It has been 50 years since the lunar module "Eagle" landed on the Moon. On board was a laser ranging retroreflector (LRR), which the astronauts positioned on the lunar surface. Several laser stations continue to use it today to analyze orbital characteristics of the Earth-Moon system. One of them is the Geodetic Observatory Wettzell, which is operated by the Technical University of Munich (TUM) in cooperation with the Federal Agency for Cartography and Geodesy. Prof. Karl Ulrich Schreiber explains why the reflector remains important for science today.
Professor Schreiber, how big an impression did the Moon landing make on you when you were young?
It wasn't just the Moon landing as such. What amazed me most were the steps that led up to it. It was a huge technical challenge. For example the question: How do I accelerate a rocket to reach the Moon? Back then, the technical possibilities were still quite limited. Each individual aspect of this megaproject involved all kinds of challenging issues. It was the amazing adventure that you could live through as a young person in the 1960s. My siblings, my classmates and I ate, slept and dreamed of the lunar missions. I think it also influenced my career choice.
What was it that fascinated you so much?
For me it was all about the technology. When I was young I was fascinated by the idea of people being able to move outside our normal habitat, in other words the Earth, and how a mission like that can be made a reality. After studying physics I received an offer to work at the observatory in Wettzell. And the Moon fever took hold of me once again. The work being done there involves laser-based distance measurements, which I was very interested in. But what motivated me most of all was the technical challenge.
There is also a link between the Moon landing and laser distance measurements.
To signal that the mission was not driven by military objectives, there was already a scientific component with the first Moon landing. There were basically two experiments: a seismometer, which was to collect data to learn about the internal structure and composition of the Moon, and a laser reflector. In the meantime there are already five reflectors on the Moon, placed at widely distributed locations. They can be used to make very precise measurements of the distance between the Earth and the Moon. It is not enough just to transmit laser pulses to the Moon because I have no way of knowing where the echo is coming from. Is the pulse bouncing back from the bottom of a crater or the top of a mountain, for example? Nor can I be sure that I am hitting the same point today as yesterday. When measuring distances to an accuracy in the centimeter range, the start and end points must be precisely defined.
How does this measurement actually work?
Laser ranging is a very elegant technology for measuring separations over very large distances. The principle is simple: I generate short laser pulses that take a certain time to cover the distance to the reflector. They bounce off it and return by the same path. On the ground, I can measure the total time very precisely. I multiply this time interval – approximately 2.7 seconds – by the speed of light, which is around 300,000 kilometers per second. Because we are measuring the distance to the Moon and back, I have to divide the result by two. Before I can use this measurement of the momentary separation, I have to apply some correction factors, for example for refraction. After the first Moon landing, it was possible to achieve accuracy within a few meters. Today we have a resolution of less than one centimeter. These precise distance measurements have many advantages when determining orbits, especially for satellites.
But how can this be used to calculate the Moon's orbit as well?
The Earth-Moon system can be seen as a laboratory for gravity experiments. This includes the ability to check some phenomena predicted by the theory of relativity – for example the weak equivalence principle. That principle relates to the gravitational behavior of bodies consisting of different materials. Do they exert the same forces of attraction on one another? There is a famous experiment in which David Scott, an astronaut on the Apollo 15 mission, dropped a hammer and a feather on the moon at the same time. The film record of Scott's experiment suggests that the equivalence principle is true. The two objects can be seen falling at the same speed. Our measurements enable us to observe what is happening thousands of times faster. The Earth has an iron core, while the Moon's core consists mainly of silicon. The laser measurements to the Moon would show whether the differences in composition of the two heavenly bodies results in a variation in the orbit. This would only become clear by tracking the Moon's orbit to an accuracy in the centimeter range over an extended period.
What was the result of the calculations?
Over the series of measurements lasting 50 years, the margin of error has become smaller all the time. Nevertheless, the theory of relativity has stood up to the test. The predictions of general relativity are so excellent that there is no sign so far of any contradiction.
You're also measuring the distance to the moon in Wettzell?
In Wettzell we are pursuing what is known as a fundamental station concept. It involves implementing as many different geodetic measurement procedures as possible at a single location. This makes it possible to identify systematic errors by comparing the methods and to combine the measurement procedures. Under that concept, it also made sense to incorporate the measurements of the distance to the Moon. This has a geodetic component because it involves the calculation of the orbit of a natural satellite of the Earth. But it also permits testing of the predictions of relativity theory. At present, only four or five observatories worldwide can provide these measurements because the required technology is so complex. The data are then combined. Wettzell is not a major player in this group, but one thing sets us apart: We deliver the smallest variance. For about a year, we have had the ability in Wettzell to use laser pulses with a duration of 10 picoseconds. That results in a theoretical resolution of 3 mm. This should make it possible to achieve greater precision.
- Geodetic Observatory Wettzell (In German)
Prof. Dr. Karl Ulrich Schreiber
Technical University Munich
Tel.: +49 9941 603113