NASA has some exciting news: a crewed landing on Mars is less than 20 years away. But NASA also has some less exciting news: a crewed landing on Mars has been less than 20 years away for the last fifty years.

That’s the problem when a government agency is in charge of your space program. You can go only as far as the people in Congress and the person in the Oval Office let you go—which hasn’t been very far since the last Apollo astronaut left the moon.

For that reason and more, you should pay attention to the April 27 announcement from Elon Musk, the founder and CEO of SpaceX, that he intends to launch his first unmanned Mars mission in just two years and will beat NASA’s goal of putting astronauts on the surface in the 2030s by up to a decade.

Musk’s plan, which he announced in—what else?—a Tweet from SpaceX, was straightforward:

Out of the gate, that was promising. A Dragon—specifically a Red Dragon—is just the kind of versatile ship you want for a Mars journey. The Dragon is the cargo vehicle that has made numerous uncrewed supply runs to the International Space Station, and will begin carrying astronauts as early as next year. Picture an Apollo spacecraft, but big enough to seat seven people instead of just three.

Red Dragon is an in-development variation that lands on legs, under the power of engines, rather in the ocean under a parachute. The engines, which have performed well in early tests, are liquid-fueled and throttleable, which means you can step on the gas or ease back as needed—the kind of flexibility required for a soft landing on Mars.

SpaceX has historically worked at a brisk clip and it’s not unrealistic to believe that the engines could be ready in time for a 2018 launch. That still leaves the landing legs to develop and test, but the company has already proven itself adept at that kind of technology, having twice used legs and foot pads to bring the first stage of its Falcon 9 rocket safely home after a launch.

The real challenge for SpaceX is less the spacecraft than the rocket that will be used to get it off the ground in the first place. The company uses a modular system for its boosters: The first Falcon rocket had a single engine. The Falcon 9, as its name suggests, uses nine of them, and is what the company has used for its space station missions. A deeper-space mission would require a bigger rocket—the so-called Falcon Heavy, which will use three clusters of the same nine rockets on its first stage.

That simple math means 27 engines—and 27 is an awful lot of ordnance to strap onto the bottom of a single booster. In one way it’s an improvement over the Apollo program’s Saturn V, which had just five far more powerful engines, since a flameout in even one of them would have been a mission-breaker. Lose one out of 27, however, and you can probably make it into space with barely a hitch in your step.

The risks of a 27-engine system, however, may exceed that one benefit. For starters, there’s the complexity; the greater the number of engines you’ve got, the greater the number of variables—and parts—that can go south on you. Worse is the problem of vibration. Identical engines firing with identical thrust can set up a sort of violent harmonic—with the whole of their matching frequencies being greater than the sum of their parts. In other words, the rocket could shake itself to pieces.

The solution is to introduce some dis-harmony into the system, to design some of the engines to sing sharp or flat or otherwise off-key. That’s bad in a choir but very, very good in a rocket. For now, no Falcon Heavy has made it onto the pad, never mind into space, and the rocket is behind its originally announced schedule. Musk promises to rectify that with a test launch this year, which means that, again, while a 2018 Mars mission is not remotely a sure thing, it’s not remotely crazy either.

Musk would make a number of uncrewed Mars landings—launching one every 26 months, to match the time Mars and Earth move into closest alignment—before attempting to send astronauts. NASA, which is continuing with its own crewed exploration plans on a slower track, has more than a little skin in the Red Dragon plan. In 2014, the space agency signed what is informally referred to as a “no exchange of funds” agreement with a number of companies, including SpaceX, in which the various partners swap assets for various projects. For a Red Dragon launch, that would mean NASA providing the launch pad and tracking and communication capabilities and SpaceX providing room on board for scientific and engineering payloads, and sharing all data about the approach and landing experience.

The bigger, sexier question is whether all of this can really lead to boots on Mars in as little as a decade. Musk himself admitted one of the challenges, in a Tweet that followed his Mars announcement, conceding that no matter how good the Red Dragon is, its habitable volume is only about the same as an SUV’s, making it fine for Earth orbital or lunar missions, but way too small for a Mars trip. For that, you’d also need an attached habitation module, similar to the school-bus sized segments that make up the space station.

And that’s not remotely all: you still need another habitat on the surface of Mars and a liftoff system to get you back off Mars and proper shielding to protect astronauts from deep space radiation en route—to say nothing of planning for the physical and mental toll a two and a half year round trip journey would take on the crew. Those too are reasons a mission to Mars has always been 20 years away. Musk, like every other space planner before him, must overcome them all.

Still, the betting here is: He just misses the 2018 unmanned deadline, hits it in 2020 and has a better than even-money chance of getting astronauts on Mars in the early 2030s, beating NASA but not by much. Those are absolutely reliable predictions—unless they’re not.

Write to Jeffrey Kluger at jeffrey.kluger@time.com.