What? Asteroids also have osteoporosis?

As people age, if they do not pay attention to calcium supplementation, they will suffer from osteoporosis.

But have you heard that many asteroids are not as dense as rocks or metals as we imagine? Most asteroids are actually "osteoporotic" and contain a lot of voids. Some asteroids even have a porosity of up to 80%.

The rubble surface of the asteroid Bennu

In fact, most asteroids are not dense single rocks, but loose and porous structures, or even gravel pile structures, with non-negligible porosity. Asteroid porosity is defined as the proportion of macroscopic voids in the volume of an asteroid. The porosity of asteroids can also explain the formation mechanism of asteroids, that is, a large number of small rocks gradually form large-sized asteroids through gravitational accretion.

The distribution range of asteroid porosity is 0%-80%. The existence of asteroid porosity also causes the volume density of asteroids to be significantly smaller than the particle density. The higher the porosity of an asteroid, the greater the difference between the volume density and the particle density.

Itokawa asteroid (porosity 41%)

The porosity of an asteroid also indirectly reflects the structural information of the asteroid. If the porosity of an asteroid is zero, it is a single rock structure. If the porosity of an asteroid exceeds 30%, it is likely to be a loose rubble pile structure. The porosity of Itokawa is about 41%, the porosity of Ryugu is about 58%, and the porosity of Bennu is about 50%, which are considered to be rubble pile structures. The porosity of Eros is 10%-30%, which is speculated to be a broken single rock.

Ryugu asteroid (porosity 58%)

Bennu (porosity 50%)

Eros 433 Asteroid (porosity 10-30%)

The "osteoporotic" properties of asteroids have important implications for planetary defense.

First, the structural strength of a "porous" asteroid is lower, and it is easier to disintegrate and explode in the atmosphere, reducing the possibility of the asteroid directly hitting the surface. The main form of harm is the shock wave damage effect. Of course, if the diameter of the asteroid is large enough, the asteroid will still hit the surface.

Secondly, when asteroids with osteoporotic structures are hit by kinetic energy, the pits formed are wide and shallow. When asteroids with single rock structures are hit by kinetic energy, they are more likely to form deep and narrow pits, and more sputtering will be ejected at high speed. The more high-speed sputtering, the better the defense effect. Therefore, under the same mass conditions, osteoporotic asteroids are more difficult to defend. Fortunately, the density of osteoporotic asteroids is generally lower. For example, the density of Ryugu/Bennu asteroids is 1.2 g/cm3, and the mass is lighter under the same brightness conditions.

On April 5, 2019, Hayabusa 2 released a micro-impactor that accelerated a 2-kilogram copper plate to about 2 kilometers per second, eventually forming an impact crater with a diameter of about 17 meters and a depth of about 3.5 meters on the surface of the Ryugu asteroid. At the same time, a central crater with a diameter of about 3 meters and a depth of about 0.6 meters was found in the middle of the crater. This also reflects that the asteroid may have a layered structure, and its surface porosity is different from the internal porosity.

Finally, osteoporotic asteroids are more likely to break into pieces when hit at high speed, making them easier to destroy.

The above analysis is still qualitative, and more theoretical and numerical studies are needed on the impact of porosity on planetary defense effectiveness.

Osteoporosis can also reduce the economic value of asteroids. 16 Psyche is an M-type asteroid located in the asteroid belt. According to previous estimates, 96% of the asteroid is iron and nickel, and the total value of its resources exceeds 100 trillion US dollars. Psyche was discovered by Italian astronomers in 1852. It is the 16th asteroid discovered in the solar system, with an equivalent diameter of about 226 kilometers, which is equivalent to the straight-line distance from Nanjing to Hangzhou.

The asteroid is believed to be the core of a planetary embryo. Under the impact, the mantle and crust of the planetary embryo were stripped away. Originally, there should have been a large planet in the asteroid belt, but because Jupiter was too "gluttonous", there was not enough material for the planetary embryo to develop into a large planet.

Since the Earth's core is very deep below the surface, humans do not yet have the ability to directly detect it. Exploring Psyche may be an extremely rare opportunity for humans to explore the core of a planet, which is of vital importance to the study of the formation and evolution of large planets.

NASA plans to launch a spacecraft to explore Psyche in 2022, and it is expected to arrive at Psyche in 2026 to conduct scientific exploration.

However, according to a recent study, researchers at the University of Arizona found that Psyche's porosity could be as high as 35%, which exceeds the critical porosity of a rubble pile asteroid. Compared with the previous belief that Psyche is a hard planetary core, it is more likely to be a rubble pile asteroid similar to Bennu.

The 35% porosity means that its mineral content is about 35% less than previously estimated. According to the University of Arizona researchers, 80% of its composition is metal, 7% is low-iron pyroxene, and 10.5% is carbonaceous chondrite. This is far from the previous estimate that its composition is 96% iron and nickel.

In this case, I am afraid its economic value will be greatly reduced.