Too Small for Our Solar System

The Faith of Interplanetary Dust Particles.


Interplanetary Dust Particles, abbreviated IDPs, are very small meteoroids, also called micrometeoroids. They range from one micrometer (1μm = 0,0001cm) to a few micrometers. In the previous blog, we have discussed dust from which our Solar System was born. This dust is not to be mistaken for IDPs. The source of interplanetary dust is mainly asteroids and comets inside our Solar System. The dust is ejected when for example asteroids collide or when comets pass the atmosphere. In rare cases, the source is the interstellar environment in our galaxy the Milky Way.


Every celestial body like a planet or a natural satellite has its own entry and escape velocity, determining whether a particle can enter or exit the system. Jumping in the air obviously doesn't make you escape the Earth, your kinetic energy is too small hence you land back on the ground. On the surface of the Earth, the escape velocity is approximately 11,2 km/s. On the Moon, it is 2,38 km/s and in order to escape our Solar System, a velocity of at least 42,1 km/s is needed.  In the same way, an object from outer space needs certain minimal velocity, entry velocity, for successful movement into and through the gases of an atmosphere of a planet. Being very small it is easy to ablate upon flight through the atmosphere due to high temperatures generated at the surface. However, although interplanetary dust particles are very small,  their surface is relatively large compared to their masses, helping them to radiate the heat upon the atmospheric entry fast enough to remain cool and not to ablate before they land on the ground. Yet regardless of the mass volume ratio, particles still need to be at least 50 micrometers in diameter not to completely burn out upon the entry. 


Now that we know, that a diameter of at least 50 μm is needed in order for a particle to land safely on Earth´s ground, one could easily assume that all particles smaller than that are sooner or later going to ablate upon the entry to the celestial body. But if the particle is very very small, that is smaller than 1 μm, a different future might be ahead of it. Instead of burning out upon unsuccessful atmospheric entry, the particle that small is thrown out of our Solar System. The reason hides in Sun´s radiation pressure, which is together with the pressure of the solar wind producing a force in a radial direction away from the center of our Solar System. If the particle is less than one micrometer in diameter, Sun´s gravitational force does not attract such an entity strong enough to fight the present radial force, causing the movement of the particle in a direction out of the System. It is indeed very interesting, that early spacecraft has encountered such IDPs near Uranus traveling towards the exit.  


Still small, but ranging from at least a few micrometers to several millimeters in diameter, the Sun's gravitational force now exceeds the force of the Sun's radiation pressure, meaning that those particles are now subject to the gravitational force instead of radiational force, making them move towards the Sun. If one such particle is now between Mars and Jupiter, it will reach the Sun in 60 million years. To get an idea, every year 250 million metric tons of IDPs fall into the Sun.