Mars is about 34 million miles from Earth — and that’s every 26 months, when the planets are at their closest distance. Their elliptical orbits, with Mars taking nearly twice as long to orbit the sun, mean that there are times when they are significantly closer to each other than others. Say an unmanned spacecraft headed for the Red Planet traveled in a direct path, did not need life support systems or extra fuel, and encountered no random flying objects, killer radiation, or other cosmic obstacles.
That obviously isn’t happening.
NASA has always launched spacecraft at the optimal time so they can take advantage of the Hohmann transfer orbit, which is the fastest way we know to send something to Mars without anyone on board. Rovers like Opportunity and Curiosity out of the atmosphere are typically loaded onto spacecraft and launched into an elliptical solar orbit that overlaps that of Mars. The spacecraft burns fuel to accelerate on that trajectory, using its velocity to burst out of Earth’s gravitational field and then slow down so the Martian orbit can capture it. It then rides the orbital wave until it finds a place to land in the red dust.
Just think that this 260-day trip (factoring in a manned spacecraft) is still the best shortcut we know of, but NASA and private companies are constantly trying to come up with alternatives to chemical rockets. Nuclear Thermal Propulsion (NTP) is fueled by low-enriched uranium and could possibly double the speed of the journey. It could also make missions to Mars possible more often than every two years, even when the planets have yet to align. Then there’s the dream of the future called DEEP IN.
DEEP IN, or Directed Propulsion for Interstellar Exploration, could possibly make a three-day trip to Mars more than science fiction. The idea is that a spacecraft without the extra burden of fuel and propellant will be powered by a propulsion system that never leaves home. Reflectors on the craft would be blasted with a stream of photons to accelerate it to near light speed. While scientists have been reaching those speeds with a particle accelerator, it probably won’t be sending anything bigger than a proton 34 million miles away anytime soon.
Until we can figure out how to zoom through space at relativistic speeds, it’s going to be a long and possibly hazardous ride.