If the telescope you got for Halley's comet is gathering dust, now's the time to try it out again. For the next week or so, Mars will be closer to the Earth than it has been in 15 years and a prime target for observation.

Astronomers say anyone with a decent small telescope should be able to make out Mars' south polar ice cap and see light and dark variations on the rest of the planet's red surface.

Mars, now about 37.5 million miles away, should be the third brightest object in the night sky after the moon and Venus. It can be sighted low in the southern sky near the constellation Sagittarius.

The distance from Earth to Mars varies greatly as each planet circles the sun at its own rate. Sometimes the two are on opposite sides of the sun. Roughly every 26 months, however, Earth laps Mars, albeit on an inside track.

Because of eccentricities in the orbits, however, the event usually happens with the planets farther apart than they are now. In 1978, for example, Mars and Earth were 61 million miles apart.

This time, Mars happens to be in the part of its orbit closest to the sun and, therefore, closest to Earth's path.