We can’t be certain which meal or train ride would be our last. But if there are space rocks drifting towards us from millions of kilometres away, we can certainly make efforts to identify them before they wipe us out. There are an infinite number of rocks floating in space, some of which are orbiting the sun. If our planet’s orbital path happens to cross their’s, we could have a problem (putting it mildly). However not all rocks can cause catastrophic damage – there’s a set of criteria that the bodies need to fulfil before being deemed as dangerous.
Bodies the size of around 25 metres across are more likely to burn up before coming close enough to the surface to cause damage. Objects between 25 metres and one kilometre will most likely cause damage to the area of impact. Any object bigger than that or around one to two kilometres across might have global effects. Near-Earth Objects (NEOs) are asteroids or comets that have a perihelion (nearest point of an object with the Earth in an elliptical orbit) distance of less than 1.3 au (Astronomical Unit which is approximately the average distance between the Earth and the Sun, i.e., about 150 billion metres). NEOs are further divided into Near-Earth Comets (NECs) and Near-Earth Asteroids (NEAs). NECs are comets which have an orbital period (the time taken by the comet to complete one orbit around the Earth) of less than 200 years. NEAs are further classified into three groups – Atira, Apollo and Amor – based on their distances explained in the image below.
NEAs are redefined into Potentially Hazardous Asteroids (PHAs) based on whether they are capable of making potentially harmful close approaches to our planet. The minimum distance is around 0.05 au or roughly 7,480,000 km and the minimum size of the asteroid is around 140m. Considering the infinite number of objects floating around space and orbiting the sun, you’d wonder whether there are asteroids already on their way to hit Earth. According to a report by NASA’s Near-Earth Object Wide-Field Infrared Survey Explorer (NEOWISE) in 2012, they identified about 4,700 PHAs, plus or minus 1,500, and their diameters were larger than 100 metres. And it is estimated that only 20 to 30 percent of these objects have been discovered. To be completely sure about objects which are on a collision course towards the third rock from the Sun, we need to scour the expanse of the observable universe. Once labelled as potentially dangerous, such bodies are marked and continuously monitored.
Monitoring hazardous asteroids
When it comes to monitoring for threats from space rocks, we are only interested in the potentially hazardous objects. Predicting or rather calculating their collision course is vital since surviving the impact would be on top priority if the object happens to be headed towards Earth. To be on top of the game, NASA pushed its asteroid-hunting efforts under the Planetary Defense Coordination Office (PDCO). Their primary responsibilities include early detection of PHAs, tracking and characterising them and issuing warnings about possible impacts. If detected, they are supposed to provide accurate information about the PHAs periodically and coordinate with the US Government to plan while dealing with an actual threat. NASA has set up automatic impact detection systems where Scout is used to locate and alert us to incoming threats from space rocks. The other program is called Sentry which looks at bigger and potentially dangerous objects. Apart from this, astronomers across the world, whether amateur or professional are contributing to the discovery of asteroids
Defending ourselves from an asteroid impact
Nihilists aside, most of us on Earth want to live longer. Coming up with plans to defending ourselves from an incoming asteroid has always been an agenda since we discovered that any object from outer space can simply drop in and annihilate everything. The only two ways to go forward from protecting ourselves is to destroy the asteroid or deflect its path.
The first and obvious measure would be to destroy the asteroid before it comes near. Since asteroids are huge, you will need a really powerful bomb and nothing is stronger than a nuclear explosion. Point the nuke towards the asteroid and blow it up, easy. However, it isn’t as simple as it sounds. This might not work because we don’t know whether the asteroid will blow up to smithereens or just breakups into pieces. The debris will still continue a crash course to Earth and with more objects, the damage might be scaled up. So, it’s a big no to blowing up incoming space rocks.
Following Newton’s first law of motion, if an asteroid travelling at a high speed collides with another object, the impact will change its path. So, instead of completely blowing up the asteroid, we could send a probe (known as kinetic impactors) right into the asteroid and change its trajectory enough to avoid a collision. Of course, there’s a lot of calculation required to accomplish this like figuring out the speed, trajectory, and size of the asteroid, finding the optimal location for impact, etc. We wouldn’t want to mess this up otherwise the deflection might just change the course of the asteroid to a new location on Earth. NASA’s Double Asteroid Redirection Test (DART) is an ongoing mission to demonstrate the kinetic impact technique for the first time.
Another way to deflect the path of hazardous asteroids is by using gravitational tractors. These don’t exist at the moment obviously but are part of a number of theoretical methods being considered to effectively tackle an impending asteroid thread. In this method, a probe blasts off towards the asteroid and starts orbiting around it. The probe will exert a minor magnitude of gravity on the rock and after a long period of time, the course of the asteroid would have changed. If that seems too slow, a rocket can be sent to directly attach itself to the asteroid and slowly pull it out of its regular orbit. One shortcoming of these methods is we’d have to be aware of the potential trajectory way before so that we can have enough time to send the probe.
Another method being considered is the use of solar sails. These take advantage of the phenomenon called solar or radiation pressure, where the photons from the Sun pushes any object in space. It’s a form of spacecraft propulsion where the light will fall on large mirrors, eventually applying enough pressure to accelerate the object. This phenomenon can be applied on an incoming asteroid by tethering a solar sail system around the asteroid and ultimately changing its course. This method also requires an advance intimation about the incoming asteroid.
Many asteroids have frozen materials present on its surface and sometimes they are completely made of a frozen substance. When they enter the Earth’s atmosphere, they evaporate and long tails are formed. Similarly, by using powerful lasers or maybe focusing sunlight on the asteroid, the frozen materials can be melted or evaporated. Eventually, until it reaches the Earth, it would either completely deteriorate or reduced in size which won’t be a threat anymore.
In conclusion, if an asteroid was to impact Earth in the next few years, we are almost defenceless. With effort being made to identify potentially dangerous objects approaching for impact, we might be able to discover threats way in advance. But unless we identify all the hazardous objects lurking in space, it’s difficult to prepare against them. Meanwhile, a couple of missions and projects are underway to try out different techniques that could divert the danger of humankind going extinct.