The Economist UK - 10.08.2019

The EconomistAugust 10th 2019 Science & technology 65

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sociated with such a de-orbiting is much

larger than that of a properly controlled re-

entry. It is still possible to arrange for this

zone to have lots of oceans and few big cit-

ies. But there is not the certainty of no casu-

alties that the South Pacific Ocean Un-

inhabited Area brings with it. Also, though

more economical than the fully controlled

variety, semi-controlled re-entry is not

free. Saving fuel for it shortens mission

lengths. Adding a drag sail adds to launch

weight. In practice, therefore, almost all

spacecraft re-enter the atmosphere at ran-

dom. But this has not prevented experts

from working out the probability that the

random re-entry of a given mission will

cause casualties. And that is useful infor-

mation, because it can be used to decide

whether a mission should go ahead in the

first place.

Re-entry-survivability analysis, as it is

known, is done using software that

crunches data on the size, shape, configu-

ration, composition and thickness of a sat-

ellite’s components. That provides an esti-

mate of the number, weight, size and

shape—and therefore potential harmful-

ness—of pieces that atmospheric friction

will not reduce to dust. The probability of

casualties can then be calculated in light of

the population density under the space-

craft’s orbit.

Hyperschall Technologie Göttingen

(htg), a German firm, charges about

€50,000 for such an analysis. Its clients in-

clude three European satellite manufactur-

ers—ohbSystem of Germany, Elecnor of

Spain and Airbus—as well as several space

agencies. For their money, these organisa-

tions get a bespoke assessment of the likely

fate of a particular spacecraft, based on dig-

ital files of its design, and using programs

with names like “Spacecraft Entry Survival

Analysis Module” and “Debris Risk Assess-

ment and Mitigation Analysis” that have

been calibrated by experiments in the plas-

ma wind tunnels owned by Germany’s

space agency.

If these calculations come back show-

ing that the risk of a satellite killing or in-

juring someone during re-entry is greater

than one in 10,000—which roughly half

do—then permission to launch will proba-

bly be denied unless the craft is redesigned

or can be rigged for a semi-controlled entry

at more favourable odds. The idea of setting

the acceptable risk at 10,000 to one, though

derided by some as arbitrary, was adopted

by America’s space agency, nasa, in 1995,

by Japan in 1997, by France in 1998 and by a

dozen or so other places in the years since.

Feeling the heat

Having to do such calculations at all,

though, is suboptimal. The best solution to

the problem of re-entering space debris is

to build spacecraft so that nothing will

reach the ground in the first place. One way

to “design for demise”, says Ettore Perozzi,

an expert on debris at Italy’s space agency,

is to build a spacecraft “like a chocolate

bar”, so that it snaps easily into pieces. The

idea is for specially positioned weak parts

to fail early during re-entry, ripping the

thing apart at an altitude of about 125km,

rather than the standard 80km or so. This

exposes the spacecraft’s guts to greater de-

structive heat for additional seconds.

One promising means of getting a

spacecraft to rip open early, according to

Charlotte Bewick, head engineer for debris

at ohbSystem, is to forge screws, nuts and

other parts for couplings out of special

“shape memory” alloys. When heated,

these alloys return to a “remembered”

shape they once held—which, in this case,

will facilitate a rapid wiggling apart early in

re-entry. Thales Alenia Space, a Franco-

Italian firm, sees more promise in another

way of accelerating a spacecraft’s break-up.

It has patented a “demisable” coupling

that, thanks to a special washer, comes

apart quickly when heated. Engineers are

testing prototypes in a plasma wind tunnel

and reckon the winning design will con-

tain a low-melting-point alloy of zinc.

Another way to reduce what reaches the

ground is to substitute refractory materials

such as titanium and steel, used to make

things like fuel tanks and fly wheels, with

substances such as aluminium and graph-

ite epoxy that vaporise more easily. Accord-

ing to Lilith Grassi, a debris expert at Thales

Alenia, this approach is bearing fruit.

Even these measures, though, will not

bring every spacecraft into compliance

with the one-in-10,000 rule. So engineers

have thought up additional ways to lower

the likelihood of a casualty. Those at ohb

System, for example, have proposed fas-

tening together with strong cabling any

components expected to survive re-entry.

That will prevent them from fanning out—

meaning, as Dr Bewick puts it, that the sur-

viving debris will hit Earth like a single bul-

let instead of a shotgun blast, thus reduc-

ing the chance that anyone will be struck.

ohbSystem has yet to find a customer

for a satellite fitted with such containment

cabling. It would add weight, and thus cost.

Moreover, some dislike the notion of in-

creasing the amount of material that will

strike Earth, even if that increase reduces

the chance of a death. But a related ap-

proach is under study at Thales Alenia.

This firm may begin encasing in a single

package the lenses and other components

of optical systems that currently often hit

the ground as a spray.

Something no one seems to be asking in

all this, is what an appropriate level of safe-

ty for satellite re-entries actually is. The

original reason for picking 10,000 to one as

an acceptable risk level has been lost in the

mists of time. To a given individual in

Earth’s human population of 7.5bn, it

translates into one chance in 75 trillion per

re-entry. This is vanishingly small, even in

a world where re-entries are numbered in

the hundreds per year.

On the other hand, any death delivered

from outer space in this way would be

headline news, and might result in calls for

the rules to be tightened still further. So far,

the satellite business has a pretty good

safety record. It would like to keep things

that way. 7

A

fter 29 hoursof uninterrupted nego-

tiations the latest report from the In-

tergovernmental Panel on Climate Change

(ipcc), on how alterations in land use are

contributing to such change, was gavelled

through in Geneva on the afternoon of Au-

gust 7th. When, minutes later, your corre-

spondent asked to speak with some of the

researchers, she was informed they had

“gone to bed”. The report these exhausted

delegates produced—all 1,300 pages of it—

fires another warning shot about the state

of the planet and the way people are trans-

forming virtually every corner of every

continent. Human activities affect roughly

three-quarters of Earth’s ice-free land, with

huge consequences for the climate.

Land masses are natural carbon sinks,

absorbing greenhouse gases by a variety of

processes, including photosynthesis. They

also produce such gases—for instance,

when vegetation decomposes or burns. By

Gloom, but not complete doom, from

the climate-change front line

The IPCC land-use report

Il faut cultiver

notre jardin