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

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

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."

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. Science@NASA’s online satellite
tracking utility, JPass
written by Patrick Meyer
, can tell you when and where to

Russia’s fabled space station is easy to see, but don’t wait
–because the end of Mir … is near.

Editor’s Note: NASA public affairs officer Kirsten
Larson says she’s been receiving plenty of phone calls asking
about NASA’s role in bringing Mir down. In fact, NASA is just
a bystander. The Russian Aviation and Space Agency, Rosaviakosmos,
is in complete control of the reentry procedure. However, noted
Nicholas Johnson, the U.S. is providing tracking data to Rosaviakosmos
as a courtesy. American radar data added to those of the European
Space Agency and the Russians themselves, will help Rosaviakosmos
make precision adjustments to Mir’s orbit and land it squarely
on target.