Today, SpaceX simultaneously fired up all 27 engines on its new massive Falcon Heavy rocket — a crucial final test for the vehicle before its first flight in the coming weeks. An hour after the test, SpaceX CEO Elon Musk confirmed that the test was good, and that the Falcon Heavy will launch in “a week or so.” When SpaceX gives an official target day and time, it’ll be the first time a definitive launch date has been given for the rocket’s inaugural voyage, a flight that was initially promised to happen as early as 2013.

Today’s test, known as a static fire, is meant to assess the performance of a rocket’s engines prior to launch. It involves restraining a rocket on a launchpad while igniting its engines to simulate the initial stage of launch — but without the rocket taking off. The rocket was tested at SpaceX’s launch site LC-39A at NASA’s Kennedy Space Center in Cape Canaveral, Florida, where the Falcon Heavy is slated to launch. SpaceX typically does a static fire prior to every launch, but today’s is particularly special: it’s the first time the company has fired up so many engines at once.

It finally means a launch is imminent after years of waiting

It means a launch is finally imminent after years of waiting. First announced in 2011, the Falcon Heavy has always been just beyond the horizon for SpaceX. A couple of rocket failures in 2015 and 2016 forced the company to push the flight. And over the last couple of years, the company has consistently updated the rocket’s target launch date, from late 2016 to early 2017 to late 2017. But now, the Falcon Heavy has at last been assembled and it’s working. This time, the launch date is a concrete plan.

It’s been a struggle to get to this point. SpaceX has been trying to conduct a static fire since the beginning of January, but the test was delayed multiple times over the last two and a half weeks. SpaceX at one point had to stand down while another rocket from the United Launch Alliance took off from the Cape, and the short-lived government shutdown also slowed the process. Most of the other delays occurred without explanation, though testing an entirely new rocket is often slow.

First static fire test of Falcon Heavy complete—one step closer to first test flight! pic.twitter.com/EZF4JOT8e4 — SpaceX (@SpaceX) January 24, 2018

The Falcon Heavy is the heavy-lift version of SpaceX’s premiere Falcon 9 rocket. It consists of three Falcon 9 cores strapped together, which provide a combined 5 million pounds of thrust at liftoff, according to the company. This power will allow the Falcon Heavy to carry around 140,000 pounds (63,800 kilograms) of cargo into lower Earth orbit, nearly three times as much as the single-stick Falcon 9.

And just like the Falcon 9, the Falcon Heavy is partially reusable. All three cores are meant to return to Earth and land upright in order to be used for future flights. For this upcoming launch, the two outer cores will break away from the center and return to ground-based landing pads at the Cape. The center core will then break away from the vehicle’s upper stage and return to land on one of the company’s drone ships out in the Atlantic Ocean. In fact, the two outer cores being used for the Falcon Heavy’s first flight have done this landing routine before: one flew in May 2016 and the other flew for a space station resupply mission in August 2016.

just like the Falcon 9, the Falcon Heavy is partially reusable

SpaceX CEO Elon Musk has said that the Falcon Heavy turned out to be harder to engineer than the company originally thought. “At first it sounds real easy: you just stick two first stages on as strap-on boosters. But then everything changes,” Musk said at a space conference in July. “All the loads change; aerodynamics totally change. You’ve tripled the vibration and acoustics.”

Because of this, Musk has said it’s possible the Falcon Heavy won’t achieve its desired orbit. “I hope it makes it far enough beyond the pad so that it does not cause pad damage,” Musk said at the conference. “I would consider even that a win, to be honest.”

That’s why this first flight isn’t launching any science payloads. Instead, the vehicle is lofting Musk’s personal Tesla roadster. If all goes well, the Falcon Heavy will send it into an orbit around the Sun. The trip will take the car out to the same distance as Mars’ orbit (though it won’t get very close to the Red Planet at all). Last year, Musk said the company would launch “the silliest thing we can imagine” on the first flight of the Falcon Heavy, and this seems to fit the bill — especially since David Bowie’s Space Oddity will supposedly be playing on the roadster’s stereo during launch.

The car is merely a stunt, but the flight is truly significant. The outcome will determine what lies ahead for the Falcon Heavy: if it fails or does not achieve orbit, then it will mean some amount of reworking on the part of SpaceX, and the timeline of the rocket’s next launches will be uncertain.

But if it succeeds, then the Falcon Heavy will become a commercially available rocket, ready for customers. The vehicle already has a few missions lined up: the next Falcon Heavy launch is meant to carry a communications satellite to orbit for Saudi Arabia, and a third flight will carry a bundle of research satellites, including the Planetary Society’s experimental LightSail spacecraft — meant to test out a sail that can propel vehicles through space using sunlight. That latter mission will also help certify the Falcon Heavy for use in future launches for the US Air Force.

The Falcon Heavy could open up even more possibilities for ambitious commercial missions to deep space. SpaceX claims the rocket will be taking two tourists around the Moon at some point in the years ahead, and the vehicle could potentially send cargo to the surfaces of the Moon and Mars. SpaceX originally wanted to use the Falcon Heavy to send the company’s Dragon cargo capsule to the surface of the Red Planet. Though those plans have been canceled, it still means the Falcon Heavy will be more than capable of ferrying large cargo, such as robotic landers, to distant locations in space.

Still, there aren’t that many customers for the mega-rocket yet, and only a few deep-space missions have been proposed for the vehicle. But that all could change once the Falcon heavy is seen in action.

Update January 24th, 1:44PM ET: This article was updated to include confirmation of a good static fire from Elon Musk.