Terraforming scenarios revolve around increasing the temperature and atmospheric density of Mars, giving it breathable air, liquid water and temperatures conducive to human and plant life, and there are three main ideas for how to do this. In the first, giant space mirrors would orbit the planet, reflecting sunlight back at Mars and heating it. They would weigh on the order of 200,000 pounds, too large to create and launch from Earth. The second is building greenhouse gas factories on the surface of Mars, using them to warm the Martian climate in a similar sense to how current fossil fuels are causing warming on Earth. The third is perhaps the most extraordinary: capturing large, nitrogen-rich asteroids, attaching engines to them, and flying them to Mars until they collide with its surface. It is estimated that four such impacts would be sufficient to create Earth-like conditions on Mars.