Every 26 months, Mars and Earth are at their closest distance, which is about 57 million kilometers apart. During this time, if an unmanned NASA spacecraft were to leave Earth, traveling at 58,000 kilometers per hour, it would take a little over 40 days to get to the Red Planet. Not bad, right? Well, a quick trip would only be possible if the spacecraft followed a direct path, undisturbed by the wrath of the solar system and weight of life sustaining resources. Unfortunately, space travel is far more complicated than a straight line.
The solar system is constantly shifting, and the orbital mechanics behind each of the planets makes space travel really complex. Because Earth and Mars have elliptical orbits, the distance between them varies considerably, and in order to send a spacecraft to Mars, the planets have to line up just right. It takes Earth one year to orbit the Sun, while it takes Mars about 1.9 years. So, every time Mars completes a single orbit, Earth goes around the sun almost twice.
Historically, for NASA, the best launch window to Mars occurs every 26 months. During this time, a spacecraft can follow what is considered the most energy efficient path to Mars known as the Hohmann transfer orbit. After launch, the spacecraft follows an elliptical orbit around the Sun that intersects the orbit of Mars. It uses fuel to increase speed and velocity to break free of Earth's gravity field. And the spacecraft uses more fuel to decelerate in order to be captured into the Martian orbit. Then, it can ride Mars’s orbital wave until it’s ready to touch down on the surface.
While the Hohmann transfer is considered the most efficient trajectory to get to Mars, the journey is still estimated to take roughly 260 days or eight to nine months with a manned spacecraft powered by chemical propulsion, so NASA and private companies are trying to develop more efficient propulsion systems that can get humans to Mars faster than the chemical rockets used in the past. Specifically, a type of electric propulsion which propels spacecrafts by accelerating a stream of electrically charged atoms, known as ions. And Nuclear Thermal Propulsion which uses low-enriched uranium as its power source.
But even if we do engineer a faster way to get to Mars, a round trip is still estimated to take multiple months. And that means we also have to solve problems of human health and resource requirements during extended deep space missions. Clearly, time is just one of the many obstacles standing in the way of our Martian dreams. But still, NASA says it plans to send humans to Mars in the 2030’s.