Objects in Space
Despite the unbelievable size of space, on local scales there is quite a lot of variety. In our Solar System there are not only our Sun and the planets. In addition, there are further, smaller objects, also called minor bodies, which we briefly introduce in the following.
Asteroids and Meteoroids
An asteroid or a meteoroid are small objects in our Solar System. They orbit the Sun and are mainly composed of rock and metal. Asteroids are smaller than dwarf planets but larger than meteoroids.
According to a new definition, agreed on by the International Astronomical Union (IAU), meteoroids are objects larger than 30 µm and smaller than 1m. Objects smaller than 30 µm are usually called dust and objects larger than 1 m are called asteroids. Most of the known asteroids are located in the asteroid belt, between the orbits of Mars and Jupiter.
If an asteroid has a perihelion smaller than 1.3 AU (1 AU = 150 Mio. km, distance Earth-Sun), it is called a near-Earth asteroid. According to estimations there are about 60 – 100 Mio. near-Earth asteroids with a size larger than 10 m. Currently (2017) about 16 000 of those are known. Some of them (about 500) have a small chance to hit the Earth in the next 100 years. Predictions for a longer time period are unreliable. Intensive studies are carried out to discover more near-Earth asteroids. Furthermore, the risk that the object hits the Earth is computed. Potential countermeasures are planned.
The smaller the object, the more frequent they are. Meteoroids smaller than 30 cm originate mainly from comets. Larger meteoroids from asteroids. Satellites in space are being hit continuously from meteoroids. That is the reason why special protective measures for satellites are needed. They have specially designed thick walls or special protection shields.
In the process of outgassing, comets release meteoroids into space. First, the meteoroids are located along the orbit of the comet as a meteoroids stream. In the course of some 100 or 1000 years they spread and become part of the background distribution. Now they are called sporadic meteoroids.
A comet is an icy body in our Solar System. In general, it has a diameter of some kilometers. A comet is composed of ice, dust, and loose stones. If it passes close to the sun the comet warms up. As a consequence, the ice below the surface is converted into gas. This is called outgassing and produces a visible coma. The different components of the coma are spread by different effects. On the one hand, the solar winds affect the ions. On the other hand, the solar radiation affects the dust particles (meteoroids). This way, two tails are produced, a plasma trail and a dust trail. The common understanding is that comets originate from the protoplanetary disk. Till today they are composed of nearly unchanged material from the origins of our Solar System. That is the reason why comets are very interesting research objects of our Solar System.
Since the start of the space age in 1957 thousands of objects have been launched into space. At the end of their lifetime many objects were left in space. In addition to these “dead bodies” there are explosions of rocket stages and collisions creating many more space debris particles. Currently, there are about 20 000 objects in space, which are larger than roughly 10 cm. They are observed continuously from the ground. It is estimated that there are about 700 000 space debris particles larger than 1 cm and even more smaller ones. If these particles hit a satellite, they cause damage like meteoroids. In low Earth orbits (LEO, < 2000 km altitude) and in the range of the geostationary orbit (at a height of around 36 000 km) the flux of space debris particles is already higher than the flux of meteoroids. Beneath 800 – 1000 km height space debris is slowed down by the residual atmosphere and falls back into the atmosphere. This can take decades, centuries, or even millennia, depending on the height.
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Distance determination of dust particles to Rosetta using OSIRIS Camera Data
In cooperation with ESA, ESTEC in Noordwijk, the Netherlands, and the OSIRIS Team at the MPS in Gottingen.
After a ten year long journey the ESA space probe Rosetta reached the comet 67P/ Churyumov-Gerasimenko in August 2014. The spacecraft escorted the comet for more than two years and landed on its surface in September 2016. In this project data were analyzed which were collected with the scientific camera system onboard Rosetta called OSIRIS (Optical, Spectroscopic, and Infrared Remote Imaging System). OSIRIS consists of two cameras, the wide angle camera WAC and the narrow angle camera NAC. In some image sequences dedicated to observing dust particles in the coma of the comet both cameras were operated simultaneously. In these image sequences the distance of the particles to Rosetta can be determined by using the parallax effect. This is possible, due to the fact that the cameras are mounted about 70 cm apart from each other on the spacecraft. If the particles are in a certain distance range away from Rosetta, the signals as seen by both cameras are shifted. From this shift the distance of the particle to the cameras can be computed. More than 250 dust particles were analyzed this way. They are in a distance range of 200 m – 6000 m from the spacecraft.
With the distance of the particle and the brightness of its signal it is possible to determine the size and mass of the particle using only few assumptions. The deduced mass distribution includes particles with some centimeters in diameter and masses in the range of mg – kg.
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