Take a moment to picture the apocalypse.

There’s a good chance your mind might have conjured up an image of an enormous asteroid barrelling down through the atmosphere, wreathed in fire, slamming into the earth and creating worldwide dust storms, heat, and general death.

This is a fairly accurate doomsday scenario – one that has happened before and will happen again. For over four billion years, the Earth has been constantly clobbered by asteroids and other objects zooming around the solar system. While the majority have burned up harmlessly in the atmosphere, others have smashed into the surface and caused global devastation.

We’ve only really begun to understand the real threat of these impacts in the past few decades. In an iconic 1980 paper, for example, physicist Luis Alvarez and his geologist son Walter reported the discovery of a thin band of extra-terrestrial sediment between layers of Cretaceous and Tertiary rock – compelling evidence that an asteroid caused a mass extinction 65 million years ago.

Unfortunately for the dinosaurs, they didn’t have the capability to detect, predict or prevent asteroids. Luckily for us, we’ve learnt a thing or two from their misfortune.

Since the late 1990s, a host of telescopes around the globe have been scanning the Earth’s neighbourhood to discover and monitor threatening asteroids. These survey programs are primarily funded by NASA and are mostly ground-based, although the near-infrared NEOWISE space telescope also spent most of 2010 spotting near-Earth objects (NEOs) from orbit.

Together, these programs have discovered nearly 20,000 NEOs so far, with the total increasing every month. That’s a lot of fast-moving objects to keep tabs on – so how exactly do we know which ones might kill us?

Figuring out risk in any field is a tricky task. Consider an everyday example: how dangerous is a car on a given road? This answer depends on a range of slippery factors, from vehicle speed to road conditions to driver skill, and even if we were able to draw solid connections between these factors, risk is a function of probability so we would still need to figure out a statistical tool to quantify the danger.

Now consider the much more complex task of putting a number on the risk posed by an asteroid. For starters, it’s difficult to measure its exact orbit, and, of course, that orbit can change over time as the asteroid interacts with other celestial bodies.

Knowing an object’s size, speed and other characteristics are also crucial in risk calculations, but in space these are generally only estimations.

Determining the risk sounds like trying to figure out the chances of a car having an accident without knowing who is driving, where, or at what speed.

But evaluating asteroids in this way is crucial to alert scientists and the world to potentially dangerous impacts.

To get a better handle on things, boffins have worked out several scales to classify the danger.

The first, the Torino Impact Hazard Scale, was hashed out in 1999 by a group of scientists from the International Astronomical Union (IAU), at a meeting in the Italian city of Turin. The result is akin to the Richter scale: it assigns incoming events an integer from zero to 10, where zero means a negligible threat and 10 means a massive, inevitable collision that would cause global devastation.

Simple, right? It’s meant to be – the Torino scale was designed to clearly communicate risks to the public.

But the model takes into account more than just whether an object is making a beeline for the Earth. It also considers the probability of impact; the date of collision, which dictates how much we need to worry right now; and the energy of the impact – deduced from the object’s size, density and velocity – which indicates how significant the devastation might be.

An object with a Torino score of zero, for example, might be a small object that would burn up completely in the atmosphere and therefore have no chance of doing damage.

Bear in mind, however, that the Chelyabinsk meteor that exploded over Russia in 2013 would have scored zero on this scale. Although its effects injured 1500 people, its impact energy was comparatively small and it didn’t survive its fiery entry into the Earth’s atmosphere.

As the numbers on the Torino scale climb, the risks begin to rise. When an asteroid is assigned a score of three, astronomers are alerted. This doesn’t necessarily mean that the object is dangerous – just that it has a 1% or greater chance of collision and could cause localised damage, so it must be monitored.

The potential for destruction only goes up from here. A score of five indicates close encounter events that pose a serious but still uncertain threat, capable of doing serious damage across a large region.

Events with a score of six or seven could inflict global damage. Not only would these need to be studied closely to see if they continue on a collision course with Earth, but also – depending on how imminent the collision is – NASA would likely notify relevant governments worldwide to start creating contingency plans.

From eight upwards, the forecast begins to look grim. Objects in this range are certain to smash into the Earth. A score of eight is a once-in-50-year event that would cause local destruction if it impacted on land and a possible tsunami offshore. An event of nine would cause unprecedented regional devastation on land and a major tsunami, and is estimated to occur once every 10,000 to 100,000 years.

And then comes the cherry on top: a perfect score of 10. This would be given to an asteroid certain to slam into the Earth and cause the kind of planet-wide climatic catastrophe that could destroy civilisation as we know it – like the Chicxulub impact, destroyer of the dinosaurs.

This type of event is predicted to occur once every 100,000 years, which doesn’t seem like that long in the grand scheme of things.

However, it’s heartening to note that the vast majority of NEOs discovered thus far have been slapped with a score of zero.

When first discovered, some asteroids can appear to be on course with the Earth according to complex orbital calculations. But the numbers are normally based on only a few days of observations, and further monitoring can give a better indication of the object’s true path and allow it to be either upgraded or downgraded on the scale.

In this sense, the Torino scale is analogous to hurricane forecasting, where predictions of the storm’s path and ferocity are updated as more data is collected.

Four was the highest score ever assigned to an object: the 370-metre diameter asteroid called 99942 Apophis. In December 2004 it caused a mild panic when astronomers calculated that it had a 2.7% probability of smashing into earth in 2029. But follow-up observations refined the orbit and eventually eliminated the potential for an impact, downgrading the rock to level zero.

Everything else discovered so far has also been downgraded to zero.

But this means the same level on the Torino scale contains thousands of asteroids all with diverse properties, and so scientists came up with more technical and precise tool to tease out further information.

Developed by NEO specialists in 2002, the Palermo Technical Impact Scale quantifies threats in more mathematical detail. Like the Torino model, it looks at an object’s predicted impact energy and date of collision, but it aims to prioritise NEOs according to how much they deserve observation and analysis.

The scale is logarithmic for scientific convenience: objects that pose no threat are assigned negative values, while more threatening ones are given positive values.

The scale also compares the risk posed by an asteroid to the “background risk”: the average risk posed by other objects of a similar size over the years between now and the predicted impact. This background risk is taken as the status quo, so when an approaching object rises above the background level, corresponding to a positive Palermo scale score, it indicates an unusual and concerning event.

This means an object that scores a minus-2 is only 1% as likely to occur as a random background event of the same size in the intervening years. A score of zero means that the event is just as likely as a background event, and a score of plus-2 means the event is 100 times more likely than a background event.

This allows scientists to fine-tune their study of possible impacts, and in particular to carefully prioritise and analyse events that score zero on the Torino scale.

Neither scale is perfect. Both, for example, tend to flag asteroids as concerning before orbital data is detailed enough to accurately predict their paths. This creates a communication issue around the urgency of the risk, sometimes inciting panic where none is warranted.

An alternative scale was proposed in 2003 by Brian Marsden of the Minor Planet Centre. Called the Purgatorio Ratio, it balances the accuracy of an asteroid’s orbit with the amount of time before its predicted impact. An object with an uncertain orbit and a predicted impact date many decades away would get a much lower rating, even if at first glance it seems to be on a collision course with Earth.

The Purgatorio Ratio is appealing because it eliminates some of the unnecessary urgency attached to asteroids in the vicinity of Earth, potentially reducing media sensation. Yet since the ratio isn’t promoted by NASA, which funds the bulk of asteroid search programs, it isn’t widely used.

But it is certainly complementary to the other scales. Assessing asteroid risk is a multi-dimensional and complex problem, so it’s a good idea to prepare a range of tools to deal with it.

And in the end, none of the scales can decide how we should act if faced with a serious and imminent asteroid threat. Ultimately, if we need to act to mitigate disaster then that terrifying and crucial choice is completely up to us.