Space Travel and Risk

The recent crash of Virgin Galactic's SpaceShipTwo was sad news for anyone interested in space travel. Most reports focus on the possible cause of the crash. This is important, but it conceals another issue. And that is the matter of risk.

A good number of people -- me included -- are uncomfortable when flying. It's not surprising. To be stuck in a confined metal and plastic tube, several miles up in the air, packed in with strangers and with no visible means of support is not a naturally relaxing experience. But though many fliers think about the possibility of crashing, few consider the risk in a quantitative way. And that's a problem when we move to pleasure-flights in space.

As human beings, we are very poor at handling probability, which is what risk is all about. It doesn't matter how many times, for instance, we have the "birthday paradox" explained to us, it still seems strange that we only need to list 23 people to have a better than 50 percent chance that two have the same birthday. And this probability blindness is illustrated brilliantly by a probabilistic puzzle known as the Monty Hall problem.

The details of the problem aren't particularly important -- but when Marilyn vos Savant publicized it in Parade magazine in September 1990, her mailbox was flooded with complaints from academics. Comments included:

You blew it, and you blew it big! There is enough mathematical illiteracy in this country, and we don't need the world's highest IQ propagating more. Shame! -- S. S. PhD, University of Florida


You are utterly incorrect about the game show question, and I hope this controversy will call some public attention to the serious national crisis in mathematical education. If you can admit your error, you will have contributed toward the solution of a deplorable situation. How many irate mathematicians are needed to get you to change your mind? -- E. R. B. PhD, Georgetown University

Crowned, after a string of negative letters from Ph.D.s, by this entertaining summary:

You're wrong, but look at the positive side. If all those Ph.D.s were wrong, the country would be in very serious trouble. -- E. H. PhD, US Army Research Institute

The reason these letters illustrate our problem with probability is that vos Savant was right and all the academics were wrong. What should be a simple probability problem had them fooled.

When we consider risk, we are applying probability to our own safety, and all the evidence is that we still struggle. For a plane flight, this doesn't matter. The risk that you won't step off alive when taking a flight on a commercial airliner is reassuringly low -- around 1 in 10 million. It's a risk we have to be able to ignore, or we wouldn't do anything like crossing the road.

However, spaceflight has a very different risk profile. Richard Branson has rightly pointed out that commercial space ventures are still in their early days and this inevitably means that brave test pilots must take on higher levels of risk to ensure that the experience is safer for passengers. He makes a comparison with aviation, which was also high risk to begin with. And this is true. But it does not mean that by the time space tourism starts, safety levels will be up to those of current civil aviation. It took around 60 years to get from the Wright Brothers in 1903 to airliners that had modern levels of risk. While we might compress the timeframe, it is not going to happen for spaceflights in a year or two.

We have known for a long time that "real" space missions are relatively dangerous. Estimates for the risk astronauts face range between a chance of 1 in 10 and 1 in 100 of not surviving a mission. A realistic value might be around 1 in 30. The question that Virgin Galactic and other commercial space operations need to face up to is what level of risk is acceptable for their passengers.

We know that some commercial passengers -- those who have booked a trip to the International Space Station -- are prepared to accept the high risks that comes with a conventional mission. Presumably this has been discussed with them. But unless those offering sub-orbital trips are open about the risk of taking one of their flights, we aren't going to react in the right way to future problems.

I don't have the information needed to make an estimate of that risk, but we can assume it will be lower than that for a true space mission, and higher than that for flying in an aircraft. Even if we had the decades of experience that we have with civil aviation, spaceflights face three big challenges. Space is a uniquely harsh environment if something goes wrong, there is more to fail in a space vessel than in a plane, and rocket motors are usually more prone to issues than jets.

Let's say, for instance, that the risk was estimated at 1 in 1,000 flights failing. That's much better than traditional space missions. If a suborbital flight took off once a week, that's a failure once every 20 years. Is it a risk people would be willing to take for a pleasure trip? Quite possibly. Think of those who annually face significant dangers in the Himalayas, for instance. But the risk needs to be made explicit.

I don't know what the real numbers are. No one can know them with huge confidence -- there isn't enough data yet. But companies offering these flights should be able to make initial estimates. And those are essential for the public to know.

Brian Clegg ( is a popular science author. His most recent book is Final Frontier: the pioneering science and technology of exploring the universe (St Martin's Press).