In 1993, J. Richard Gott III computed with scientific certainty that humanity would survive at least 5,100 more years. At the time, I took that as reason to relax, but Dr. Gott has now convinced me I was wrong. He has issued a wake-up call: To ensure our long-term survival, we need to get a colony up and running on Mars within 46 years.

If you’re not awakened yet, I understand. It’s only prudent to be skeptical of people who make scientific forecasts about the end of humanity. Dr. Gott, a professor of astrophysics at Princeton, got plenty of grief after he made his original prediction in 1993. But in the ensuing 14 years, his prophetic credentials have strengthened, and not merely because humanity is still around.

Dr. Gott has used his technique to successfully forecast the longevity of Broadway plays, newspapers, dogs and, most recently, the tenure in office of hundreds of political leaders around the world. He bases predictions on just one bit of data, how long something has lasted already; and on one assumption, that there is nothing special about the particular moment that you’re observing this phenomenon. This assumption is called the Copernican Principle, after the astronomer who assumed he wasn’t seeing the universe from a special spot in the center.

Suppose you want to forecast the political longevity of the leader of a foreign country, and you know nothing about her country except that she has just finished her 39th week in power. What are the odds that she’ll leave office in her 40th week? According to the Copernican Principle, there’s nothing special about this week, so there’s only a 1-in-40 chance, or 2.5 percent, that she’s now in the final week of her tenure.