Introduction

Paradoxically, the “empty space” surrounding the Earth is filled with a large number of objects which, owing to their very high velocity, represent a collision risk for space missions that cannot be ignored. These objects also represent a risk on Earth itself, in the event of pieces falling to the ground.

These objects are either of natural origin, such as meteoroids, or man-made space debris created by human activities in Space.

The following text describes the situation in Earth orbit, the corresponding risks and, with regard to space debris, the possible solutions and the current regulatory picture.

LES METEORITES

In addition to the Sun and the Planets, the solar system is populated by a large number of bodies of all sizes, from grains of dust up to objects spanning several kilometers. When the trajectory of one of these bodies intersects with the Earth’s atmosphere, it creates a luminous phenomenon called a meteor, which is the origin of the term meteorite, used to designate the remains of these objects which can be found on Earth. By extension, the term meteoroid applies to the objects themselves in the Earth’s environment.

These bodies are left over from the formation of the solar system 4.5 billion years ago. Asteroids, the size of which can vary from a few meters to several hundreds of kilometers, can mainly be found in the asteroid belt in the ecliptic plane between the orbits of Mars and Jupiter. Smaller objects have been created by impacts between these asteroids. Comets are also a source of meteoroids. They come from the far reaches of the solar system and follow highly eccentric orbits.

The meteoroid flux in the Earth’s atmosphere or its immediate environment can be split into two parts:

• The sporadic flux, which is the permanent, omnidirectional flux created by the particles randomly distributed around the solar system by the various upheavals.
• Meteor Showers: certain very short phenomena (from a few minutes to a few hours) take place on regular dates (the Perseids in the month of August, the Leonids in November). These phenomena occur when the Earth passes through showers of meteoroids, which are clouds of particles ejected by certain comets when they pass through the perihelion. These phenomena can sometimes occur on a considerable scale, in which case they are called meteoroid storms.

The following graph gives the value of the sporadic meteorite (and debris) flux at an altitude of 940 km according to the size of the particles (cumulative flux = number of impacts per m2 and per year by particles higher than the value on the X-axis):

The composition of the meteorites varies according to their origin, depending on whether they are from the centre or the periphery of a fragmented asteroid. There are various types of meteorites: stony, iron and stony-iron (part-way between the two previous types).

 In the Earth’s environment, these bodies have extremely high velocity, on average 20 km/s although this can reach 72 km/s for meteorites produced by comets. At these very high speeds, in the event of a collision with a space vehicle, the mechanical effect is compounded by an electrical effect: a plasma is generated by the impact and can damage the electronic equipment.

 SPACE DEBRIS : Main characteristics

Since the beginning of human activities in Space, a very large number of objects of all sizes have been generated in Space. Recent evaluations have counted about 20,000 objects larger than 10 cm. Statistical models estimate that there are about 300,000 objects between 1 and 10 cm in size and several tens of millions of objects of between 0.1 and 1 cm. Particles smaller than 0.1 cm are of course far greater in number. For most of the object sizes in Space, the pollution created by Man is now greater than the natural environment due to meteorites (see Figure 1).

 Objects larger than about 10 cm in low Earth orbit and 1 m in geostationary orbit are identified and catalogued by the US Space Surveillance Network. The following graph shows the breakdown of these catalogued objects.

 

Debris

The largest debris can be observed from the ground using radar or optical systems. The smaller debris can only be observed in-situ using sensors carried on-board satellites.

 

Debris impacts

Debris impacts can also be observed on items which have spent time in space and been returned to Earth (Space Shuttle, LDEF, Eureca, samples on MIR, solar panel from the Hubble space telescope for example).