Asteroids are remnants of the Solar System’s youth. When the planets were forming more than 4.5 billion years ago, gas and dust molecules clung together to form larger objects, which in turn collided and stuck together to make yet bigger things. At the end of the process, we were left with the big planets, moons, and a huge number of smaller bodies which contain the raw chemicals we see on Earth.

Some asteroids could contain significant amounts of rare metals such as platinum, rare-earth elements, and other materials. Even water is a valuable resource in space, since it is useful as fuel (broken into hydrogen and oxygen components) and necessary for astronauts, but very heavy and therefore expensive to carry into space.

Now, President Barack Obama signed a bill into law granting private companies ownership over materials they extract from asteroids or the Moon. The bill also extends the period of time private corporations can develop spacecraft without direct government oversight, to help speed the process of getting more rockets into space.

But don’t pull up stakes for the asteroid-mining gold rush just yet.

The United States law can’t grant private ownership of asteroids themselves, or of any planetary body such as the Moon—that’s forbidden by the 1967 Outer Space Treaty. In fact, that treaty may nullify the new asteroid mining law, though scholars seem to disagree about that.

The Outer Space Treaty does explicitly allow private companies to build rockets and fly them into space, though, as long as they don’t claim to own land on other worlds: no high-rise developments on the Moon. However, it’s less clear whether mining is allowed, and who owns anything produced by mining. It may take a lawsuit or some other challenge to settle that question, and other countries may dispute the United States’ right to allow companies to exploit resources that the spirit of international law says belong to all humanity.

Leaving the legal issues aside, it’s not clear how quickly or easily asteroid mining could begin. The difficulties are partly technical, but also relate to the nature of asteroids themselves.

Rocketry is still difficult, even to reach the relatively low orbits used by many satellites and the International Space Station. The most successful companies still have rockets blow up, and the best plans involve redundancy: having a second rocket ready to go if the first fails for any reason. That adds to the cost of any flight, without even taking destination into account.

Private companies such as SpaceX and Blue Origin have made great progress at getting rockets into low-Earth orbit, and Orbital Sciences (now Orbital ATK) successfully sent the LADEE probe to the Moon. Getting to an asteroid is a significantly harder problem, since asteroids are literally moving targets, orbiting the Sun on their own trajectories.

On the plus side, would-be asteroid miners don’t need to travel all the way to the Main Belt, the region of the Solar System between Mars and Jupiter where most asteroids live. The better bet is to send spacecraft to near-Earth asteroids, or NEAs, which orbit the Sun closer than Mars. Some NEAs even cross Earth’s orbit, and a few are fairly large. Astronomers track these to make sure none pose a threat to Earth, but they are also potential targets for mining—especially those that are closer than Mars.

Asteroids are also potentially easier to mine than the Moon or other worlds, simply because they have less gravity. The biggest cost of rockets is weight: the more mass you have to shift, the heavier the rocket is, both in terms of payload and fuel. The effort to lift material from an asteroid is far less than the effort required to launch something from Earth or the Moon: The gravity on many asteroids is weak enough you could literally jump off their surfaces and never come down.

But asteroids could be inherently difficult to mine. Most NEAs are “rubble piles” rather than solid rock: they are aggregates of small particles loosely held together by gravity. The water, platinum, and other valuable chemicals are mixed in with materials like iron, silicon, and carbon compounds. Right now, it’s hard to tell how easy it will be to extract the stuff we want, since we aren’t likely to find rock seams or other deposits like we have on Earth.

That question at least can be answered by scientific space probes. Both the OSIRIS-Rex and Hayabusa 2 missions are designed to study asteroid structure in greater detail than ever. OSIRIS-REx will even bring a sample of asteroid material back to Earth, if all goes well. Since that’s a mining project in miniature, it’s a sort of first feasibility study on what it would take to extract any usable minerals.

Overall, asteroid mining will be profitable if all the money isn’t used up just in getting to the asteroid and bringing valuables back to Earth. That already precludes using human miners: Astronauts are far too expensive, thanks to our pesky need for food, water, air, and room to move around on a spaceship. Even without people involved, asteroid mining involves getting to the asteroid, getting material out of it, returning it to Earth, and processing enough of it to turn a profit. It doesn’t matter how much platinum or rare-earth minerals an asteroid has if the per-ounce price is higher than that of similar materials we can mine here.

The asteroid gold rush is still a few years in the future, in other words. That’s not to say it won’t happen: Humans are ingenious, and cleverness could be enough to overcome the technical challenges to asteroid mining. What it does mean, though, is that the new United States mining law may end up being more of a test of a treaty than a real change in the way we get valuable metals.