From Abracadabra to Zombies
UFOs (unidentified flying objects)
In a nutshell: UFOs are unidentified flying objects that some people think are spaceships from other planets.
A UFO is an unidentified flying object that many people think might be spaceships run by aliens.
Most scientists think that UFOs are most likely things like meteors, satellites burning up, the planet Venus, flocks of birds, aircraft, car lights, weather balloons, reflections, or tricks. One of the tricks is to make fake photos of flying objects. Another trick is to set loose balloons with candles in little crates beneath them.
The closest star to Earth (other than the Sun) is Alpha Centauri. It's about 4 light-years away. Traveling at the speed of light is easy for a photon, but our fastest spacecraft, Voyager, travels at about 40,000 miles an hour. It would take 70,000 years to get to Alpha Centauri on Voyager. If you had been born in Stone Age times and were able to live until you counted to 70,000 by months, you'd still be alive.
Even traveling at one million miles an hour, it would take more than 2,500 years to get there. (Imagine the ancient Greek philosopher and mathematician Pythagoras getting on a spaceship in 500 BCE. Some 50 generations later one of his descendents is born on the ship as it pulls into orbit around a star in Alpha Centauri in 2011.) To get there in twenty-five years would require traveling at more than 100 million miles an hour for the entire trip. If you could travel at 100 million miles an hour you could make it to the Sun and back in about an hour and 45 minutes. The spacecraft would have to be built of some mighty fine stuff to endure such speeds for such a length of time. And there'd be no repair shops on the way. But the main problem for travel between stars is the fuel, the energy needed to get there. Estimates range from one to 100 times the total energy output of our entire planet for a year would be needed to travel to a star 4 light-years away. We couldn't do it with nuclear fission because our spacecraft would have to carry thousands or millions of nuclear power plants on its wings! We could do it if we could produce energy the way the Sun does, by nuclear fusion. If we could collect and use all the energy the Sun produces in just one second, we'd have enough energy to last about 500,000 years! Unfortunately, we'd burn to a crisp if our spaceship produced energy the way the Sun does. Maybe someday we'll figure out a way to provide enough energy to travel to a planet outside of our solar system.
Despite the problems that would face anyone on any planet from traveling to other star systems or galaxies, it is not impossible. Perhaps there are beings who can travel at very fast speeds and have the technology and the raw materials to build vessels that can travel here from other solar systems.
Given the size and age of the visible universe, though, it seems likely that if any creatures were able to travel to distant solar systems some of them would have done it by now. If any have done it by now, the chances of them picking our star (the Sun) and our planet out of the zillions to pick from seems pretty slim.
It's kind of sad to think that there might be millions of planets with all kinds of interesting creatures on them that we'll never know about. And they'll never know about us. That won't stop us from using our imagination and making up stories about space travel to far off galaxies. We may not be able to explore other planets in other galaxies, but we can explore possibilities with our minds. In the meantime, there is still a lot to explore on planets in our own solar system like Mars. And don't forget, we still don't know everything about planet Earth!