The Economist UK - 10.08.2019

64 The EconomistAugust 10th 2019

1

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very day a tonne or two of defunct sat-

ellites, rocket parts and other man-

made orbiting junk hurtles into the atmo-

sphere. Four-fifths of it burns up to become

harmless dust, but that still leaves a fair

number of fragments large enough to be le-

thal. It is testament to how much of Earth’s

surface is sea, and how sparsely populated

the remainder remains, that the only re-

corded victims of this artificial hailstorm

are five sailors aboard a Japanese vessel,

who were injured in 1969, and a woman in

Oklahoma who was grazed by a piece of

falling rocket in 1997. But it is also testa-

ment to luck—and the odds of that luck

holding are shortening.

Population growth means that the frac-

tion of Earth’s surface which space debris

can hit harmlessly is shrinking. At the

same time, more spacecraft are going up (111

successful launches in 2018, compared

with 66 a decade earlier, and with many

launches carrying multiple payloads). And

payloads themselves are increasingly de-

signed so that equipment which has ful-

filled its purpose falls out of orbit years or

decades sooner than it otherwise would,

lest it collide with functioning spacecraft.

In light of all this, more attention is be-

ing paid to the safe disposal of satellites

and other space junk. To do that, space

agencies and private companies alike want

to steer craft to the least risky impact-desti-

nations possible, and also reduce the num-

ber of fragments that will survive re-entry

and endanger people and property.

A drop in the ocean

One tried and tested solution is to plunge a

re-entering craft into a zone known as the

South Pacific Ocean Uninhabited Area.

This is the expanse between Chile and New

Zealand. It is island-free, little sailed and

little overflown. Such controlled re-entries

are not a completely precise science. Any

ships and planes heading into the vicinity

at the time will be advised to steer clear of a

potential impact area that may exceed

10,000km^2 —roughly the size of Lebanon.

But if everyone takes these warnings seri-

ously, then controlled re-entries are as safe

as it gets, according to Holger Krag, head of

the European Space Agency’s Space Debris

Office in Darmstadt, Germany.

Job done, you might think. Yet only a

few controlled re-entries are carried out

each year. The reason is cost. If a spacecraft

is to be put into the steep descent needed to

aim it reasonably precisely at a particular

spot on Earth’s surface, it will need to carry

two or three times as much fuel as is re-

quired for standard orbital adjustments. It

will also require larger thrusters. That fuel

and those thrusters add to a mission’s

weight, and therefore its launch costs.

Ground controllers are also necessary to

supervise the re-entry. Ending a mission

with a controlled re-entry can thus add

more than €20m ($22m) to its cost.

A cheaper alternative is a “semi-con-

trolled” re-entry. Instead of diving towards

a pre-arranged target, a satellite is lowered

gradually into the atmosphere using either

what thruster-fuel remains to it or a spe-

cially designed drag-sail. This sail inter-

cepts air molecules that have leaked into

space from the atmosphere, slowing down

the satellite it is attached to and thus de-

creasing the craft’s altitude until it reaches

a point where air resistance to the body it-

self pulls it into the atmosphere.

The trade-off is that the danger zone as-

Space debris and human safety

Stopping a hard rain

Technologists are working out ways to lessen the likelihood that debris falling

from space will kill people

Science & technology

65 The IPCC land-use report

66 Elephants, ants, trees and fire

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