Laser-based observations at Metsähovi support satellite operations and monitor space debris

There is an enormous amount of space debris, everything from obsolete satellites and remains of booster rockets to tools used by astronauts and metal debris from collisions between objects in space. For the safety of society it is important to investigate the contents of the debris and how it moves.

Scientists at the Finnish Geospatial Research Institute are currently investigating if the satellite laser at Metsähovi could be used to observe space debris. This research is a part of the project "Characterization of objects in near-Earth space using the Metsähovi satellite laser ranging system" or MATINE SLR.

'Our task is to characterise objects in near-Earth space and to investigate their orbits, size, shape and spin', says Research Scientist Arttu Raja-Halli.

More precise data on objects in near-Earth space is useful, for example, during operational satellite disruptions, forecasting when and where a satellite is going to return to Earth and in attempts to remove space debris.

'When you know the spin of space debris, it improves forecasts on where objects returning to Earth are going to land', says Raja-Halli.

New technology at Metsähovi

Within a year, a satellite laser ranging system is going to be built at the Metsähovi geodetic research station. Metsähovi is important, because it will be the only geodetic fundamental station in the Nordic countries that is a part of the global network of research stations. In addition, its satellite laser ranging system is the only national measuring instrument for observing objects in near-Earth space.

The satellite laser ranging system sends short laser pulses into space, which are reflected by prism reflectors mounted on satellites. From the time the pulse takes to reach the satellite and return, it is possible to calculate the distance to the satellite within a few millimetres. With repeated measurements, it is possible to calculate the position and orbit of an object very accurately.

With a satellite laser ranging system, it is easier to observe objects that have reflectors than dark pieces of debris. According to Raja-Halli, slightly more than 70 no longer operational navigational or research satellites have reflectors on them. Satellites can turn into debris when they run out of fuel or their communications stop working, for example.
 
'Objects with reflectors reflect laser light a million times more brightly than other pieces of debris', estimates Raja-Halli.

The properties of the satellite laser determine which objects can be observed. The project has studied the performance of the current system and considered alternatives to improve the performance. The dataset consisted of 17,000 pieces of debris, whose orbit altitude and cross-sectional area are known.

'With our current laser, we can observe very large objects, such as the non-operational Envisat satellite, at an altitude of approx. 760 kilometres', says Raja-Halli.

It would, if necessary, be possible to replace the current laser with a more powerful one. This would make it possible to observe smaller objects at greater altitude than before.

The Sun or laser as a light source

'The ranging of objects in space can be split into active and passive ranging. Active ranging is done using, for example, satellite lasers, when the laser pulses reflected from an object are observed. Passive ranging means that sunlight reflected from the object is used for ranging. In passive ranging, ordinary optical telescopes are used', says Senior Research Scientist Olli Wilkman.

Last year, Wilkman processed ranging data from European and Asian satellite laser ranging stations. Wilkman has estimated how many photons that are reflected by different sources are observed using the system at Metsähovi, how the reflectors could be improved and how the data should be processed. He has also compared the differences between optical and laser ranging.

'Through optical or passive ranging, it is only possible to observe objects in low orbits immediately after sunset or just before sunrise, while the objects are in sunlight', says Wilkman.

Wilkman has compared two methods of determining the spin of pieces of debris: photometry from passive ranging and satellite laser ranging. Photometry is used to study changes in the total brightness of the object, or its light curve. Changes in total brightness as shown by light curves can be used to calculate the object's rotational speed and axis. In satellite laser ranging, changes in distance can be distinguished as the reflector moves closer to or further away from the observer. From this change in distance, it is possible to determine the spin of the satellite.

Debris threatens operational satellites

Pieces of space debris are being searched for and monitored around the world using large radar antennas. The advantage of satellite laser ranging when used in conjunction with radar ranging is quick reaction speed and high precision.

'If a possible collision between a satellite and a piece of debris is foreseeable, it is possible to use satellite laser ranging quickly to calculate accurate orbits and distances between objects. In this way, it is possible to obtain a more accurate estimate on whether the objects are on a collision course or not', says Raja-Halli.

The project, which receives financing from the Ministry of Defence's MATINE started last year and continues until the end of 2018. Next, the project will compare different methods for determining the attitude of different objects, combine different ranging observations and study the spin of objects that do not have reflectors.

More information

Senior Research Scientist Olli Wilkman, +358 50 576 1064
Research Scientist Arttu Raja-Halli, +358 50 411 8882
The e-mail addresses are of the format firstname.lastname@nls.fi

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