Imagine that one day the observatories of the world will all as one confirm: an asteroid is approaching the Earth, a collision is inevitable. Space nations must agree on how to stop it. Boulders flying through space can cause catastrophic damage to our planet. What happens next depends on how much time the asteroid leaves us to think. None of the options will be easy; nuclear weapons may be required. What are we going to do when that day comes?
Large asteroids rarely fall. The last of these to cause severe damage to life was the Tunguska meteorite in 1908. It is believed that it was a meteorite that exploded 10 kilometers above the remote Siberian region.
This kind of fall happens every few centuries. But Siberia is far away; even today, its population is small and scattered over a vast territory. If the same object had arrived four to five hours later, it would have fallen on St. Petersburg and produced an explosion that is equivalent to a megaton nuclear explosion.
We had the honor to observe a scaled-down version of this nightmare scenario quite recently. In 2013, the Chelyabinsk meteorite, which collapsed at an altitude of 30 kilometers, shattered glass and injured 1,400 people in a Russian city. The explosion he caused was the equivalent of 500 kilotons - about 30 bombs dropped on Hiroshima - but it was high enough to be okay. Such falls occur quite often, on average three times a year. Most of them occur over the ocean or in remote locations, so they are not noticed. And yet the question that worries us will be "will such a fall happen at all and when will it happen?"
States are taking this issue very seriously and are taking the first steps to prevent dangerous falls. In January, NASA formed the Planetary Defense Coordination Office to act as a focal point for observing asteroids and working with other space agencies on how to deal with a possible collision of large space rocks with the Earth.
PDCO currently spends most of its efforts on detection, coordinating various surveillance programs, says Lindley Johnson, NASA's planetary defense officer. Because you can't fight space stones if you don't know where they are. “We are trying to find anything that could become a threat in the coming years and even decades, in advance,” he says. As soon as a dangerous asteroid is discovered, work begins on plans to stop this particular object.
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The simplest method involves a kind of planetary billiards that uses a space probe to direct a heavy object (or the probe itself) to collide with the object. Then the asteroid is believed to change its course and fly past the Earth.
A joint mission from the European Space Agency and NASA will have to test such technology in the next few years: it's called Asterod Impact and Deflection Assesment (Aida). The mission consists of two spacecraft, one called the Asteroid Impact Mission (Aim), which will launch in late 2020, and the second, the Double Asteroid Redirection Test (Dart), which will launch in 2021.
In 2022, they will arrive on the double asteroid 65803 Didymos, which is flying with its companion Didymoon. Didymos is 780 meters across and Didymoon is 170 meters across. The younger one turns around the older one every 11.9 hours, and they are close to each other - only 1100 meters away. The Aim spacecraft will meet the asteroid and study its composition. As soon as Dart arrives, he will crash into Didymoon, and Aim will study the consequences for the orbit of the younger rock. The objective of the mission is to find out how you can redirect the asteroid so as not to put it on a dangerous trajectory. This, in fact, is the starting point for mission planning.
To understand the promise of such a mission, the famous Arizona crater in the US state of Arizona was probably formed by an object three times smaller than Didymoon, and its diameter is 1.18 kilometers. A rock the size of Didymos that hits the Earth at 125 meters per second will cause an explosion equivalent to two megatons; that's enough to destroy the city. And this is the minimum speed. At its maximum speed (about 186 meters per second), it will eject four megatons of energy - that's about four million tons of TNT.
"We want to change the orbit of this satellite," says Patrick Michel, senior researcher at the French National Center for Scientific Research and one of the leaders of the Aida team, "since the satellite's orbital speed around the main body is only 19 centimeters per second." Even small changes can be measured from Earth, he adds, by changing Didymoon's orbital period by four minutes.
It is also important to see if the explosive element will fire. “All of the collision models we are working on are based on an understanding of collision physics that has only been tested on a laboratory scale at centimeter targets,” says Michel. Whether these models will work on real asteroids is not yet entirely clear.
Johnson adds that this technology is the most mature - humans have already demonstrated the ability to reach an asteroid, in particular with the Dawn mission to Ceres and the Rosetta mission to comet 67P / Churyumov-Gerasimenko.
In addition to the warhead approach, there is also the gravitational approach - simply place a relatively massive spacecraft in orbit near an asteroid and let their mutual gravitational pull gently guide the object onto a new path. The advantage of this method is that essentially you only need to deliver the spacecraft to its destination. NASA's ARM mission can indirectly test this idea; part of this plan is to return the asteroid to near-Earth space.
However, time will be a key element of such methods; it will take a good four years to assemble a space mission beyond Earth's orbit, and it will take the spacecraft an extra year or two to reach the desired asteroid. If time is short, you will have to try something else.
Quichen Zhang, a physicist at the University of California, Santa Barbara, believes lasers will help us. The laser will not detonate an asteroid like some Death Star, but it will vaporize a small part of its surface. Zhang and colleagues worked with experimental cosmologist Philip Lubin to present a suite of orbital simulations to the Astronomical Society of the Pacific.
This plan may seem ineffective, but remember that if you start early and work for a long time, you can change the course of the body for many thousands of kilometers. Zhang says the advantage of the laser is that a large laser can be built in Earth orbit without having to fly to an asteroid. A one-gigawatt laser, operating for a month, can move an 80-meter asteroid - like the Tunguska meteorite - two Earth radii (12,800 kilometers). This is enough to avoid collision.
Another variation of this idea is to send a spacecraft equipped with a less powerful laser, but in this case it will have to reach the asteroid and follow it relatively close. Since the laser will be smaller - in the 20 kW range - it will have to operate for many years, although Zhang's simulations show that a satellite chasing an asteroid could knock it off course in 15 years.
Zhang says that among the benefits of using Earth's orbit is that chasing an asteroid or comet is not as easy as it sounds, despite the fact that we have already done it. “Rosetta was originally supposed to fly to another comet (46P), but the delay in launch caused the original target to leave an attractive position. But if the comet decides to head for Earth, we will not have the opportunity to change it to a better option. Keeping track of asteroids is easy, but it still takes at least three years to get there.
Johnson, however, notes one of the biggest problems associated with the use of a laser of any kind: no one has ever launched a kilometer-long object into orbit, let alone a laser or the whole array. “There are many immature moments in this regard; it is not even clear how to reliably convert solar energy into laser energy so that it functions long enough."
There is also a "nuclear option". If you've seen the movie Armageddon, this option seems simple to you, but in reality it is much more complicated than it seems. "We'll have to ship the whole infrastructure," says Massimiliano Vasile of Straitclyde University. He offers to detonate a nuclear bomb at some distance from the target. As with a laser, the plan is to vaporize some of the surface, thereby creating thrust and altering the asteroid's orbit. “When detonated, you get the benefit of high energy efficiency,” he says.
While lasers and nuclear bombs can go off when the asteroid is closer, even in these cases, the composition of the object will be important, since the evaporation temperature will differ from asteroid to asteroid. Another issue is flying rubble. Many asteroids may simply be a collection of rocks that stick together loosely. In the case of such an object, the warhead will not work. The gravity tug will be better - it does not depend on the composition of the asteroid.
Any of these methods, however, can face one final obstacle: politics. The 1967 Outer Space Treaty bans the use and testing of nuclear weapons in space, and putting a gigawatt laser into orbit could make some people nervous.
Zhang notes that if the power of the orbiting laser is reduced to 0.7 gigawatts, it will displace the asteroid by only 0.3 Earth's radius - about 1,911 kilometers. “Small asteroids that can destroy a city are much more common than planetary destroyers. Now imagine that such an asteroid is on a trajectory leading to New York. Depending on the circumstances, the attempt and partially unsuccessful deflection of the asteroid from the Earth could displace the crash site to London, for example. If there is any risk of error, the Europeans will simply not let the US deflect the asteroid."
Such obstacles are generally expected at the last moment. "There is a loophole in these treaties," Johnson says, referring to the space treaty and the total test ban treaty. They do not prohibit the launch of ballistic missiles that travel through space and may be armed with nuclear weapons. And in light of the need to protect the planet, critics may be patient.
Michelle also notes that unlike any other natural disaster, this is precisely what we can avoid. “The natural risk of this is very low compared to tsunami and the like. But in this case we can do at least something."
ILYA KHEL