How soon could humanity launch a mission to the stars? That’s the question considered today by Marc Millis, former head of NASA’s Breakthrough Propulsion Physics Project and founder of the Tau Zero Foundation which supports the science of interstellar travel.

This is a question of increasing importance given the rate at which astronomers are finding new planets around other stars. Many believe that it’s only a matter of time before we find an Earth analogue. And when we do find a place with the potential to host life like ours, there is likely to be significant debate about the possibility of a visit.

The big problem, of course, is distance. In the past, scientists have studied various factors that limit our ability to traverse the required lightyears. One is the speed necessary to travel that far, another is the cost of such a trip.

By looking at the rate at which our top speed and financial clout are increasing, and then extrapolating into the future, it’s possible to predict when such missions might be possible. The depressing answer in every study so far is that interstellar travel is centuries away.

Today, Millis takes a different approach. He looks at the energy budget of interstellar missions. By looking at the rate at which humanity is increasing the energy it has available and extrapolating into the future, Millis is able to estimate when we will have enough to get to the stars.

To make his extrapolation, Millis looked at the amount of energy the US has used to launch the shuttle over the last thirty years or so, as a fraction of the total energy available to the country. He assumes that a similar fraction will be available for interstellar flight in future. He then calculates how much energy two different types of mission will consume.

The first mission is a human colony of 500 people on a one-way journey into the void. He assumes that such a mission requires 50 tones per human occupant and that each person will use about 1000W, equal to the average amount used by people in the US in 2007.

From this, he estimates that the ship would need some 10^18 Joules for rocket propulsion. That compares to a shuttle launch energy of about 10^13 Joules

The second mission is an unmanned probe designed to reach Alpha Centauri, just over 4 light years away, in 71 years. Such a ship would be some three orders of magnitude less massive than a colony ship so it’s easy to imagine that it would require less energy.

But Millis places another constraint on this mission. Not only must it accelerate towards its destination, it must decelerate when it gets there (although why this isn’t a requirement for a colony ship isn’t clear).

That changes the the numbers significantly. Millis estimates that the probe would require some 10^19 Joules.

The final step in is to determine when humanity will have this kind of energy available for these kinds of missions. By extrapolation, Millis calculates that the required energy will not be available until at least the year 2196. “This study found that the first interstellar mission does not appear possible for another 2 centuries centuries,” he says.

That’s necessarily a crude calculation but a sobering one nonetheless. It implies that while we will soon be able to gaze with wonder upon other Earths, it will not be possible to visit them within the lifetime of anybody alive today.

In other words, for the foreseeable future, we’re trapped.

Ref: arxiv.org/abs/1101.1066: Energy, Incessant Obsolescence And The First Interstellar Missions