Much of the research into advanced propulsion is focused on finding fuels with a higher specific impulse—the amount of thrust provided per unit mass of fuel—to take us faster and farther than conventional rockets.

In 1947 that search led mathematician Stanislaw Ulam to propose using nuclear bombs to propel rockets. Designs for the ship varied, but they all had one thing in common: a metal plate at the base of the craft that could deflect the energy of the bombs as they detonated. The U.S. government believed that a bomb-based design, called Orion (not to be confused with the recently scrapped NASA craft designed to go to the moon), could bring a crew to Mars, but the Nuclear Test–Ban Treaty of 1963 put an end to the project.

Still, the idea of a nuclear-powered spacecraft lives on. In the early 1990s, researchers at Lawrence Livermore National Laboratory developed a concept craft called the Vehicle for Interplanetary Space Applications (VISTA). In it, pellets of deuterium and tritium (isotopes of hydrogen) would explode to create a cloud of plasma that would interact with a magnetic field to propel the ship. In another scheme, nuclear reactors could provide the necessary energy. One such craft, designed by Franklin Chang Díaz, a former NASA astronaut and the founder of rocket company Ad Astra, the Variable Specific Impulse Magnetoplasma Rocket, or Vasimr, could use the electricity generated by nuclear reactors to heat gas to a plasma. The plasma jets out of the back of the rocket with some help from powerful magnets, providing the rocket with thrust.

Vasimr has been tested in vacuum chambers on Earth, and in 2014 it will be tested from the International Space Station in the harsh conditions of space. To help fund development of the craft to go to Mars and beyond, Vasimr could deploy and reposition satellites and clean up space debris on a contract basis to private companies and space agencies.

HOW PROJECT ORION WOULD HAVE WORKED

1. A series of nuclear bombs, each with the power equivalent to a few kilotons of TNT, is ejected though a hole in the center of the spaceship. Several bombs detonate per second.

2. A large metal plate and shock absorbers deflect the enormous energy and convert it into thrust.