Australian Geographic - 09.2019 - 10.2019

I

N THE LAST EDITION OF AG,

this column looked at new

ideas about the origin of Earth’s

Moon. But science’s fascination

with planetary satellites doesn’t stop

there. Even the furthest planet from

the Sun is now giving up its secrets

to inquisitive astronomers.

Neptune boasts 14 known moons,

with a truly extraordinary range of

properties. The seven innermost are

the most normal, with orbits closely

aligned to Neptune’s rotation,

suggesting they formed with the

planet. Their diameters range from

10s to 100s of kilometres. The outer

seven are thought to be captured

asteroids and revolve in a much more

chaotic manner, with four orbiting

backwards – including Triton, which

is by far Neptune’s largest moon.

Making headlines now is the

most recently discovered Neptune

moon. Its orbit has been confirmed

by scientists at the SETI Institute in

California, which allows an official

name to be bestowed.

Convention demands that

Neptune’s moons are all named

after Greek and Roman water gods,

so the new one is called Hippocamp

after a mythological horse-like

sea-creature. At 35km in diameter,

it is the smallest of Neptune’s

moons, and all but invisible from

Earth. A specially planned sequence

of images from the Hubble Space

Telescope was needed to establish

its orbit.

Hippocamp tracks close to the

largest of Neptune’s ‘normal’ moons,

Proteus, and conventional wisdom

says that it should have been swept

out of its orbit by Proteus’s gravita-

tion. But a clue to Hippocamp’s

origin comes from a huge crater on

Proteus. This was probably caused

by a cataclysmic collision with a

comet or asteroid early in the Solar

System’s history.

The researchers believe

Hippocamp is a by-product of that

collision, having coalesced from the

resulting debris cloud.

Q: Does gravity travel faster than light?

If light can’t escape a black hole does

this mean gravity at the singularity is

pulling light back faster than the speed

of light? Is the accelerating expansion

of the Universe due to gravity having

less of an influence on spacetime?

John Brooks, Callala Bay, NSW

A: Great gravity questions! First: No. It’s

been proved gravity travels at exactly

the speed of light. Second: No. It means

the black hole (aka the singularity) has

an escape velocity greater than the

speed of light, so light never leaves it.

Third: Sort of. We think the reason we

see the Universe expanding faster today

than in the past is that the gravitational

pull of galaxies on each other is smaller

now, due to greater distances between

them. That allows ‘dark energy’ to

become the dominant driving force,

producing the accelerating expansion.

Hippocamp
ILLUSTRATION COURTESY NASA, ESA AND A.FEILD (STScl)

buzz

Scientists have discovered

14 moons orbiting Neptune,

eight of which are shown here,

since the first was seen by

19th century English amateur

astronomer, William Lassell.

FRED WATSON

is Australia’s

Astronomer-at-Large.

September. October 19

FR ED A NSW ERS

YO U R Q U E S T IONS

The moon that shouldn’t be there

SPACE