Zenia Frantz, age 10, of Allentown, Pennsylvania for her question:

Flow long does it take light to reach us from Pluto?

Little Pluto is invisible to unaided human eyes but it can be seen and photographed with the help of a powerful telescope. Its dim light takes about four hours to reach the earth. Actually the time schedule varies from day to day and from year to year. This is because everything in the solar system is on the move and the distance between us and Pluto shrinks and stretches. Sometimes the travel schedule is eight minutes shorter and sometimes it is 16 minutes longer than usual.

In 1930, astronomers spotted a dim dot of light on a telescope photograph and announced the discovery of the ninth planet of the solar system. It was named Pluto and its average distance from the earth was figured to be 3,660 million miles. Its light, of course, whips across space at 186,000 miles per second. At this distance, its beams travel about four hours to make that dim dot on a telescope picture.

Later in the 1970s, we hope to launch a spacecraft called TOPS to relay back a closeup look at the little planet. In fact, NASA plans to launch several TOPS to explore the outer planets. This is possible because once every 177 years, these planets line up in a row. Giant Jupiter and mighty Saturn can be used to boost a spacecraft on a grand tour. A TOPS can thus reach Pluto in about nine years, instead of taking 41 years.

Naturally, no spacecraft can match the speed of light    but even light takes years and even centuries to cross the vast oceans of space. The time, of course, depends on the distance it travels. The sunbeam that just kissed your cheek took eight minutes to travel 93 million miles from the sun. Pluto is 30 times farther from the sun than we are, so every sunbeam takes 30 times longer to get there. Some of its light is reflected out into space and we catch a few stray beams on this return trip.

Our orbit is 186,000 million miles wide. When the earth and Pluto are on opposite sides of the sun, its light takes 16 minutes longer to reach us. Pluto takes 248 earth years to complete its huge, oval shaped orbit. When it is closest to the sun and the earth, the distance between us is 2,670 million miles. Then light from the sun takes about four hours to get to Pluto    and its pale reflected beams take about three hours and 52 minutes to bounce back to the earth. Pluto's farthest distance from the sun is 4,567 million miles. From this point it takes a beam of light 16 minutes longer to travel from Pluto to the earth.

This is easier to imagine if we scale down all the sizes. Let's shrink the sun to a two foot beach ball. The earth is a pea, 215 feet from our model sun. Pluto is a half sized pea. Its closest distance to the beach ball is one and a quarter miles. Its farthest distance is one and a half miles.