Will we ever be able to send spacecraft to the stars in a reasonable amount of time? The big problem—and it’s huge—is distance. The nearest star system to the Sun, Alpha Centauri, lies about 4.3 light-years (40 trillion kilometers) away, or roughly 10,000 times the distance of Pluto. At its present speed of about 17 kilometers per second (60,000 km/hr), Voyager 1, which is the fastest of the five spacecraft currently on exit trajectories from the Solar System, would take more than 75,000 years to reach Alpha Centauri (assuming it was heading in the right direction, which it isn’t).
The highest speed ever attained by any spacecraft from Earth, relative to the Sun, was 240,000 km/hr in the case of the Helios 2 Sun probe in 1976. But even at this rate the nearest star would be 19,000 years away, or the equivalent of more than 600 generations.
Clearly, a mission that takes many thousands of years to reach its goal is not acceptable. Not only would no one be interested after all that time, but such a slow spacecraft would inevitably be overtaken by much faster vehicles launched at later dates with more effective methods of propulsion, rendering it obsolete.
Having said that, small steps toward interstellar exploration have already been taken and more ambitious interstellar precursor missions have been proposed. These early flights will provide valuable scientific data about conditions at the edge of the Solar System and in the space between the stars that lies beyond.
The twin Voyagers, 1 and 2, launched in 1977, together with Pioneers 10 and 11, and New Horizons (at present on course for a rendezvous with Pluto in 2015), are all on trajectories that will ultimately take them into the interstellar void. Both Pioneers have fallen silent, their power reserves drained past the point at which they can transmit a detectable signal. Both Voyagers, on the other hand, are still returning information as part of what in 1989 became known as the Voyager Interstellar Mission. Having crossed the terminal shock, where the solar wind slams into the interstellar medium, the two Voyagers have entered the heliosheath, heading in different directions. The latest data from Voyager 1, which take 17 hours to travel to their home planet, suggest that the spacecraft has entered a region dubbed the “magnetic highway,” in which the magnetic field between the stars is making itself increasingly felt. In a matter of months, or at most a couple of years, mission scientists believe the probe will break through into interstellar space.
Proposals have also been put forward for faster spacecraft that would be launched specifically to explore the near interstellar medium. The first detailed analysis of an interstellar precursor mission was made at the Jet Propulsion Laboratory in 1977. It focused on a probe that could reach a distance of 370 astronomical units (55 billion kilometers) from the Sun in 20 years after launch, and 1,030 AU (155 billion kilometers) in 50 years after launch, using a nuclear-electric propulsion system. In 1990, the Interstellar Precursor Mission was revised as one of three NASA “frontier probes” concepts to explore the heliosphere and local interstellar space. Of these three, the “fields and particles” Interstellar Probe, powered by a solar sail, was endorsed for further study. However, neither this nor any subsequent proposed interstellar probe has so far gone beyond the drawing board.
This is the first part in a three-part series that details what it will take for humanity to truly become a space-faring species.