We are not at all vulnerable to a dinosaur killer asteroid. They have already found all the asteroids of 10 km and larger and none are headed our way at least until 2200.

We can’t map out all the comets in the same way, but comets at present are very rare. As of 3rd August, we know of 106 comets that do regular flybys of Earth out of 16,468 total so one in 155 is a comet (these numbers change frequently, as they find new asteroids, for instance, they found five new asteroids since 1st August)). The chance of an impact by a 10 km asteroid per century is about 1 in a million. So that makes it a tiny negligible chance of 1 in 155 million that Earth is hit by a comet as big as 10 km in the next century. We’d spot a comet that large many years in advance, the much smaller Siding Spring 300 meter comet that did a close flyby of Mars was spotted three years in advance.

Siding Spring had a tiny chance of hitting Mars but as they refined its orbit, they found it would miss. Since Mars is a very small target, that’s what you’d expect. Same for Earth. If we found a small comet headed our way, we’d know many years in advance and to start with we might not know if it would hit, but as they refined the calculation the most likely thing always is that it misses.

Asteroids as large as a hundred kilometers can’t hit us at all. The craters on the Moon are from well over 3 billion years ago and those big asteroids have all been cleared out of Earth’s orbit long ago. The only ones left of that size are in the asteroid belt. We are protected from ones beyond Jupiter by Jupiter itself. For details, see Debunked: Earth could be struck by a huge asteroid hundreds of kilometers across

Yes there are people monitoring them. What’s more, the publish all their observations as they do them. Everything is done in public. It involves teams of professional astronomers from many countries world wide and then many amateurs. The asteroids are found by big telescopes like Pan Starrs.

CURRENT IMPACT RISKS

It is easy to keep up to date with the latest news by visiting this page:

Current Impact Risks.

This is the first place to go if you see one of those stories. It is automatically updated with all the new observations. Look to see if any of the entries in the table are orange or red. As of writing (August 2017), this has never yet happened.

DETAILS OF HOW THIS WORKS

If any of them go red, a collision is certain, but how much of a threat depends on the number.

If the number is 8, it is localized on the land, large enough to form a small crater, or if it falls in the sea, it could cause a tsunami. Sooner or later we are bound to see a level 8 red alert, as these are expected to happen about every fifty years.

Often these are not even noticed, because the impacts are in remote parts of the world. Much of the world is still desert or sparsely inhabited. But with our population growing, there's more and more of a chance that one of these impacts will come close to a populated area. If one of these hit a city, it would be devastating.

It's a bit like being able to predict a normal terrestrial hurricane, volcanic eruption, earthquake or tsunami - with the extra twist that we can actually predict it decades in advance with enough data. And, with our space technology, we may be able to deflect the asteroid and prevent it altogether.

If the entry is red, and the number is 9 or a 10, then it's a much more serious threat, either regional, threatening an entire country, or global. These are rare, and the chances are that we probably won't see one of these in this century.

If the number is orange, then there is a significant possibility of a major impact, but it is not yet confirmed. With again the severity depending on the number.

If any entry is yellow, then it merits attention by astronomers, but there is at least a 99% chance that it will miss (for levels 3 and 4) and it may be very unlikely to hit (for level 2). It may be of significance to the public if the potential impact is less than a decade away - but since it is so improbable, the chances are that it will soon be reassigned to level 0.

The Near-Earth Asteroid 2004 MN4 briefly reached level 4 in 2004, so it went right up to the highest level in the yellow section, setting a record. But it was soon reassigned first to level 1 and then by 2006, to level 0. See 99942 Apophis (on wikipedia) for the details (2004 MN4 was a provisional name, and it was renamed to 99942 Apophis).

This should be no great surprise as, after all, there was a 99% chance of it missing Earth even when the news first broke.

If (as is usual) all the entries are coloured blue (very small objects), white or green then there is no confirmed impact threat for the next 100 years.

It's quite common for an asteroid to reach level 1, green briefly. This means that it will do a close flyby of Earth, with a collision extremely unlikely.

Another thing to look out for. Sometimes astronomers maks an announcement saying that an asteroid is definitely going to miss Earth, but that it could be either by a few thousand kilometers to a few million kilometers. How can astronomers be so uncertain of the flyby and yet know it will miss?

Well that’s because the uncertainty is about its exact speed along its orbital track - and so, where Earth will be when it crosses close by to Earth’s orbit.

An analogy I use - it’s like driving along a road that crosses a railway track on a bridge. If you cross that bridge every day at the same time, and a train also goes under it every day at that same time, say 6 pm, then from time to time you will cross the bridge exactly at the same time the train goes under the bridge. But it just needs the train to be delayed by a minute, or you to be delayed by a minute, and the train, even traveling at a modest speed of say 60 km per hour will be a kilometer away when you cross the track.

But in none of those encounters will you ever be hit by the train because you are on a bridge and the train track is below the bridge. It’s like that.

For more about all these levels, see Torino Scale.

For more on this see my Giant Asteroid Headed Your Way? - How We Can Detect And Deflect Them