Re: At what altitude do space re-entry vehicles begin to burn in the atmosphere

Greetings:

There are many thousands of pieces of space junk that will eventually reenter
the earth’s atmosphere. There is not a distinct altitude were space debris begins to
burn. To study this growing problem the Center for Orbital and Reentry Debris Studies
(CORDS) at The Aerospace Corporation, located in El Segundo, California. CORDS was
established in 1997 to focus on this problem. You will find a great deal of current
information and FAQs on the CORDS web site at:

http://milcom.aero.org/cord .

The following discussion on this conplex problem is from the CORDS web site:

OUOTE
Spacecraft Reentry Breakup Overview

Many satellites do not remain in their orbits indefinitely, but gradually return to Earth.
This is because Earth's atmosphere does not end abruptly, but becomes progressively
thinner at higher altitudes. In fact, there is still some atmosphere several hundred
kilometers up, where some satellites orbit Earth. Because the atmosphere is so thin at
those high altitudes, satellites can take a long time to come down.

For satellites in low Earth orbit (hundreds of kilometers in altitude), it may take years or
tens of years to return to Earth. Higher altitude satellites are of less concern because they
can stay in orbit much longer—hundreds or even thousands of years.

As a satellite loses altitude it enters denser regions of the atmosphere, where friction
between the satellite and atmosphere generates a great deal of heat. This is due to the high
velocity of orbiting satellites, which can be more than 29,000 km/hr. The tremendous
amount of heat generated can melt or vaporize the entire satellite or portions of the
satellite. A similar effect occurs during a meteor shower, where streaks of light (meteors
or "shooting stars") are generated by bits of natural materials (meteoroids) as they burn up
in the atmosphere.

Although many people believe that satellites burn up during atmospheric reentry,
some satellite components can and do survive the reentry heating (of course, satellites like
the space shuttle orbiter survive reentry entirely because they are protected by specially
designed heat shields). Component survival on an unprotected satellite can occur if the
component's melting temperature is sufficiently high or if its shape enables it to lose heat
fast enough to keep the temperature below the melting point.

During reentry, the object is decelerating quickly and the loads on the structure can
exceed 10 Gs (10 times the acceleration of gravity). These loads combine with the high
temperature to cause the structure to break apart.
When the satellite components lose enough speed, the heating rate is reduced, the
temperature decreases, and the objects begin to cool. By this time, the objects have fallen
to even denser regions of the atmosphere and fall virtually straight down from the sky.
They impact the ground at relatively low speeds, but still represent a hazard to people and
property on the ground.

It is very difficult to predict where debris from a randomly reentering satellite will hit
Earth, primarily because drag on the object is directly proportional to atmospheric density,
and atmospheric density varies greatly at high altitudes. In general, we can predict the time
that reentry will begin to within 10 percent of the actual time. Unfortunately, reentering
objects travel so fast that a minute of error in the time is equivalent to many miles on the
ground.

If a satellite or rocket body has propulsive capability, it can use rocket motor burns
to target the reentry into a desired area, such as the ocean. This technique was recently
used by NASA to ensure that debris from the 14,000-kg Compton Gamma Ray
Observatory impacted in the ocean.

Reportedly, only one person has been struck by debris from a reentering satellite in
the history of our use of space—about 40 years. Fortunately, this person was hit by a
lightweight object and was not injured.

The risk that an individual will be hit and injured is estimated to be less than one in
one trillion. To put this into context, the risk that an individual in the U.S. will be struck
by lightning is about one in 1.4 million.

Reentry risk estimates are supported by the fact that, over the last 40 years, more
than 1,400 metric tons of materials are believed to have survived reentry with no reported
casualties (of course, it is possible that casualties have occurred somewhere in the world,
but have not been reported). The largest object to reenter was NASA's Skylab, which
weighed 70,000 kg.
END QUOTE

Best regards, Your Mad Scientist
Adrian Popa