With NASA committed to faster, cheaper, and smaller space missions, a leading researcher at the agency's Jet Propulsion Laboratory yesterday described a proposed low-budget mission to one of the solar system's most enigmatic planets.

The destination is Mercury -- closest planet to the sun, no larger than Earth's moon and yet possessed of a full-scale magnetic field and a density as least as great as that of Earth and Venus.

Long intriguing to astronomers, Mercury was visited only once before when a mariner spacecraft flew past it 21 years ago to discover its puzzling magnetic field and photograph a portion of its cliff-studded, crater-pocked surface.

The planet is only about 37 million miles from the sun, compared with 93 million miles for the Earth. Its sunlit temperature reaches a searing 700 degrees at the equator, while in the dark of night at the poles, temperature drops to 235 degrees below zero.

At the American Geophysical Union's fall meeting in San Francisco yesterday, Robert Nelson, chief scientist of the proposed mission to Mercury, told planetary exploration colleagues how his team, in partnership with space engineers at the TRW Corp., could build the craft and fly it for four years -- at a cost of less than $140 million.

The mission, he said, would take three years to speed past both Venus and Mercury twice and then take another year to fly in orbit around the north and south poles of the tiny inner planet.

The team has dubbed its spacecraft Hermes, after the mythical winged messenger from Olympus.

The 3,200-pound bird would carry a camera to map and photograph the planet's entire surface, Nelson said. It would also focus other acutely sensitive instruments to measure Mercury's magnetic field, analyze its surface rocks and minerals, calculate the heights of its mountains and its crater depths and study the impact of the intense solar wind that has blasted it for billions of years.

The Hermes mission would carry the spacecraft as close as 125 miles from Mercury's poles and nearly 1,000 miles out from the planet's equator.

Planning for Hermes in under way, with a launch date aboard a Delta rocket scheduled for July 1999. Universities and space centers have proposed to NASA 28 small-scale missions -- some aiming at planets, some at the moon, some at comets and some at asteroids.