Adolf Hitler came close to conquering Europe. Julius Caesar conquered the Mediterranean. Genghis Khan’s empire covered most of the known world in 1200 AD.

All these are as nothing compared to humanity’s next imperial ambition: outer space.

NASA has already defined the colonization of space as the ultimate goal of space exploration. Science fiction offers countless accounts of aliens from other worlds, but for humans the dream of interplanetary proliferation remains far in the distance.

For one thing, we have significant technological and resource limitations.

The amount of fuel required to send people and the necessary supplies, even to somewhere as nearby as Mars, is staggering. More than 99 percent of the total weight of whatever we want to send to Mars would be composed of liquid fuel.

We are also limited by time. Aside from the moon, which is dead, and Venus which would quickly make dead anything that dared touch its surface, Mars is the closest planetary body in the Solar System.

But according to NASA’s Web site, the highest speed currently attainable with liquid fuel is eight miles per second per rocket stage (most rockets have two stages). In August 2003, Mars was as close to Earth as it had been in 60,000 years — 35 million miles away. Even taking that unusually close measurement, a one-way trip would take more than nine months.

That number, bad as it may look, isn’t so much different from travel time between England and the colonies during the 1600s. And the resource limitations don’t guarantee failure, either; in the kind of ideal world we’d need to be in to move humans to other worlds, they’d all be free anyway. The real problem is making the world hospitable to humans once they arrive.

The process of terraforming involves making an otherwise hostile world hospitable to humans; it might involve making the soil fit to grow plants, the atmosphere fit to breathe or transforming a desert into an oasis.

On small scales, we do it on Earth already. But the kinds of things we would have to do to make something like Mars hospitable are far beyond current human ability. There is a certain amount of conceit in the human psyche, and a backward mentality, to believe we can make Mars hospitable while actively ruining Earth without figuring out any real way to fix it.

One thing we’d have to do is construct a suitable Martian atmosphere. While scientists believe Mars once had an atmosphere similar to Earth’s, the Martian atmosphere of today is about 95 percent carbon dioxide — toxic, at those levels in particular, to humans.

However, plants on Earth convert carbon dioxide into oxygen for human use through photosynthesis. But Martian soil is hardly conducive to growing lots of green plants, and there is no rainfall or liquid water (at least at the surface). The atmosphere of Mars contains about .03 percent water vapor, against the roughly 3 percent at low-level Earth atmosphere.

One way of bringing about the necessary changes is importing the necessary ingredients, either on ships from Earth or by altering the paths of comets which contain the necessary elements so they collide with Mars. By doing this, we might bring more organic material to improve the soil and more water to feed the plants.

But even at this, there are still other issues with Mars. The planet is much colder than Earth, averaging around perhaps -60 degrees Fahrenheit. Humans would have to import greenhouse gases to the Martian atmosphere in order to raise its temperature, but one of the reasons Mars has such a thin atmosphere to begin with is its low gravity.

There is nothing in our current understanding which would allow us to adjust for this, but we don’t really need to — an artificial Martian atmosphere would linger around for around a million years. However, differences in gravity could lead to other issues. Movement from Earth to Mars, which only has about a third of Earth’s gravity, could lead to complications in human performance (they may feel severely weighed down coming from Mars to Earth) or, worse, health.

The thin atmosphere brings in a whole host of problems even aside from temperature. Mars is not very well-shielded from cosmic rays, ultraviolet radiation or solar flares, all potential hazards to life as we know it.

Humans might be able to escape all these complications by establishing artificial bases. These enclosed structures could accommodate all human needs. By rotating them, we could create an artificial gravity comparable to Earth’s. We could also create whatever atmosphere we wanted in them. They might rest on the planet’s surface or float in orbit above it, perhaps connected to the surface by what’s known as a space elevator — essentially just a long cable attaching the space station to the planet, held in place by immense tensile strength.

Technical feats being as staggering as they are, space colonization isn’t totally outside the realm of possibility. What might be more debatable is whether the colonization of outer space falls within the realm of moral permissibility.

As mentioned earlier, humans have already pretty much ruined planet Earth. This is one of the chief reasons for our desire to move out into space. NASA cites “the increasing consumption of Earth’s resources” as necessitating a move into outer space.

But given the miserable job we’ve done being stewards of the planet we were given, what gives us any right to do what we’ve done here to other worlds — particularly those such as Mars, which may already harbor life of some kind? It might just be that our shameful, reckless disregard for non-human (and in many cases, human) life would be frowned upon in the galactic community.

The moral dilemma, however, can wait. Human beings will never be capable of large-scale interplanetary travel without overcoming significant moral and technical obstacles to begin with.

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