It may sound like science fiction, but the DART (Double
Asteroid Redirection Test) is a real proof-of-concept experiment, blasting off
at 10:21pm Pacific Time Tuesday (11:51am IST Wednesday) aboard a SpaceX rocket
from Vandenberg Space Force Base in California.
The goal is to slightly alter the trajectory of Dimorphos, a
"moonlet" around 525 feet (160 metres, or two Statues of Liberty)
wide that circles a much larger asteroid called Didymos (2,500 feet in
diameter). The pair orbit the Sun together.
Impact should take place in the fall of 2022, when the
binary asteroid system is 6.8 million miles (11 million kilometres) from Earth,
almost the nearest point they ever get.
"What we're trying to learn is how to deflect a
threat," NASA's top scientist Thomas Zuburchen said of the $330 million project,
the first of its kind.
To be clear, the asteroids in question pose no threat to our
planet.
But they belong to a class of bodies known as Near-Earth
Objects (NEOs), which approach within 30 million miles.
NASA's Planetary Defense Coordination Office is most
interested in those larger than 460 feet in size, which have the potential to
level entire cities or regions with many times the energy of average nuclear
bombs.
There are 10,000 known near-Earth asteroids 460 feet in size
or greater, but none has a significant chance to hit in the next 100 years. One
major caveat: scientists think there are still 15,000 more such objects waiting
to be discovered.
15,000mph kick
Planetary scientists can create miniature impacts in labs
and use the results to create sophisticated models about how to divert an
asteroid — but models are always inferior to real world tests.
Scientists say the Didymos-Dimorphos system is an
"ideal natural laboratory," because Earth-based telescopes can easily
measure the brightness variation of the pair and judge the time it takes the
moonlet to orbit its big brother.
Since the current orbit period is known, the change will
reveal the effect of the impact, scheduled to occur between September 26 and
October 1, 2022.
What's more, since the asteroids' orbit never intersects our
planet, they are thought safer to study.
The DART probe, which is a box the size of a large fridge
with limousine-sized solar panels on either side, will slam into Dimorphos at
just over 15,000 miles an hour.
Andy Rivkin, DART investigation team lead, said that the
current orbital period is 11 hours and 55 minutes, and the team expects the
kick will shave around 10 minutes off that time.
There is some uncertainty about how much energy will be
transferred by the impact, because the moonlet's internal composition and
porosity are not known.
The more debris that's generated, the more push will be
imparted on Dimorphos.
"Every time we show up at an asteroid, we find stuff we
don't expect," said Rivkin.
The DART spacecraft also contains sophisticated instruments
for navigation and imaging, including the Italian Space Agency's Light Italian
CubeSat for Imaging of Asteroids (LICIACube) to watch the crash and its after-effects.
"The CubeSat is going to give us, we hope, the shot,
the most spectacular image of DART's impact and the ejecta plume coming off the
asteroid. That will be a truly historic, spectacular image," said Tom
Statler, DART program scientist.
Nuclear blasts
The so-called "kinetic impactor" method isn't the
only way to divert an asteroid, but it is the only technique ready to deploy
with current technology.
Others that have been hypothesised include flying a
spacecraft close by to impart a small gravitational force.
Another is detonating a nuclear blast close by — but not on
the object itself, as in the films Armageddon and Deep Impact — which would
probably create many more perilous objects.
Scientists estimate 460-foot asteroids strike once every
20,000 years.
Asteroids that are six miles or wider — such as the one that
struck 66 million years ago and led to the extinction of most life on Earth,
including the dinosaurs — occur around every 100-200 million years.
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