laser-propeled spacecraft

Figure 1. In the future, spacecraft may be able to travel to other stars in reasonably short times. Beam-propelled laser sails such as the one shown here are one form such spacecraft may take.

Dyson sphere

Figure 2. A Dyson sphere is an artificial hollow ball built around a star by an advanced, intelligent race. This imaginary Dyson sphere around the Sun has been made by taking apart the planet Jupiter.

Andromeda Galaxy

Figure 3. The Andromeda Galaxy is a spiral galaxy similar to our own. The Drake equation predicts how many intelligent races there may be in our galaxy.

It will be many years before we can send a spacecraft to a nearby star to search for life around it. The closest star to the Sun, a red dwarf called Proxima Centaur, is about 25 trillion miles away. Even our fastest robot probes, at present, would take around 100,000 years to cover that enormous distance.


Scientists know of ways to make spacecraft go much faster. But it is unlikely that any high-speed starship will be launched for at least another century.


The next best method is to try to make contact with intelligent aliens by radio waves. Here on Earth, we regularly make se of telephones to call people rather tan visit them in person. It would make sense for any alien race that knew how to send and receive radio messages to do the same. Perhaps inhabited planets in our region of space are exchanging such messages at this very moment. If so, then all we need to do is set up the right equipment and tune in!


Listening to alien conversations, though, even assuming any are taking place, is not that simple. First, just as an ordinary radio has to be tuned to pick up a particular station, so astronomers have to choose the precise wavelength at which they are going to listen for an alien signal. The wavelength is the distance between the top of one wave at the next. Radio waves may be anywhere from less than a tenth of an inch to hundreds of yards long.


The second problem is deciding where to point the receiver. There are an enormous number of stars to choose from – billions and billions in our area of space alone. Unless a receiver is trained exactly on the place from which a radio message is coming, nothing will be heard.


Project Ozma


radio dish used in Project Ozma
The radio dish used in Project Ozma


In 1960, American astronomer Frank Drake began the first serious attempt to pick up radio signals from intelligent beings around another star. The project was named Ozma, after the queen of the land of Oz in Frank Baum's famous series of stories.


Drake knew there were billions of wavelengths, as well as billions of targets, from which to choose. But he was helped by a suggestion that had been made by two scientists, Giuseppe Cocconi and Philip Morrison, a year earlier.


Cocconi and Morrison pointed out that one wavelength, about eight inches, was very useful for studying the Universe. It is the one at which atoms of cold hydrogen – the most common substance in space – give off radio waves. According to these scientists, this wavelength would be known by all intelligent beings everywhere. As a result, it might be used as a standard for sending and receiving signals.


Acting upon this idea, Drake built a receiver tuned to the special hydrogen wavelength. Then he fitted the receiver to the 85-foot radio telescope – a large, bowl-shaped instrument for collecting radio waves from space – at the National Radio Astronomy Observatory in Green Bank, West Virginia. As his first target, he chose Tau Ceti, a nearby star that is similar to the Sun.


For eight hours, Drake heard nothing but crackles. Then, a signal came in at precisely eight pulses a second! Could it be a race of Tau Cetians trying to communicate? After checking his equipment thoroughly, Drake pointed the telescope at another star. The signal stopped. Then he directed the instrument back toward Tau Ceti. But the signals were gone, never to be heard again. Later, it turned out that what Drake had probably picked up were radio signals from a passing plane.


The Warm Glow of a Super Race

One of the problems our civilization is likely to face is a shortage of energy. One way to solve this, according to astronomer Freeman Dyson, would be to build a thin, round shell of material all the way around the Sun (see Figure 2). Thousands of years from now, human beings may be able to take apart a whole planet, such as Jupiter, in order to build such a "Dyson sphere." The inside of the shell would capture almost all the energy given off by the Sun, making it available for useful purposes. From the outside the Dyson sphere would give off a strong infrared glow, since it would be much warmer than the surrounding space. Other intelligent beings, more advanced than us, may already have built such enormous structures around their own stars. If so, suggested Dyson, we ought to be able to detect their infrared glows from Earth.


Drake's Equation

A year after he began Project Ozma, Frank Drake helped organize the first large meeting of scientists to discuss the search for extraterrestrial intelligence, or SETI. Extraterrestrial means "outside the Earth."


While making plans for the meeting, Drake wrote down a formula for determining the chances of finding advanced extraterrestrial life. The so-called Drake equation has become the most popular way of calculating the chances that intelligent life exists in space (see Figure 3).


Our Sun is one of about 200 billion stars making up a huge collection known as the Milky Way Galaxy. The Drake equation predicts how many races of intelligent beings, capable of communicating with each other, the Galaxy may contain. It states that this number depends on the following items:


1. the rate at which stars are being made in the Galaxy;
2. the fraction of stars with planets;
3. the fraction of planets capable of supporting life;
4. the fraction of planets where life actually arises;
5. the fraction of life that evolves into intelligent creatures
6. the fraction of intelligent creatures with the desire and ability to communicate; and
7. the average lifetime of a race of such creatures


The Drake equation identifies the key requirements for intelligent beings in space. To begin with, it requires the right star, the right planet, and the right kind of life. Put these estimates into the equation, multiply them together, and there is the answer – the number of communicating civilizations in the Galaxy.


Yet, some scientists have suggested that Jupiter may have life – or, at least, the beginnings of life. The mixture of gases in Jupiter's atmosphere, which includes hydrogen, methane, ammonia, and water vapor, is quite similar to that on our own planet billions of years ago. What is more, Jupiter's atmosphere is almost certainly warmer at great depths. Far below the cloud-tops, there may be a layer in which conditions are not greatly different from those that existed long ago on Earth. It is here, say some researchers, hat the chemicals needed for life may have been, and may still be, formed.


The multiplication is easy. Making the estimates is the hard part. And the answer is only as correct as the estimates that are used.


For example, how many stars in the Galaxy are capable of supporting life as we know it? Very old stars are probably not suitable, since they formed at a time when there was very little carbon in the Universe. Any planets of old star would not contain much carbon, either, so the chemicals of life that we think are necessary could not build up. Stars that circle around one another are probably out of the question, too. Any planets they had would follow unsteady paths, making conditions on them swing wildly from one extreme to another. In this way, we can rule out perhaps three-fourths of all the stars in the Galaxy.


We can also rule out certain types of planets. If they circle too close to their parent star they will probably be too hot to support life. On the other hand, if they are too far away, their surfaces will be too cold. According to some scientists, there is only a narrow zone around any given star in which a planet can move if it is to support life. In the case of the Sun, the Earth falls in the middle of this zone. All of the other planets of the Solar System probably move outside it.


Today, no one can give an exact answer to the question of life on other worlds. Different scientists put different estimates into the Drake equation. The answers that come out, as a result, range all the way from hundreds of millions of planets with intelligent life to just one – the Earth!