Space station Mir, the heaviest thing orbiting our planet other than the Moon itself, will return to Earth around March 20th. When the space station Mir returns to Earth over the remote South Pacific later this month, it will be big news. And rightly so. The 135-ton Russian outpost is the heaviest thing orbiting our planet other than the Moon itself. During its 15-year stint in space, Mir has set endurance and space-adventure records that are going to be hard to beat. But among scientists who monitor the near-Earth environment, an encounter with a 135 ton object from space is, well.... all in a day's work. "Asteroids weighing as much as Mir hit Earth perhaps 10 times each year," says Bill Cooke, a member of the Space Environments team at the Marshall Space Flight Center's Engineering Directorate. "We know this because we observe the flashes of the explosions in the upper atmosphere via Department of Defense satellites." Just last year a 200-ton asteroid startled Canadians with a sonic boom and a brilliant fireball as it disintegrated above the Yukon territory. Scientists later recovered a smattering of meteorites from nearby Lake Tagish, none larger than a few hundred grams. "If a [rocky] asteroid with the same mass as Mir hit the ground it would explode like a few kilotons of TNT, gouging out a crater about the size of a football field," noted Cooke. However, Mir will never make it that close to the ground. As Cooke explained, "the atmosphere is very good protection and it breaks up meteorites and other space objects well before impact." Indeed, if Mir were an asteroid, it wouldn't merit classification as a potentially hazardous one. In the cosmic scheme of things, Mir is simply too small. Nevertheless, scientists expect the space station to put on a good show when it returns. Mir is put together much like an erector set. It's a beautiful but gangly-looking assortment of solar arrays, laboratories and living quarters -- obviously not designed for aerodynamic flight through the atmosphere. The station will quickly fall apart as it descends toward Earth. "We expect Mir to break into six or more main pieces when it hits the atmosphere," says Nicholas Johnson, chief scientist and program manager for orbital debris studies at NASA's Johnson Space Center. Each piece will resemble a blazing comet that spits smaller fireballs as the pieces crumble and burn. Cosmonauts assembled Mir piece-by-piece during a busy ten year period beginning in 1986. The station's modules include the voluminous Core, Mir's original 20-ton segment that harbors the crew's living quarters; plus Spektr, a 19-ton science laboratory famous for its 1997 collision with a Progress spacecraft; and the 19-ton Priroda Earth observatory, launched only five years ago. Five of Mir's modules are still pressurized with air inside for humans. When they explode, sky watchers (mainly sea birds) could witness a once-in-a-lifetime display as the incandescent fragments streak across the sky. "Of Mir's 135 tons, the Russians say about 20 tons might reach the surface -- mostly in small pieces," noted Johnson. Even now Mir is sinking 1.5 km each day because of atmospheric drag. Left to itself, the station would naturally plunge to Earth from its 250 km orbit no later than March 28th. But that would be an uncontrolled descent. Instead, Mir will be guided to its final resting place by a Progress spacecraft attached to the station. On March 20th Russian ground controllers plan to fire the Progress's engines when Mir is at apogee -- its greatest distance from Earth. "The engine firing will move perigee [Mir's closest approach to our planet] to a point inside the atmosphere over the south Pacific," explained Johnson. "That's where the station's fragments will land." "With a controlled deorbit it doesn't matter if 20 tons or the whole 135 tons reaches the surface -- the risk to people or property should be essentially zero," says Johnson. Mir's descent is certainly safer than the many uncontrolled encounters we experience with Mir-weight asteroids each year. No one knows more about dumping spacecraft in the remote Pacific than the Russians. Since 1978 they've deorbited 80 Progress spacecraft and five Salyut space stations in the same area. "Two Progress spacecraft have gone down there already this year," says Johnson. "Mir, which is attached to a Progress, will be the third." "The most recent space station to descend over the Pacific was Salyut 6," he added. "That weighed 40 tons and came down in July of 1982. The deorbiting technique is exactly the same -- Mir's just a bit bigger." Mir's dazzling finale won't be seen by many people, but perhaps that's just as well. The twenty tons of Mir-bits that scatter across the Pacific will be traveling 100 to150 mph when they reach the water -- a bit too fast for comfort! Fortunately, there's still time to see Mir from the safety of your own back yard. The rapidly-moving space station reflects sunlight and if you're outside at the right moment -- usually near local dusk or dawn -- Mir will appear as bright as a streaking first or second magnitude star.