Planetary Defence Part 2: Detection and Deflection

File:Impact event.jpg
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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:
1. https://www.nasa.gov/planetarydefense/faq
2. https://www.space.com/40943-nasa-asteroid-defense-plan.html
3. https://www.youtube.com/watch?list=PLTiv_XWHnOZoFCYPi25rbz5gMM01ZHY-1&time_continue=14&v=53Js-_vo3mo&feature=emb_title
4. https://www.nasa.gov/planetarydefense/neoo
5. https://phys.org/news/2018-03-scientists-asteroid-deflector-massive-potential.html
6. https://curiosity.com/topics/the-hammer-spacecraft-could-save-the-world-from-killer-asteroids-curiosity/
7. https://arxiv.org/pdf/0705.1805.pdf




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