BY ELEANOR K.
SANSOM :
NASA smacked a
spacecraft into an
asteroid – and learned
details about its
12-million-year history.
The DART mission not
only tested our ability
to protect ourselves
from future asteroid
impacts, but also
enlightened us on the
formation and
evolution of rubble
pile and binary
asteroids near Earth.
NASA’s DART mission – Double
A s t e r o i d
Redirection Test –
was humanity’s
first real-world planetary
defence mission. In September
2022, the DART spacecraft
smashed into the companion
“Moon” of a small asteroid 11
million kilometres from Earth.
One goal was to find out if we
can give such things a shove if
one were headed our way.
By gathering lots of data on
approach and after the impact,
we would also get a better idea
of what we’d be in for if such
an asteroid were to hit Earth.
Five new studies published
in Nature Communications
have used the images sent back
from DART and its travel buddy LICIACube to unravel the
origins of the DidymosDimorphos dual asteroid system. They’ve also put that data
in context for other asteroids
out there.
Asteroids are natural hazards: Our Solar System is full
of small asteroids – debris that
never made it into planets.
Those that come close to Earth’s
orbit around the Sun are called
Near Earth Objects (NEOs).
These pose the biggest risk to
us, but are also the most accessible. Planetary defence from
these natural hazards really
depends on knowing their
composition – not just what
they’re made of, but how they’re
put together. Are they
solid objects that will punch
through our atmosphere if given the chance, or are they more
like rubble piles, barely held
together?
The Didymos asteroid, and
its tiny Moon Dimorphos, are
what’s known as a binary asteroid system. They were the perfect target for the DART mission, because the effects of the
impact could be easily measured in changes to Dimorphos’
orbit. They are also close(ish)
to Earth, or are at least NEOs.
And they’re a very common
type of asteroid we haven’t had
a good look at before.
The
chance to also learn how binary asteroids form was the icing
on the cake.
Quite a few binary asteroid
systems have been discovered,
but planetary scientists don’t
exactly know how they form.
In one of the new studies, a team
led by Olivier Barnouin from
Johns Hopkins University in
the United States used images
from DART and LICIACube to
estimate the age of the system
by looking at surface roughness and crater records.
They found Didymos is
roughly 12.5 million years old,
while its Moon Dimorphos
formed less than 3,00,000 years
ago. That may still sound like
a lot, but it’s much younger
than was expected.
A pile of boulders:
Dimorphos is also not a solid
rock as we’d typically imagine.
It is a rubble pile of boulders
that are barely held together.
Along with its young age, it
shows there can be multiple
“generations” of these rubble
pile asteroids in the wake of
larger asteroid collisions.
Sunlight actually causes
small bodies like asteroids to
spin. As Didymos started to
spin like a top, its shape became
squashed and bulged in the
middle. This was enough to
cause large pieces to just roll
off the main body, with some
even leaving tracks.
These pieces slowly created
a ring of debris around
Didymos. Over time, as the
debris started sticking
together, it formed the smaller Moon Dimorphos.
Another study, led by
Maurizio Pajola from Auburn
University in the US used boulder distributions to confirm
this.
The team also discovered
there were significantly more
(up to five times) large boulders than have been observed
on other non-binary asteroids
humans have visited.
Another of the new studies
shows us that boulders on all
asteroids space missions have
visited so far (Itokawa, Ryugu
and Bennu) were likely shaped
the same way. But this excess
of larger boulders on the
Didymos system could be a
unique feature of binaries.
Lastly, another paper shows
this type of asteroid appears to
be more susceptible to cracking. This happens due to the
heating–cooling cycles
between day and night: like
a freeze–thaw cycle but without the water.
This means if something
(such as a spacecraft) were to
impact it, there would be much
more debris thrown up into
space. It would even increase
the amount of “shove” it could
have. But there is a good chance
that what lies underneath is
much stronger than what we’re
seeing on the surface. This is where the European
Space Agency’s Hera mission
will step in. It will not only be
able to provide higher-resolution images of the DART impact
sites, but will also be able to
probe the asteroids’ interiors
using low-frequency radar.
The DART mission not only
tested our ability to protect
ourselves from future asteroid
impacts, but also enlightened
us on the formation and evolution of rubble pile and binary asteroids near Earth.
(The Conversation)