Test-firing the world's largest space rocket, the 322-foot-tall Space Launch System in December 2014, NASA took its first step on the long journey to Mars. But there's just one problem: According to critics, NASA may never actually get there.

The trouble with high hopes
At last report, NASA was planning to attempt a manned Mars mission sometime in the 2030s -- about 20 years hence. The cost of this venture is uncertain, but in past columns, we've relied upon the widely cited estimate that it could cost $30 billion -- and perhaps as much as $100 billion -- to develop the technology, build the spaceship, and conduct a successful Mars mission. As it turns out, these numbers could be wildly optimistic.

At a recent House space subcommittee hearing, Congressional lawmakers and witnesses who were called to give testimony on the program's progress cast cold water on NASA's plans. Former NASA Goddard Space Flight Center director Tom Young lamented the lack of "a plan, strategy, or architecture with sufficient detail that takes us from today to humans on the surface of Mars."

John Sommerer, who chaired a National Research Council technical panel that advised NASA in the past, testified that, at the rate NASA is going, it could take as long as 40 years to put a man on Mars, and cost as much as $500 billion. Here's the problem: Congressional budget plans currently envision spending about $180 billion to fund NASA "exploration" activities over the next 20 years.

That budget covers everything from development work on the Mars program to an upcoming SLS mission around the Moon to just keeping the International Space Station up and running. But if a Mars mission will cost $500 billion, then NASA's budget wouldn't cover that cost -- even if it directs every penny it gets toward the Mars project for the next 20 (or 40) years.

This raises the very real prospect that the best we can look forward to is 180 billion taxpayer dollars being cast into a black hole -- and no astronaut on Mars at the end of it. Given this very real possibility, Congress could decide to cut bait on the Mars idea entirely. They could decide, if NASA can't afford to go to Mars, maybe we shouldn't even try.

The Red Planet -- so near, and yet so far (out of our ability to pay for a human journey there). Image source: NASA.

Don't join 'em, beat 'em
Could it be that NASA doesn't need to go to Mars? Over at SpaceX, Elon Musk's privately run space-exploration company, engineers are hard at work developing a spacecraft that could do the Mars mission cheaper, and faster, than anything NASA currently has in the works. Musk describes his craft, dubbed the "Mars Colonial Transport," as capable of sending 100 tons of supplies and/or 100 live human passengers. That's a far cry from the Lockheed Martin (NYSE:LMT)-built Orion Multi-Purpose Crew Vehicle atop the Boeing (NYSE:BA) -built SLS, which together can seat at most four astronauts.

In contrast to NASA plans calling for an SLS trip to Mars in the 2030s, Musk thinks SpaceX will be ready to make the trip within a decade. Best of all, SpaceX has already developed a rocket capable of landing on the planet, and potentially taking back off again for a return trip to Earth.

Are these realistic goals? Doubters abound. But then, people also doubted Musk's ability to turn Tesla into a successful business. (Tesla is working on its fourth new car model, by the way.) They doubted his ability to launch a rocket into space, and then land it back on Earth -- but Musk ticked that box last year, while Boeing and Lockheed were still talking about catching rockets with helicopters. If anyone's able to take a 40-year, $500 billion project and start lopping off zeros until it becomes both affordable and near-term, it's Elon Musk.

Why this matters to investors
All that said, though, even Musk's money is not infinite. And even if SpaceX succeeds in cutting the cost of a $500 billion mission down to size, it's still going to cost a lot of money -- perhaps more than privately funded SpaceX can afford on its own. If Elon Musk truly wants to make a go of a private expedition to Mars, chances are good he'll first need to IPO SpaceX to do it.

Sometime in the next decade, we may finally get our chance to own a piece of SpaceX.

SpaceX wants to occupy Mars. Investors want to occupy SpaceX stock. Image source: SpaceX.

This article represents the opinion of the writer, who may disagree with the “official” recommendation position of a Motley Fool premium advisory service. We’re motley! Questioning an investing thesis -- even one of our own -- helps us all think critically about investing and make decisions that help us become smarter, happier, and richer.