Recently, a vague source of message swiping a circle of friends: “Potentially threatening asteroids are moving at high speed toward sending earth.” “The spread of this news has caused concern among many people, after all, some studies believe that the former Earth’s dominant dinosaur was destroyed by the planet’s impact on Earth disaster.
But then, NASA’s official Twitter account, Asteroid Watch, explained the message that the asteroid, 2002 PZ39, belonged to the Apollo-type asteroid and posed no threat to Earth, and that it was passing by Earth at a safe distance of 5.77 million kilometers. Equivalent to 15 times the distance between the earth and the moon. The number is “2002 PZ39” because astronomers discovered the asteroid in 2002 at the Lincoln Laboratory in the United States.
Astronomers are divided into Amos, Apollo and many other things according to the orbit of the near-Earth object. Apollo asteroids are objects that pass through Earth’s orbit in a radial direction, but the orbital surface is above or below the Earth’s elliptical orbit (the sun’s trajectory on the celestial sphere indicates the Earth’s orbit), and they generally do not intersect the Earth’s orbit. However, their orbits change over time and may also deviate from the danger zone.
Images taken by the Arecibo Observatory on 4 and 5 February 2020, relatively clearly recorded to 2020 BX12
In fact, every year there are many asteroids and the Earth have a thrilling passing. On February 3rd another asteroid, called 2020 BX12, with a diameter of 200-450 meters, moved at a speed of about 90,000 km/h, one of the largest in recent weeks to break into the area around Earth. When it flew over the Earth, it was only 4.3 million kilometers away, but also 11 times the distance between the Earth and the moon.
The 2020 BX12, also an Apollo-type asteroid that will shuttle back and forth between Mars and Earth orbit, will continue to fly over Earth in 2022 and 2024, but both will be farther away.
Although the probability of a planet hitting the Earth is very small, but after all, there is a possibility. The frequency and intensity of collisions have always been a subject of debate. Will something hit the Earth and cause damage on the time scale we care about? There is no completely certain answer.
In Her book Dark Matter and Dinosaurs, Lisa Randall, a member of the National Academy of Sciences, makes some estimates based on data to give readers a clearer picture of how and how often and how well asteroids hit Earth.
This chart shows the relationship between the number of impacts (left ordinates) and the approximate impact interval time (right-hand ordinates) and the diameter of NEOs. The above coordinates give the amount of energy (in TNT equivalents) that the hypothetical object can release at a speed of 20 kilometers per second at impact. The horizontal coordinates also indicate the stars of the celestial bodies: the dotted line is an estimate from 2003, the circle is a new estimate, and the solid line represents the number of asteroids discovered before 2009.
The success scale in the graph is logaritht, which means that the change in the size of the object may be much larger than you might think. For example, a 10-meter-large object may visit Earth every 10 years, while a 25-meter-large object may hit the Earth only once every 200 years. This means that small changes in measurements can make a big difference to the predicted value.
The scale of the upper border of the chart shows the energy that a object of a given size can release at a speed of 20 kilometers per second, in megatons. For example, a 25-meter-sized body of energy releases 1 trillion tons of energy.
For example, the frequency of a 500-meter-sized object hitting the Earth is estimated to occur once every 100,000 years, a 1,000-meter-large object is about 500,000 years, and a 5,000-meter-sized object is 10 million years. Figure 9-3 also tells us that an object about 10 kilometers long that could be wiped out of the dinosaurs will occur about 10 million to 100 million years before it occurs.
Impacts certainly do harm, and Lisa Randall presents a table from a study by the National Academy of Sciences in Dark Matter and Dinosaurs. Large meteor impacts are much rarer than natural disasters, so they do not pose an imminent threat. But once it happens, the energy and severity they release will be devastating.
Although the threat of an asteroid hitting Earth is too small, what can we do if it happens? The two basic responses are to destroy it or shift it off the track.
Lisa Randall says destruction isn’t necessarily a good idea. If you blast an object that is about to hit the Earth into a lot of small rocks flying in the same direction, it will increase the probability of impact. Although each small piece is less destructive, it would be better if there was a way to reduce the number of impacts.
Therefore, the method of shifting the asteroid into orbit seems more reasonable. The most effective offset method is to slow down or slow down the flying celestial bodies, rather than pushing them sideways. The earth is actually very small, and it orbits the sun very fast, about 30 kilometers per second. Depending on the direction of the flying object, change its orbit by simply letting it be advanced or delayed by seven minutes – the time when the Earth moves a radius of the Earth in a orbit, which can avoid an impact and turn it into an exciting but harmless wipe. This is not a big orbital change. If we can detect this dangerous object in advance, if only a few years ago, even a small change in speed will be enough to achieve that goal.
However, if an object larger than a few kilometres in diameter hits the Earth and can cause global damage, neither destruction nor offset can save us. Fortunately, such an impact is not expected to occur for the next 1 million years.
Dark Matter and dinosaurs
If we come across a smaller object, in principle we can help ourselves. Lisa Randall says the most effective offset method is a nuclear explosion, which can prevent the impact of an object with a maximum diameter of 1,000 meters. However, the law does not allow nuclear explosions in space, so the technology has not yet been developed. Another possible approach is to hit an object with a small flying object, so that the kinetic energy (i.e. the energy of motion) of the object is transferred to the asteroid. If the amount is sufficient in advance, especially if an asteroid is hit multiple times, it will turn objects hundreds of kilometers in diameter. Other ways to offset an asteroid could be solar panels, satellites as gravitational tugboats, rocket engines, and anything that could generate enough thrust. The technology could eventually be effective for objects as large as 100 meters, but it will take decades to advance. All of these methods (and the asteroidite itself) need more research, so it is too early to say which one is more effective.