Once collisional cascading begins, the number of objects in a particular orbit
will gradually increase—and the risk to satellites and manned spacecraft will
rise accordingly. A team of researchers in Italy, collaborating with Alessandro
Rossi, a research fellow at the National Research Council of Italy, has
calculated that enough objects are already present in two popular orbits, about
600 miles and 1,000 miles overhead, for cascading to begin. By the time the
cascades have run their course, in a hundred years or so, even small spacecraft
will suffer damaging collisions after just a few years in orbit. "This is only
a projection," Rossi says, but if we keep putting objects into orbit as we have
been, "operations will not be possible anymore."
For many years NASA and the Department of Defense were skeptical about the
dangers of space debris. The problem seemed abstract, residing more in computer
models than in hard experience. And it challenged the can-do mentality of space
enthusiasts. Earth's orbit seemed too large and empty to pollute.
To its credit, NASA has long maintained a debris-research program, staffed by
top-notch scientists who have persisted in pointing out the long-term hazards
of space junk even when the higher-ups at NASA haven't wanted to hear about it.
Then came the Challenger accident, in 1986. NASA officials realized that
their emphasis on human space flight could backfire. If people died in space,
public support for the shuttle program could unravel.
Engineers took a new look at the shuttle and the International Space Station.
Designed in the 1970s, when debris was not considered a factor, the shuttle was
determined to be clearly vulnerable. After almost every mission windows on the
shuttle are so badly pitted by microscopic debris that they need to be
replaced. Soon NASA was flying the shuttle upside down and backward, so that
its rockets, rather than the more sensitive crew compartments, would absorb the
worst impacts. And engineers were adding shielding to the space station's most
vulnerable areas. At this point the modules should be able to survive impacts
with objects measuring up to half an inch across, and NASA is developing repair
kits for plugging larger holes in the walls.
But adding shielding and repair kits won't solve the real problem. The real
problem is that whenever something is put into an orbit, the risk of collision
for all objects in that orbit goes up. Therefore, the only truly effective
measure is a process known as deorbiting—removing objects from orbit when they
reach the end of their useful lives. With current technology deorbiting
requires that a satellite or a rocket reserve enough fuel for one last trip
after its operations are finished. With enough fuel a spacecraft can promptly
immolate itself in the atmosphere or fly far away from the most crowded orbits.
If less fuel is available, it can aim for an orbit where atmospheric drag will
eventually pull it to Earth.