Yo Yehudi answered on 15 Nov 2017:
It’s definitely something we’re working on and aware of. If you go to scholar.google.com and search for “asteroid deflection” a _lot_ of papers written by scientists come up in the results. I’m not enough of a physicist to really understand the papers myself fully, but it’s pretty easy to see why it’s as important as it is. In 2013, a meteor that was only around 20 meters in diameter – not all that big when you remember that an olympic swimming pool is 50 meters wide – entered the Earth’s atmosphere, and exploded in the air without even hitting the Earth. Despite this, the explosion was estimated to be 20 or 30 times as strong as the nuclear bomb in Hiroshima! Imagine what a larger meteor might do. It’s scary, really. We don’t want to go the way the dinosaurs probably did! You can read more about the 2013 meteor here: https://en.wikipedia.org/wiki/Chelyabinsk_meteor
The USA and Russia even have a treaty to collaborate with peaceful use of nuclear technologies, one of which could be asteroid defense: https://energy.gov/articles/united-states-russia-sign-agreement-further-research-and-development-collaboration-nuclear
Google Scholar search for asteroid deflection: https://scholar.google.co.uk/scholar?hl=en&as_sdt=0%2C5&q=asteroid+deflection&btnG=&oq=asteroid+de
An example paper talking about strategies to deflect asteroids: https://arc.aiaa.org/doi/abs/10.2514/1.33432?journalCode=jgcd
Oli Wilson answered on 15 Nov 2017:
Some international treaties mean countries aren’t really supposed to put weapons in space (interestingly, the USA voted against most of them), but people have thought about it. Apparently one Soviet spacecraft had a machinegun (y’know, just in case) and they designed another one to carry a laser to destroy satellites (but it was never launched), but that’s about it for weapons in space. As Yo implied, though, you don’t have to be in space to fire weapons that high – like in the Bond film Die Another Day, it’s possible to fire missiles up to destroy satellites if need be.
This could probably also work if aliens fired something big and fairly slow at us, but it would make the debris problem worse – and at the moment it’s the debris we should worry about!
At the moment there are half a million bits of debris orbiting the Earth at up to 17,500mph… I don’t know if you’ve seen the film Gravity (my wife swore never to go to space after watching it), but space debris really could cause huge problems to satellites and even the space station. Sadly, I think we’ll need a better solution than shooting stuff!
Have a look at this NASA page on space junk https://www.nasa.gov/mission_pages/station/news/orbital_debris.html
Kerrianne Harrington answered on 15 Nov 2017:
It would be really difficult, but as Yo and Oli said it is something people are discussing. First of all, it’s a big space to survey at all times!
Our nearest defense system at the moment would be NASA’s Near Earth Orbit programme. It looks at orbits for Near-Earth Objects (NEOs) to predict their future motions, look at potential danger, and they then make these results available on their website. But even they won’t catch all of them, there’s still a chance they could miss one. These objects are also good for scientific study, other than just judging their danger, to help us understand the space around us. This is usually because these are remnant debris from something interesting, like our solar system formation!