The format Drake suggested in 1962 is similar to proposals from several decades earlier - proposals that had never gained wide circulation and that had been forgotten in the meantime. Like his predecessors, Drake used mathematics. But instead of showing geometrical concepts, he focused on ways of communicating numbers in interstellar messages.

In addition, Drake drew on the likelihood that ET would also know a lot about the physical universe. In fact, many SETI scientists today still believe that if extraterrestrials have an advanced radio technology, then they would also know something about math, chemistry, and physics. Thus, these topics might provide a foundation for an interstellar "common language."

To help decode the message, Drake tried to make the format as obvious as possible. For starters, he made sure the number of characters in the message would provide a clue for displaying it as a two-dimensional picture. When he distributed this message to colleagues, to see if they could figure it out, he gave them a page of 551 0s and 1s.

The key to putting this string of numbers into the proper format is to recognize that 551 is the same as 19 multiplied by 29. And, in fact, those are the only two numbers by which 551 can be divided (except of course for 1 and 551, which don't provide much help). When this sequence of 0s and 1s is converted into black and white squares, and arranged in rows 19 squares long, stacked one on top of another for a total of 29 rows, you get the picture shown here:

But what does it mean?

If SETI scientists do detect a message from ET some day, they will be faced with the same sort of challenge you face right now. Of course, its possible that even if there is a message embedded in artificial signals from space, the message will be hard to extractat least until we develop more sensitive detection systems. (For more details, see astronomer Seth Shostaks article.)

But suppose we do find a pattern in the radio or optical signals. How will we be able to understand ET when we dont share a common natural language, like English, Spanish, or Swahili? As a hint to decoding this message, Drake relied on two kinds of information: numbers and pictures.

The next article in this series will contain the answer key. In the meantime, try your hand at breaking the code from this fictional extraterrestrial.