Because they have worked out its orbit from making a number of observations. Once you know the orbit, you can then use the gravitational effects of the known planets in the solar system and other objects like our Moon to predict where it will be for centuries into the future.
There are some uncertainties in these predictions
- If it does a very close flyby of Earth in which case it may be uncertain what it does after that because you need precision of meters to know what happens after a very close flyby.
- There are subtle effects such as the solar wind, and the Yarkovsky effect on rotating objects (heat them up and then as they spin the heat radiates away in a different direction so changing their orbits). But those just introduce known uncertainties which you can deal with using error bars unless it does something that is very sensitive to them such as a close flyby of Earth
In this case they are able to predict that it will stay at about the same distance from Earth,so 100 times the distance to the Moon, for several more centuries and by tracking backwards they figure out it has been where it is for a century or so. In the future after perhaps five centuries it will leave Earth and head off into interplanetary space again.
They can’t say for sure that it will never hit the Earth though. Our predictions are accurate over centuries or thousands of years but not over millions of years.
Actually every object in an Earth crossing orbit will get “cleared out” within about 20 million years as that’s how long it takes Earth to “clear its orbit” They don’t necessarily hit the Earth. Some hit the Moon, some hit Venus or Mars, others hit the Sun or Jupiter, others get ejected from our solar system. But one way or another they are nearly all gone in 20 million years. (Some exceptions,for instance I think trojan asteroids are longer term).
So, without detailed calculations, just based on the general population, it could do any of those. Perhaps hitting the sun or Jupiter most likely. Ejected from the solar system also. Hitting Earth is generally not that likely because Earth is so small, but because it is already in an orbit so similar to Earth’s I would expect there’s a somewhat higher chance of hitting Earth than for most asteroids. But we are okay for centuries at least with this one. And there are no asteroids currently known that are going to hit Earth in the next century. And especially we have now (just a few years back) found all the asteroids of diameter 10 km and above in near Earth orbits, and none of those are going to hit Earth in the next century.
If we do find an object headed our way, we can do something about it.
If we know their orbits well, and know a decade or so in advance, we can deflect even quite large ones enough so that they will miss Earth. It only needs a small change, push them enough so they change their speed by about 72 meters per hour , and ten years later that changes a direct hit to a miss because it all adds up, like every day they are 1.75 kilometers further away. And if they do a flyby of Earth before they hit, as most do, it becomes even more sensitive, just need to change their speed by centimeters per day and they will miss Earth. Sometimes you just need to paint the asteroid white, which for a spinning asteroid will change its velocity by the Yarkovsky effect.
The longer in advance the better. Unless it is really huge we can probably deflect it even with a year or two of notice but that would mean things like nuclear weapons or lots of impacts - there's also the idea of deflecting a smaller asteroid to hit a larger one which has a multiplier effect and often there are smaller asteroids in similar enough orbits to do that, inter asteroid billiard balls .
But even with a massive one, especially if it does a flyby of Earth first, then it's just centimeters per day, so not that hard to achieve. If it does a flyby, there's a small "keyhole" in space just a couple of hundreds of meters in diameter typically which it has to fly through to hit Earth next time around, so if you can catch it a decade before flying through a keyhole, you just need to deflect it cms per day to miss that keyhole. E.g. 100 meters is enough to get it out of the keyhole, and 10 years in advance so that's 3650 days to shift it by 10000 centimeters, so that's 10000/3650 = 2.74 cms a day is enough to shift it out of the keyhole so it misses Earth the next time after the flyby. Gives a rough idea how it works.
Note that from time to time astronomers do discover objects with a chance of hitting Earth. Example, 2002 NT7, large enough to have potentially global effects, had a risk of 1 in a million of hitting Earth in February 2019. That was a 99.9999% chance it would miss. Again, no surprise to astronomers, given that it was already a near certainty that it would miss - with more follow up observations they proved for sure that it will miss. More about these misses here:Near-Earth object (Wikipedia)
That’s usually the way it goes. If we found one headed our way, it would go the opposite direction, to start off with it’s a 1 in a million chance, as they find out more instead of it missing they find there’s a 1 in a thousand, then a 1 in 100, then eventually tehy find it will hit. It’s very unlikely that they find out right away on the day that they discover it except in the case where they spot it for the first time a few days away from Earth which has happened a couple of times with really tiny asteroids on collision course with Earth. Anything of any size, then there’s a decent chance we spot it at least a week to three weeks before it hits, plenty of time to evacuate. There’s a tiny chance of a hit from the direction of the sun, but much reduced since the ATLAS system started operating recently.
We could reduce the risk to almost nothing if we put a space telescope in orbit close to Venus’ orbit looking outwards at a cost of about half a billion dollars - it would find even really tiny ones of 40 meters upwards and even some of 20 meters upwards, and find a large percentage of even the smallest ones within 6.5 years. Then after that, nearly all possible asteroid impact would be things we know about and can prepare for decades in advance. So astronomers know how to do the search, they just need the funding which is less than the cost of a presidential campaign, indeed the 2016 presidential campaign may well cost six to twenty times the amount needed to pay for one of these telescopes. How Much Will it Cost to Become President In 2016? | Investopedia Similarly, the UK is considering spending 200 times that on renewing Trident.
So, it is well within the capabilities of even any single one of the wealthier states on Earth to pretty much retire this risk unilaterally without a significant impact on their economy. Never mind doing it collaboratively. Indeed also many individual billionaires could do this out of their own money. We could probably have several of these telescopes there indeed, even dozens of space telescopes looking out for asteroids and the world’s largest economies would hardly notice it. So it’s well within our capabilities as a technological species to do this, easily, hardly notice it.
Also the costs of spy satellites are several times this. We could launch four to six of these asteroid detection satellites, for every one of the spy satellites the US launches into orbit, judging by the cost of this declassified 1970s spy satellite, KH-11 Kennen with an estimated unit cost of 2.26 to 3.17 billion US 2015 dollars. Just one of them would be enough to find most of the asteroids of 40 meters and upwards that risk hitting Earth within 6.5 years.
I think that if we did somehow make first contact with ETs they might be astonished at how much we spend on satellites to spy on each other, and how little we spend on satellites to look out for asteroids from space (no dedicated satellites for this so far, though NeoWise is a satellite that got repurposed for asteroid detection after its primary mission was over)
See also my Giant Asteroid Headed Your Way? - How We Can Detect And Deflect Them (read online for free)
Also available on Kindle
Giant Asteroid Is Headed Your Way? : How We Can Detect and Deflect Them (Amazon)
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