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Asteroid impacts have always been a fascinating concept.
Although uncommon, even today we see news articles of asteroids reaching the surface
of Earth.
And they make for interesting subjects of disaster movies.
Why?
Well, they are likely to have caused the mass extinction which wiped out 76 percent of all
species on the planet 66 million years ago.
Was this a one off?
Or could such an event happen again?
And if a large asteroid was on course to strike Earth, could we prevent it and survive using
current technology?
I’m Alex McColgan and you are watching Astrum.
Join with me today as we try to understand asteroids and how NASA is planning to defend
Earth from asteroid impacts by seeing what happens when we launch spacecraft into them!
Major impact events have significantly shaped Earth's history.
Going back to the very early solar system, one of the biggest impacts in our history
may have formed the Moon, where a planet sized object named Theia collided with the young
Earth.
As time progressed, we may now have oceans full of water thanks to collisions with asteroids.
And since then, asteroids have also potentially caused several mass extinctions.
One of the most well-known impact events left the Chicxulub crater, which is believed to
have caused the extinction of dinosaurs 66 million years ago.
There is an inverse relationship between the size of an object and the frequency of such
events.
Small objects frequently collide with Earth, while large object collision events are very
rare.
And as the solar system ages, large asteroid impacts become less common.
You see, when asteroids collide with something, they either fragment into smaller chunks,
or they get integrated into the larger bodies like the planets.
It is estimated that Earth gains 15,000 tons per year this way from meteors entering its
atmosphere.
Impacts actually happen all the time on Earth, a lot more than you may think, but most are
barely noticeable.
Meteors, or shooting stars, are tiny objects with sizes ranging from sand grains to stones
a few centimetres across.
These burn up as they enter Earth’s atmosphere.
So, how many do you think hit Earth’s atmosphere on a daily basis?
The figure may surprise you.
25 million.
And guess what the biggest meteor is likely to be on any given day?
40cm across.
The biggest one on a yearly basis is around 4m.
And every century we get one that is at least 20m across.
The objects that manage to survive the Earth's atmosphere and land on the surface are called
meteorites.
These have to be somewhat larger than normal shooting stars to survive, and most of the
time what's left of the object upon reaching Earth’s surface is about the size of a brick.
These objects cause large trails in the sky with spectacular colours, known as fireballs.
Every 2,000 years or so, a meteor about 100m across hits Earth, blasting through the atmosphere,
causing significant damage to the area from the impact itself but also from the airburst
associated with it.
And then you have objects large enough to threaten civilizations on Earth, objects over
10km, that only come around once every several million years or so.
The chances of anyone alive today encountering such an event are very close to zero.
One of the good things about larger objects is that they are also easier to spot, and
scientists think they know of all nearby asteroids above the size of 10km with a very high degree
of certainty.
But smaller, less easy to spot meteors are still a big threat on a more local scale.
About 1,500 people were injured in 2013 when the Chelyabinsk meteor exploded over Chelyabinsk
in Russia.
Around 7,200 buildings across six cities were damaged by the explosion's shock wave.
Luckily, no one was fatally injured during this incident.
But imagine if such a disaster were to happen in a highly populated region.
Needless to say, the results would be catastrophic.
This is why it is very important for us to be able to detect and prevent this before
it has a chance of happening.
One of the ways to do that is for us to find and track objects in space that might be a
threat, and we are doing this faster and more efficiently than ever before.
We’ve discovered over 20,000 new potentially dangerous asteroids in just the past two decades
with about 30 new discoveries added each week.
Most of them have harmless orbits, but what would we do if ever we found an asteroid that
was on a collision course with Earth?
This is what NASA will be trying to find a solution for with its DART mission, or the
Double Asteroid Redirection Test.
DART will be visiting Didymos, a relatively large near-Earth object, with a diameter of
about 780 metres.
It has a moon named Dimorphos, which is about 160 metres across, and it orbits Didymos from
about a kilometre away.
The orbit of Didymos ranges from about just 1 AU outside the orbit of Earth to a bit beyond
the orbit of Mars, and it takes 2.1 Earth years to make an orbit around the Sun.
As it orbits, the asteroid comes close to where Earth orbits, and when our orbits align,
it can get pretty near.
In 2003, it passed only 0.048 AU from Earth, just over 7 million kilometres, which is actually
when it was close enough that we could detect its small moonlet Dimorphos.
Didymos spins rapidly, rotating about once every 2.26 hours, while the moonlet orbits
around its parent body about once every 11.9 hours.
The DART spacecraft trip to Didymos will be fairly short for space mission standards.
It will launch on a SpaceX Falcon 9 rocket on November 24, 2021, from the Vandenberg
Space Force Base in California.
It will travel a distance of 11 million kilometres and will reach Dimorphos by September 2022.
DART is a very simple and low-cost spacecraft when compared to other orbiters or space missions.
It’s also very small, only about the size of a refrigerator, and its payload consists
of a single instrument, called DRACO.
DRACO is a high-resolution imager, which will help DART with its navigation and targeting,
to measure the exact size and shape of the asteroid target, and to help scientists determine
the best impact site and to provide a geological context to it.
DART will also carry the LICIACube, a CubeSat contributed by ASI, the Italian Space Agency,
as a secondary spacecraft.
The LICIACube weighs just 14 kilograms and is only about 30cm long.
LICIACube will be stowed away in the mothership during the interplanetary cruise and will
be released around 10 days before the impact.
It will flyby the asteroid system using an autonomous navigation system in order to witness
the DART spacecraft impact Dimorphos and to acquire images and data of the target after
the event.
The DART spacecraft itself will also be running autonomously.
Due to the latency between sending and receiving instructions caused by the distances between
Earth and the spacecraft, DART must itself locate the moonlet, detect a perfect spot
for the impact to happen, and then aim itself appropriately.
This is where DART’s SMART Nav comes in handy.
This autonomous optical navigation system will identify and distinguish between the
two bodies and will detect the target, and then direct the spacecraft toward it.
And surprisingly all this happens within only an hour before the impact.
During DART’s impact in 2022 the distance between Earth and the Didymos system will
be near its minimum.
The last time Didymos was this close to Earth was in 2003 and the next time will be in 2062.
This timing allows scientists to make higher quality telescopic observations of the Didymos
system after the collision.
The impact itself will be magnificent, similar to the Deep Impact mission of 2005.
The impact’s energy will excavate a new crater, and blast between 10,000 to 100,000
kgs of asteroid surface material, called ejecta, into space.
It is estimated that the impact of the 500 kg DART spacecraft at 6.6 km/s will produce
a velocity change on the order of 0.4 mm.
This doesn’t sound like a lot, but it will lead to a small change in trajectory of the
asteroid system, and over time, this will lead to a large shift in its path.
Not to worry though, even with large margins of variability, scientists know that even
with this collision, the orbit of Didymos won’t change enough to ever cross paths
with Earth.
But if there was an Earth-bound asteroid, this little nudge early enough in advance
can impact its orbit enough to take it off a collision course.
The images acquired by DRACO onboard DART before the kinetic impact will be streamed
back to Earth in real time.
In its final moments, DRACO will help characterize the impact site by providing high-resolution,
scientific images of the surface of Dimorphos.
LICIACube is equipped with two optical cameras, which will inform the microsatellite’s autonomous
system by finding and tracking the asteroid throughout the encounter and capture scientific
data along the way.
All the data produced in this phase of the mission will be fundamental in verifying the
effectiveness of moving the asteroid’s orbit through this method.
It will also study the formation of the plume generated by the impact, in particular, to
characterize its structure, morphology and evolution.
Hera is a follow up mission.
It will provide a detailed post-impact survey that will turn this grand-scale experiment
into a well-understood and repeatable planetary defence technique.
Hera is ESA’s contribution to this mission.
It is due to launch in 2024, and it will reach the Didymos system by 2026.
And upon reaching the asteroid system, it will perform high-resolution visual, laser
and radio science mapping of the moonlet, to build detailed maps of its surface and
its interior structure.
Because even with the most powerful telescopes, Didymos and Dimorphos still appear no more
than a single point of light from Earth.
Scientists will try measure the brightness of that point of light over and over again,
building up a plot of the light curve and looking for changes in brightness when the
impact happens, however, this technique by itself is not very effective in measuring
the change that DART’s impact caused to the asteroid’s orbit.
What ESA is trying to do with the Hera mission is perform its own ‘crash scene investigation’
of the moon’s impact crater and surrounding surface in great detail.
The data collected will put into numbers exactly how much of an impact we can have on an Earth-bound
asteroid, should the need ever arise.
So, there we have it.
Almost everything you would want to know about the DART mission, and what our best hopes
are at diverting potentially dangerous asteroids away from Earth.
Another reason scientists want to learn about asteroids is because they might have other
uses for humanity in the future too.
Companies are already setting their sights on mining asteroids, due to them being rich
in various metals.
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end up being preferred to mining on Earth, and how far away we are from seeing this becoming
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