New Scientist - USA (2019-06-22)

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LOSE to the sun lie a pair of sizzling
coals. You could be forgiven for
thinking these strange worlds were
two circles of hell: Mercury, a black and
blasted plain, and Venus, a sweltering world
beset by rain of pure acid.
But for all the terror of their outward
appearance, their insides are remarkably
familiar. Along with Earth and Mars, they
form the solar system’s only rocky planets, a
stark contrast to the bloated gas giants that
make up its outermost worlds. In the language
of those hunting for extraterrestrial life, such
planets count as potentially habitable. In fact,
if  we found a Mercury or a Venus in a solar
system far from our own, we might even
call them Earth-like.
So what caused these seemingly Earth-like
planets to become so resolutely, well, not? Was
it some accident of composition, or millions of
years spent in an inhospitable environment?
Getting to the bottom of these questions, and
understanding the diversity of rocky planets,
is of paramount importance for astronomy.
Without clear answers, we aren’t only ignorant >

straight for the planet doesn’t guarantee you

will arrive. “The second you point your rocket

toward Mercury, you are going far too fast to

get caught in Mercury’s gravity,” says Paul

Byrne at North Carolina State University. The

sun has so much gravity that a direct mission

to Mercury will always miss.

We didn’t know how to get into orbit around

Mercury until the mid-1980s, when Chen-Wan

Yen at NASA’s Jet Propulsion Laboratory in

California figured it out. A spacecraft has to

take the scenic route, with loops around Earth

and Venus and multiple swoops past Mercury

to slow it down before it can enter orbit.

The first probe to visit was Mariner 10, which

flew past, rather than orbiting, just three times

in 1974 and 1975. The second was Messenger,

which orbited for four years from 2011. “We

had Mariner 10 go by, and it was craters and all

a little bit boring,” says David Rothery at the

Open University, UK. “We would still wonder

of our own solar system’s history, but blinkered

in our search for life elsewhere.

Part of the problem is that the innermost

planets are incredibly hard to visit. Only two

spacecraft have ever made it to Mercury and

of the numerous Venus landers, none survived

longer than 90 minutes. Painting a more

detailed picture will involve a new generation

of probes, such as the European Space Agency

and Japan Aerospace Exploration Agency’s

BepiColombo, currently on its way to Mercury.

While the chances of finding any signs of life

are close to zero, the data it sends back will

help us look for it elsewhere.

To find out anything about Mercury, though,

we first have to get there.

The biggest problem when it comes to

planning a trip to Mercury is the sun. When

your next-door neighbour is a star 6 million

times heavier than you with a gravitational

field 80 times stronger, visitors have a

tendency to get redirected.

If a spacecraft flies by Mercury too quickly,

it will be trapped by the sun’s powerful gravity

and dragged to its doom. But even heading

22 June 2019 | New Scientist | 43

Mercury (left) and Venus (right)

remain largely mysterious

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