Mars is the most frustrating of the planets. The fourth planet from the sun orbits tantalizingly close to the third planet, Earth. Mars isn’t that much farther from the sun than we are, and its climate shows remarkable similarities to Earth’s. For hundreds of years, telescopic observations suggested the possibility of life, perhaps even the intelligent variety. The proximity of the red planet is deceptive, however. Mars is only about 4,100 miles wide, about half the diameter of our planet. Mars is so small it that shows surface features only when Earth is between Mars and the sun, a condition that astronomers call a Mars opposition. As Earth laps Mars in their cosmic race, for a few weeks every 2 1/2 years or so, Mars is only 35 million to 50 million miles from us, and its polar caps and mysterious green markings become visible.