If humans are going to get to Mars, they're going to need rockets with some serious liftoff power. NASA’s Space Launch System is the most powerful rocket in the world—it has twin five-segment solid rocket boosters, four liquid propellant engines, and a minimum of 70 metric tons of lifting power—but engineers won’t know until June 28th if it’s really going to work.

At 8:05 am MDT on Tuesday, SLS will undergo a qualification ground test at Orbital ATK’s facilities in Utah that will see if its systems are up to snuff. And scientists are setting a high bar. The qualification testing has over 80 objectives—basically, everything but launching skyward—to determine whether SLS is ready to send the Orion spacecraft on the first leg of Exploration Mission-1, an unmanned mission planned for 2018. EM-1 will take Orion 40,000 miles beyond the moon, which is further than any spacecraft built for humans has ever gone. But EM-1, and the manned missions planned for the 2020s, can't happen unless SLS can get them off the ground.

Granted, everyone's pretty sure that it will be able to. This test is the fifth of five. And it's the second of two qualification tests, which are more about how the boosters will perform than if. Tuesday's burn is mostly a test of the engines to see how much power they provide when the propellant is at a low temperature of about 40 degrees Fahrenheit (so that's Florida cold, not space cold). The last qualification test back in March tested hot motor performance, with propellant at 90 degrees.

NASA

Which is not to say that designing, building, and proving SLS—which draws on the retired shuttle program's launch systems—has been a cinch. "The rocket looks a lot like the one in the museum, but that's about all that's the same: the outside," says Fred Brasfield, Orbital ATK's SLS Boosters lead. Not only did engineers have to determine whether the space shuttle tech was still up to the task, they also overhauled the avionics system (Brasfield calls the '70s version "basically analog"), updated the nozzle, and pretty much everything going on inside.

Take just the insulation: NASA and Orbital ATK had to make it safer by getting rid of voids that might expand under high temperatures. They also had to make it more environmentally friendly (and 10,000 pounds lighter) by eliminating asbestos. But the rubber asbestos replacement had a tendency to evolve gases—and you really don't want any unexpected additions to your rocket fuel.

Computing power and environmental impact are only a small part of the story. SLS has to be capable of so much more than its shuttle program predecessors. "It's almost a totally different mession," says Alex Priskos, NASA SLS Boosters Manager. "One is designed to go the store, and one is designed to go halfway across the country." The massively increased velocity and power—both boosters produce 3.6 million pounds of thrust each—means massively increased loads. SLS has to be very precisely controlled if it's going to avoid ripping itself to pieces.

That's why NASA has been taking a more tortoise than hare approach with these deep space missions. "This test is part of a deliberate buildup approach," says Mike Sarafin, EM-1 mission manager. "We're going to go past the GPS system constellation, beyond the Tracking and Data Relay Satellite, beyond the Earth's magnetic field. Until we demo, there's going to be a lot of uncertainty." If this test goes well, it should remove another whisper of that uncertainty, and bring Mars that much closer.