Planetary Defence Part 2: Detection and Deflection
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In Part 1
(phys-talk.blogspot.com/2020/03/planetary-defence-part-1-introduction.html),
I looked at the impacts of asteroids and comets and how to distinguish between
them.
The first
step to protecting our planet from potentially dangerous asteroid and comet attacks
is detection. This allows scientists to predict whether an object poses a
threat to the Earth through orbit determination. If a hazardous near-earth
object is found, the next step is the ‘implementation of measures to deflect or
disrupt’ it. [1]
Detection
The main
method of detecting near-earth objects is through telescope observations. For
instance, NASA currently supports ‘several ground-based observatories that scan
the skies for asteroids’ (e.g. the Catalina Sky Survey in Arizona) [2]
as part of its ‘Near-Earth Observation Program.’
At these observatories,
the sky is scanned and images are taken where an asteroid, which reflects
light, is seen as a white dot. Initially, this is not very useful since it is sometimes
difficult to distinguish between stars and asteroids. However, more images are
taken over time. If a white dot is seen to be moving, computers ‘automatically
check it against a database of known objects.’ [3] If the object
cannot be classified, it is placed into a list of objects that need to be
confirmed. Then, if it appears that the object may pass in the Earth’s
vicinity, observatories from across the globe scan the sky. NASA’s Center for
Near-Earth Object Studies in California uses the data reported ‘[compute] high-precision
orbit paths.’ [4] As more and more data is collated, better
predictions about its trajectory can be made.
Space
telescopes are also used to detect small bodies. For example, NASA has
repurposed the Wide-field Infrared Survey Explorer to ‘survey for [near-earth
objects] and other small bodies using the remaining infrared channels on the
telescope.’ [4] As part of ‘The NEOWISE Project’, the telescope has
helped in ‘determining [near-earth object] sizes with a precision not possible
using optical measurements.’ [4]
Deflection
NASA claims
to be developing technologies that could be used ‘against a predicted asteroid
impact threat.’ [1] However, they have not released information
about official measures that have been put in place; they acknowledge that ‘an
asteroid […] could not be shot down in the last few minutes or even hours
before impact’ [1] with the Earth.
In 2018, a
team of scientists (including those from the Lawrence Livermore Laboratory in
California) ‘designed a conceptual spacecraft to deflect Earth-bound asteroids.’
[5] In particular, they wanted to assess whether their spacecraft, known
as HAMMER (Hypervelocity Asteroid Mitigation Mission for Emergency Response
vehicle), could deflect the 79 billion kilogram 1010955 Bennu asteroid which is
approximately 500 metres wide. Currently, the asteroid in question has a 1/2700
chance of hitting the Earth in 2135. However, as more data is compiled, this
probability could either increase or decrease. The proposed spacecraft is 9
meters tall and weighs around 8.8 tonnes. It would act either like ‘a battering
ram […] or as a transport vehicle for a nuclear device.’ [5] The ‘battering
ram’ method involves using a device known as a ‘kinetic impactor’ to strike the
asteroid multiple times in order to alter its path. The scientists believe that,
even if planned 25 years in advance of impact, ‘it would take at least seven
HAMMER spacecraft to deliver enough energy.’ The alternative option is to ‘detonate
a nuclear bomb in space, nearby Bennu.’ In this situation, X-rays from the
explosion would vaporise the surface of the asteroid and so ‘the asteroid would
be pushed away into a new orbit away from earth.’ [6]
Unfortunately,
many flaws with this approach have been identified. Since multiple spacecraft
would be required for the ‘battering ram’ method, ‘mission success becomes more
difficult, due to the failure rate associated with each individual launch’ (Megan
Bruck Syal). Additionally, construction of numerous spacecraft is expensive and
time-consuming. As a result of this, the scientists determined that the kinetic
impactor concept would be unsuitable. Meanwhile, the ‘window of opportunity
[for nuclear detonation] would be very tight’ if an object’s trajectory is not
100% known. [5]
Conclusion
At the
moment, sky scanning and asteroid/comet detection is relatively advanced, and scientists
should be able to predict whether a large object would impact the planet. However,
new and better methods of deflection need to be developed to fully protect our
planet as current ideas seem to be inadequate.
It is not
all doom and gloom however as new ideas are being developed by NASA and
scientists globally and major attacks in the near future are unlikely. One
suggested solution which I find interesting is based on ‘the landing of multi-screw
rockets, powered by mini-nuclear engines.’ The rockets would ‘dig a small
fraction of the soil surface to use as an exhaust propeller’ to shift an asteroid
away from Earth. [7] All concepts require vast amounts of testing and
development in order to be considered reliable options.
Sources:
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