New approach to defense from asteroids: nuclear explosions in space

Scientists show that a nuclear explosion can alter the course of a dangerous asteroid

American scientists have simulated an attempt to change the trajectory of a dangerous asteroid by detonating a nuclear device. According to a study published in the journal Nature Physics, the blast's X-rays can actually affect most potentially dangerous asteroids.

Recent radioisotope studies have shown that the impact of asteroid Chixulub, which killed flightless dinosaurs, caused global warming that lasted about 100,000 years. Scientists consider the impact of a large comet or asteroid in a near-Earth trajectory one of the most realistic threats that could destroy modern civilization^[1].

The recent DART space experiment has shown that a kinetic impact can divert the trajectories of potentially dangerous asteroids. However, this method has the disadvantages of high mission costs, significant mission preparation time, and low effectiveness in repelling high-mass asteroids.

One of the most promising ways of eliminating this threat is by intensively heating the asteroid's surface with X-rays while a nuclear charge explodes nearby. The heated material will vaporize intensely, creating a force in the opposite direction of the explosion.

Successful first experiment

Scientists led by Nathan Moore have been able to test the effectiveness of this method on Earth using the Z machine, currently the world's largest pulsed X-ray source, owned by Sandia National Laboratories in the US.

In an experimental set-up, the researchers observed the behavior of two 12 mm samples made of quartz and silicon suspended in a vacuum on special metal rings under short but powerful (1.5 megajoules) X-ray pulses^[2].

In a series of experiments, varying the pulse duration used, the researchers observed the evaporation of the material from the samples, which created short-term pressures of up to 10 gigapascals with a plasma velocity of up to 23 km/s as it flew from the surface.

The scientists then used the results to run numerical simulations to work out how a nuclear explosion would affect a real giant asteroid.

"Our experiment demonstrated a scaled-down version of the asteroid reflectivity scenario using X-rays from a nuclear explosion at a distance. To show that this method is suitable for planetary defense, we illustrated how the experimental results could be modified to predict the maximum diameter of a spherical asteroid," the study says.

Assuming an X-ray emission of one megaton of nuclear charge, the researchers calculated that its detonation could effectively change the trajectory of asteroids up to 4 kilometers across. This figure includes the most well-known and potentially dangerous celestial bodies that appear close to Earth.

What are these mysterious asteroids?

Asteroids (Gr. asteroeidēs, "starry"), small bodies (up to 1000 km across) in the Solar System orbiting in elliptical orbits (mean values of the orbital elements are about 0.15 for the eccentricity and about 10° for the inclination of the plane of the orbit) around the Sun, closer than Neptune. Asteroids have typical rotation periods of 4-20 h around their axis. They are spherical, elongated or irregular in shape; the surface is hard and covered with impact craters. Asteroids have no atmosphere. The total mass of known asteroids is about 5% of the mass of the Moon. The first and largest asteroid, Ceres (diameter about 950 km, mass about 30% of the mass of all asteroids) was discovered in 1801. Other larger asteroids include Palade, Vesta, Hygiea and Davida.

Around 30 asteroids with diameters greater than 200 km have been discovered, and around 250 asteroids with diameters greater than 100 km. The combined mass of the latter asteroids represents about 90% of the total mass of all asteroids discovered. Large asteroids are thought to have formed similarly to asteroids through the merger of planetesimals, while small asteroids are mostly fragments from the collision of asteroids themselves. Asteroids are divided into families according to their orbital elements (semi-major axis, eccentricity, inclination of the orbital plane) (classified by K. Hirayama in 1918). Families are thought to consist of pieces of the same asteroid from collisions. Asteroids are divided into groups according to their distance from the Sun at perihelion and aphelion: there are Apollo, Amur, Aten, Trojan, Centaur and others. The amount and spectrum of energy of the reflected light from the Sun distinguishes between carbonaceous (C-type, dark - albedo 0.02-0.07; 75 % of asteroids), silicate (S-type, albedo 0.08-0.22; 15 %), and metallic (M-type, albedo 0.10-0.18; about 5 %) asteroids. The highest asteroid albedo is above 0.5. Most asteroids (90-95%) are found in the so-called asteroid ring between Mars and Jupiter (Figure). They have distances from the Sun of 2.1-3.3 au and orbital periods of 3-6 years. The asteroids in the ring are unevenly distributed, with several Kirkwood gaps discovered in 1867; the largest are at distances of 2.06, 2.50, 2.82, 3.28 au from the Sun. Asteroids with such large orbital semi-axes have orbital periods of 1/4, 1/3, 2/5 and 1/2 of the period of Jupiter. The different types of asteroids are distributed differently in the asteroid ring, with about 60% S and 10% C asteroids in the inner part of the ring and about 80% C and 15% S asteroids in the outer part. M-type asteroids are concentrated in the middle part of the asteroid ring. It is thought that the strong gravitational pull of Jupiter did not lead to the formation of a planet at the site of the asteroid ring. The orbits of asteroids outside the ring vary widely, with some (Icarus) coming closer to the Sun than Mercury, and others (Chiron) moving closer to the orbit of Uranus. Several asteroids, such as Gaspra, Ida, Matildas and Eros, have been photographed and studied by spacecraft. Asteroids are only given names and sequence numbers after the orbital elements have been reliably identified (the name is given by the discoverer). Over 15 000 asteroids are already known, and it is thought that this is only about 1% of the objects (larger than 1 km in diameter) in the Solar System. Over 400 objects have been discovered (the first in 1992) orbiting in the Kuiper belt beyond Neptune's orbit (the largest in 2001, KX76, with a diameter of more than 1200 km). The mass of the small solar system bodies orbiting there is thought to be much greater than the total mass of the asteroids in the ring.