by
Author｜Ling Zijun
Editor｜Jing Yu
One day in the 21st century, two astronomers discovered that a comet the size of a Himalayan was about to hit Earth in six months’ time with an energy equivalent to a billion atomic bombs. Without action, the planet would be destroyed.
They reported the news to NASA, the President of the United States, and then to the public through television news.
Unfortunately, no one paid attention.
The president was concerned about his election, and the public took the news as an interesting headline. So the earth that “doesn’t look up at the sky” ushered in destruction.
This satirical comedy “Don’t Look Up” (Don’t Look Up), which was launched on Netflix at the end of last year, although the story is purely fictional, one detail is in line with reality-two astronomers were handed over by the NASA administrator to the director of the “Planetary Defense” Office , led by the supervisor to see the president overnight.
In reality, NASA does set up a similar department with a similar name:Planetary Defense Coordination Office. The office’s main task is to keep an eye out for potentially threatening near-Earth objects, issue warnings, and devise preventive plans. Therefore, if a comet the size of the Himalayas in the movie really threatens the earth, it should be discovered by this department first, less than two ordinary astronomers.
Just today, September 27, 2022, Beijing time, this office has just performed the first asteroid defense exercise in human history – impacting an asteroid with an artificial satellite to change its orbit.
The impact mission was part of the Double Asteroid Redirection Test. Satellites sent to space on missions are called DARTs. Dart, also means dart in English. It may also be the most expensive “dart” to date.
DART roadmap and schematic diagram of the binary star system | Source: NASA
Running for a long time, colliding head-on
At 7:00 p.m. local time in the United States on September 26, 2 minutes before the “last blow”, the motor went out. Before that, it ran to the target at a speed of about 6.6 kilometers per second.
At this point, the DART satellite had locked onto the asteroid Dimorphos, getting closer and closer, just one last blow. It had been in flight for 10 months before this impact. In November 2021, the satellite was launched into space by a Falcon 9.
The DART satellite is targeting Didymos (Greek for twins), a binary star system 11 million kilometers away from Earth.
There are two asteroids, one large and one small, in this binary star system, the large one is 780 meters in diameter and the small one is 160 meters in diameter. This little brother asteroid is the moon of another big brother asteroid, and it orbits the larger of the binary star system in 11 hours and 55 minutes per revolution. The asteroid, named Dimorphos, is the ultimate destination for the DART satellites.
At 3:00 p.m. on the 26th of the impact mission, after the satellite received the last course correction from Earth, DART went into autopilot mode—one of the technologies tested on this mission. The scientific research team equipped the satellite with an optical navigation system. An hour before impact, the navigation system guided the satellite DART to lock on to its younger brother in the binary system.
In the last few minutes, the camera on the satellite sent back photos to the earth, recording the “running” from the satellite’s perspective:
2.5 minutes before impact, this was also the last time the binary system was in view. 570 km from the target.
11 seconds before impact, 68 km from the target.
Last full picture from before impact, 12 km from target.
About 1 second before impact, 1 km from the target, the satellite tenaciously sent back the last photo.
On the way back to Earth, the satellite “kissed” the target, so only a fraction of the information was returned.
At 7:14 p.m., mission control in Maryland, USA, announced that the impact mission was successful.
The reason why this binary star system was chosen as the experimental object is that it is at a moderate distance from the earth, which is convenient for aircraft to reach, and convenient for observation by various astronomical telescopes.
The choice of the smaller brother in the binary system is not entirely because of “persimmons first find a soft pinch”, but also because after the impact, by observing the changes in its orbit around the older brother, the impact of the impact can be calculated.
Another reason,Asteroids with a diameter of more than 140 meters may pose a potential threat to the earth. At 160 meters in diameter, the younger brother planet has reached the “lower volume limit” for a potential threat, making it a suitable candidate for re-orbit experiments.In other words, this binary system is an ideal testing ground for asteroid re-routing.
In terms of volume, the DART satellite is only 19 meters in diameter, less than one-eighth the size of the impact target. Scientists estimate that the impact will change the orbit of Dimorphos by 7 minutes.
Workers remove the DART from the container | Source: NASA
The probability of “planet hitting the earth”
While the satellites themselves can no longer transmit any information back to Earth, scientists are still waiting for more images. A small box may be the best audience for this “artificial astronomical phenomenon”.
Fifteen days before the impact mission, a small cube “box” (LICIA Cube) was isolated from the DART satellite. It was provided by the Italian Space Agency and was an imager with its own wings. After separation, it will also automatically adjust its attitude to reach the sky above DART, witness the historical moment of the impact, and capture images of the asteroid ejecta formed in space during the impact.
However, since the LICIA Cube is not equipped with a large antenna, the images will be sent back to Earth one by one in the next few weeks. With the imagery it provides, combined with the imagery returned by the satellite itself, one can expect a more vivid and comprehensive record of the impact in the future.
Size comparison of the two planets and DART | Source: NASA
Scientists predict that this impact may eject 9980 kg to 99800 kg of asteroids by material and form craters on the surface. At the moment of collision, millions of tiny rocks will be squeezed, and the combined thrust they create will add to the effect of the DART impact.
Scientists studying collision models found that,The main factors affecting the impact result are the speed of the satellite DART, the angular velocity when the two collide, and the internal structure or material properties of the impacted asteroid itself.
The strength of the rocks that make up the asteroid, and the porosity of those rocks (that is, the volume of air in the middle of the rocks), will have a significant impact on the outcome of the impact. However, because people do not know the situation of the asteroid rock itself, in addition to calculating different models, scientists are also waiting for the pictures returned by the LICIA Cube to get more information.
Results calculated by scientists under different porosity | Source: NASA
Over the past few decades, there has been growing evidence that many asteroids do not exist as solid rocks or rocks with millions of microscopic pores. They exist in the form of rubble piles, that is, loose piles of boulders. Gravitational pulls the stones together to form an asteroid. They are much like giant rock fields floating in the universe. Scientists suspect that Dimorphos is the same.
Humans define the closest distance from the earth to the sun as 1 AU. If there is a celestial body with a perihelion distance of less than 1.3 AU, it is considered a near-Earth object.
The currently observed near-Earth objects are mainly asteroids and comets, as well as the most common meteors.
As of July 20, 2020, there were 23,118 known near-Earth asteroids, according to NASA’s in-house Center for Near-Earth Object Research. A small half of the asteroids are 140 meters in diameter. These are all asteroids that may pose a potential threat to the Earth, but little is known about them at present.
Every once in a while in the news, there are predictions that an asteroid will hit Earth in the future, but after a while it turns out to be a miscalculation. In 1998, news broke that an asteroid would hit Earth in 2028. Later, it was discovered that the calculation was wrong, and the asteroid would pass the earth.
It is speculated that in the early days of the solar system, about a billion years ago, the earth was hit by a large number of asteroids. At present, about 200 asteroid impact craters have been found on Earth, as well as suspected impact craters.
Calculate the probability of a near-Earth object hitting the earth according to the currently available information,Small objects of the order of 200 meters in diameter appear once every thousand years. The impact that wiped out the dinosaurs 65 million years ago was an asteroid 15 kilometers in diameter.The probability of it happening is calculated in billions of years. This has only happened once in known history.
For humans, an asteroid with a diameter of 1,000 meters can bring a devastating blow to the earth – the dust from the impact will cover most of the land, and it will also cause tsunamis in the ocean.
According to the data currently available, in the next 100 years, no known asteroid larger than 140 meters in diameter may hit the earth. However, there are still blind spots in the human field of vision, and some asteroids have not been observed.
In addition, there are individual asteroids that are included in the list of “potential threats”. One asteroid is believed to have a one-in-1,000 probability of hitting the earth in more than 100 years; 1 in 1 chance of hitting the Earth. Fortunately, humans are not dinosaurs. In the face of small probability events, they will think about plans hundreds of years ago.
By the way, to commemorate the first human asteroid diversion experiment, you can use “DART NASA” as a code word today to trigger a web page that was tilted after being hit by DART.
Crooked Memorial | Source: Screenshot
The impact of DART is just a starting point, and the plan will continue in different ways over the next few years.
In addition to observations of the binary system by observatories on Earth, in 2024, the European Space Agency will launch a spacecraft called Hera, which will arrive in the binary system in 2026. The main spacecraft and its two LICIA Cube-like imagers will take a close look at the crater, the crater left by Dimorphos, and measure its mass.
At that time, more data on this binary star system will be in the hands of scientists – at that time, Dimorphos may become the asteroid that humans know best in the universe.
