2021-09-13

To prevent the destruction of the earth, NASA plans to use spacecraft to hit asteroids to gain experience

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This task is called “Dual Asteroid Redirection Test” (DART), an unmanned spacecraft project Launched as early as November 24 (latest February 2022), and it will take one year to reach the target-Dimorphos. The asteroid is about the size of a stadium and orbits a larger planet named Didymos.

NASA plans to use a car-sized DART spacecraft weighing about 330 kg to hit Didymos at a speed of 6.5 kilometers per second, changing the orbital time of nearly 12 hours around Didymos by a few minutes. Five years later, the European Space Agency’s Hera mission spacecraft will arrive at the scene to check whether the mission is successful. The impact will only have a small impact on the orbit of Dimorphos, but it should be enough to deviate the asteroid from its orbit of crashing into the Earth in the future. NASA DART project scientist Tom Statler said: “We are doing this to be able to prevent a real disaster.”

To prevent the destruction of the earth, NASA plans to use spacecraft to hit asteroids to gain experience

Scientists have fully studied the potential changes in the orbit of Dimorphos. But so far, we don’t know what changes will happen to Dimorphos itself after the impact. Researchers led by Harrison Agrusa, a professor at the University of Maryland, simulated the changes in Dimorphos itself by calculating how the momentum of the impact changes the roll, pitch, and yaw of the asteroid. The result may be dramatic, Agrusa said: “It may start to spin and enter a state of chaos.”

This unexpected rotation brings many interesting challenges, for example, it will increase here. Difficulty of landing on an asteroid. The European Space Agency hopes to use two small spacecraft to try to land the asteroid in its Hera mission. It may also complicate future attempts to de-orbit an asteroid that hits the Earth, because any rotation may affect the path of the asteroid in space.

When the DART spacecraft collided with Dimorphos, its impact energy was equivalent to the explosion of three tons of TNT explosives, spraying thousands of fragments into space. Professor Statler described it as a golf cart crashing into a football field at a speed of 24,000 kilometers per hour. Agrusa and his team stated that the force of the impact will not immediately change the rotation of Dimorphos, but within a few days, the situation will begin to change drastically.

Soon, Dimorphos will start to shake slightly. This swing will become larger and larger, because the momentum from the impact makes the Dimorphos spin out of balance, and there is no friction in the vacuum to slow its speed. Dimorphos may start to rotate in a single direction, or they may rotate along a long axis similar to a barbecue grill. For observers, this seemingly calm satellite will take on a new form, beginning to swing back and forth violently, and its previously hidden sides will be revealed.

Within a few weeks, Dimorphos’s rotation will become more violent, so that it will fall into a chaotic tumbling state and uncontrollably rotate around its axis. In more extreme cases, Didymos’s tidal effect may be completely broken, and Dimorphos may begin to completely flip.

What happens depends on many factors. The shape of Dimorphos will play an important role: if it is more slender than spherical, it will rotate more chaotically. So far, radar observations have shown that Dimorphos is a slender type, but we will not know until a few hours before the DART spacecraft hits it.

The location of the impact is also critical. The DART spacecraft will aim at the center of Dimorphos, with the goal of applying maximum force to change its orbit. But the more off the center of the impact point, the more chaotic the spin produced by Dimorphos will be. However, in most cases, Dimorphos should swing back and forth dramatically or roll in multiple directions within a few weeks.

Five years from now, when the European Space Agency’s spacecraft arrives, the scene may be quite dramatic. Due to human influence, Didymos may spin wildly in its orbit. It may take decades or even centuries for Didymos’ gravity to restore Dimorphos to its original state, which is locked by the tide.

The arrival of the Hera spacecraft will be the only way for us to determine how Dimorphos itself has changed, because the DART spacecraft will be destroyed by the impact, and Dimorphos is too small to be observed in detail from the earth. The small Italian-made LICIACube satellite will be deployed before the impact and will take images when the impact whizzes past, but it can only take a few minutes of images, not enough to observe the subsequent impact of the impact.

The Hera mission also plans to deploy two smaller satellites in an attempt to land on the surface of Dimorphos. The tumbling motion is not expected to hinder these efforts, but may make them more difficult. Without proper planning for this chaotic rotation, the satellite may bounce around and eventually fail to reach the position the scientist wants. Patrick Michel of the French National Center for Scientific Research (CNRS) said: “In any case, landing on such a small celestial body is very difficult.”

The rotation of Dimorphos is not expected It will affect the rehearsal of the DART spacecraft to save the earth, and it will not pose any danger to the earth, but it will provide a lot of scientifically useful information. The spin state of asteroids may affect other properties, such as how much sunlight they reflect, which may affect their orbit, which may also be a factor that needs to be considered in future asteroid orbit deflection missions. Astronomer Paul Wiegert of the University of Western Ontario said: “It’s not as simple as letting a spacecraft hit an asteroid. You need to understand a lot of physics.”

Decades or even centuries of observations will also give us an unprecedented opportunity to see how the double asteroid system will change after such an impact. The Hera mission spacecraft can observe how strong the tidal effect is in the process of restoring the system to a normal state, and help us understand the gravitational relationship between two asteroids like this.