Lunar laser ranging

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Retroreflector of the Apollo 11 mission

With Lunar Laser Ranging (abbreviated: LLR ), starting from ground stations on earth, runtime measurements of laser pulses to retroreflectors on the moon and back are carried out. LLR measurements provide information about the earth-moon system , the moon rotation and the verification of gravitational physics .

The accuracy of the measurements is currently a few centimeters in the earth-moon distance. This distance averages around 384,400 km over time.

Reflectors on the moon

Replica of the reflector from the Apollo 11 mission in the Science Museum

Retroreflectors have the property of reflecting incoming light in exactly the same direction from which the radiation comes. The retroreflectors currently in use consist of up to 300 triple prisms, each 4 cm in diameter, mounted on an aluminum frame.

The first retroreflector was installed on the lunar surface in July 1969 by astronauts on the Apollo 11 mission, and two more reflectors were installed in 1971 by the Apollo 14 and 15 . In 1970, a reflector with the Soviet Lunochod-1 mission could be placed on the moon. Signals from this reflector could no longer be detected since the 1970s. The reasons for this were unknown for many years. After the American lunar probe Lunar Reconnaissance Orbiter took pictures of Lunochod 1 in 2010 and therefore the position could be determined more precisely, the laser reflector was successfully aimed again for the first time. In 1973, a further reflector could finally be deposited by the Lunochod-2 mission, which can still be used for measurements.

Improving the measurement accuracy in the millimeter range is not possible with these retroreflectors. Their individual prisms are not all equally distant, if only because of the libration ; the many short echo pulses overlap and form a longer pulse. A new concept provides for fewer, but larger, prisms or hollow cube-corner mirrors to be placed so deeply in the distance that the echo can be resolved into a comb .

LLR ground stations

Laser ranging system of the geodetic observatory Wettzell in Bavaria

Since the Apollo 11 mission placed the first reflector on the moon, measurements have been taken at the McDonald Observatory near Fort Davis , Texas . In 1984 another station of the Lure Observatory on Haleakalā on the island of Maui , Hawaii and a laser station of the Observatoire de Calern were put into operation. In 2005 the Apollo system (Apache Point Observatory Lunar Laser-Ranging Operation) was put into service in New Mexico . Occasionally, observations were made on the geodetic fundamental station Wettzell in the Bavarian Forest , as well as on the Australian station Orroral.

The stations use telescopes with aperture diameters of up to 350 cm, which are used in time division multiplex for both sending and receiving. When transmitting, the laser beam is widened in order to be able to illuminate a small area on the moon around the selected reflector location, limited by the unrest in the air to about 70 km². The pulse energy is typically 100 mJ (APOLLO project: pulse duration 90 ps FWHM, pulse energy 115 mJ), which corresponds to about 3 · 10 18 photons at a wavelength of 532 nm ( Nd: YAG , frequency doubled ) . On average, less than one of these (with the 75 cm Wettzell telescope) or a few (with the 1.5 m telescope on the Côte d'Azur) are registered by the detector. Both stations work with a pulse rate of 10 Hz.

The exact positions of the telescopes in the ITRS are determined by time-of- flight measurements to navigation satellites. Some GPS and GLONASS and all Galileo satellites have retroreflectors.

Evaluation of the LLR measurement

Despite the use of narrow-band interference filters , the useful signal is completely lost in the stray light if you don't know where to look. The expected running time for a certain epoch and its rate of change (around 100 m / s) must be deducted. The running times reduced in this way should be stable over thousands of pulses (several minutes of measurement duration) and stand out as a sharp line from the background in a histogram . The position of this line represents the measured value for the epoch, a so-called normal point. Since 1970, more than 20,000 normal points have been determined. The analysis of the normal points succeeds with the help of extensive program packages. These contain an ephemeris part for the movement of the astronomical bodies on the one hand and a part which is used to estimate the parameters .

Results from LLR measurements

Laser distance measurements provide information on various aspects of the Earth-Moon system, such as geocentric coordinates of stationary or mobile laser stations, which can be determined with an accuracy of 3–5 cm. Since some of the laser stations are located on different continental plates, station movements can optionally be estimated in the form of drift rates with an accuracy of around 0.4 cm. Furthermore, important information on the shape of the moon ( inertia tensor , tidal deformation) can be derived from LLR data , as well as the increase in the distance to the moon by approx. 3.8 cm per year due to the tidal friction, which also slows the earth's rotation .

In addition, relativistically significant quantities and earth rotation parameters can be derived from laser distance measurements to the moon.

See also

Web links

Individual evidence

  1. ^ Richard A. Kerr: Decades-Old Soviet Reflector Spotted on the Moon. Science, 2010.
  2. ↑ Porcelli et al .: Lunokhod vs. MoonLIGHT retroreflectors. Conference contribution to the 21st IWLR, 2018, Canberra.
  3. ↑ M.Schneider et al .: The high-precision measurement of the moon movement (TU Munich) ( Memento from January 7, 2014 in the Internet Archive ) Status ~ 1997.
  4. ↑ Benoît Dasset et al .: Méthode historique d'évaluation de la distance Terre-Lune et du diamètre de la Lune. University of Aix-Marseille, 2005.
  5. ↑ ILRS: Standard for Retroreflector Arrays at GNSS Altitudes.
  6. ↑ International Laser Ranging Service: Normal Point Data .