since leaving the agency in 2013, says this was no coincidence.

"It ended up that the prime industry contractors got together with a few senior NASA and Capitol Hill people to modify the Ares V architecture just enough that they could keep all their existing contracts and change the name," she told me.

Behind the bill

Garver is hardly alone in her charges. SLS critics decry it as a pork-barrel project designed by legislators and the aerospace industry to keep government money and jobs flowing through traditional space districts.

But the alternative was not politically feasible, according to Jeff Bingham, one of two former senate staffers largely credited with drafting the authorization bill. Without a replacement program, the loss of both Constellation and the space shuttle represented a potentially devastating blow to some of NASA's southern centers, which impact their local economies by billions of dollars.

"You've got the pressure of the current workforce, and horror stories about peoples' life savings being sucked up because they no longer have a job, and no prospects of getting one because you're basically doing away with an industry," Bingham said.

An Ares V-style rocket, then, would solve certain political challenges while still providing the heavy lift vehicle NASA had sought for decades. But why does the law impose such detailed requirements for the vehicle, such as payload capabilities?

"The pushback was there, such that if we weren't very specific, [SLS opponents] would find a way to go in a different direction," he said.

I asked Garry Lyles, the SLS chief engineer, what he thought about those 70 and 130-ton payload numbers that Congress wrote into the law. He said it likely goes back to studies NASA had been conducting since the days of Magnum—and possibly earlier.

"We had been playing around with concepts for years that ranged anywhere from a 120 to 150-ton capability," Lyles said. "It wasn't surprising that something like 130 tons showed up."