Climate research with gravitational wave technology

The two GRACE Follow-On satellites have been measuring the Earth’s gravitational field since mid-2018 to obtain important data for geophysics and climate research. On board is a laser interferometer that will serve as a model for future satellite missions and represents a step toward the LISA gravitational wave observatory.

An article by Gerhard Heinzel

GRACE Follow-On is the continuation of the highly successful German-American mission GRACE (Gravity Recovery and Climate Experiment), whose satellites orbited the Earth about 90,000 times between 2002 and 2017, measuring the gravitational field of our home planet. In the successor of the experiment, again, two satellites move around the Earth on identical orbits, 200 kilometers apart from each other and at an altitude of about 490 kilometers. The satellites orbit the Earth every 90 minutes on paths that cross the poles. Because of the Earth’s rotation, the satellites survey adjacent strips of the surface during successive orbits.

Over a large accumulation of mass, such as a mountain range, the Earth’s gravitational field is a tiny bit stronger than over open water, for example. This causes the first satellite to accelerate, increasing the distance between the two by a fraction of a millimeter – when the trailing satellite reaches the spot, it catches up again. From these small distance variations, collected over many orbits, the gravity field and thus the mass distribution of the entire Earth can ultimately be reconstructed.

Two satellites orbit the Earth at a distance to one another

GRACE Follow-On is a tandem of two satellites orbiting the Earth at a distance of 220 kilometers on the same orbit at an altitude of 490 kilometers above the Earth’s surface. The mission measures the distance between the satellites using microwaves (blue) and a new laser interferometer (red). © Earth: NASA “Blue Marble,” satellites: Schütze/AEI

Temporal changes in the gravity field are of high interest to geophysicists and climate researchers, as they can be used to measure indicators of climate change. These include, for example, the melting of the polar ice caps and changes in groundwater levels. However, these temporal changes in the gravity field are also much weaker than the static gravitational anomalies that result, for example, from the flattening of the Earth. The satellites must therefore measure their distance down to micrometer or even nanometer accuracy, to allow to distinguish the temporal changes from the static ones. The previous mission between 2002 and 2017, GRACE, successfully used microwaves to this end, with an accuracy of one to two micrometers.

A follow-up mission with future technology

The GRACE data, and in particular the long-term trends that could, for the first time, be measured with it, have become irreplaceable for climate research and related fields. Thus, around 2008, it was decided to launch a follow-up mission called GRACE Follow-On. It was supposed to continue the stream of valuable measurement data with as short a gap as possible after the end of the GRACE mission in 2017.

To save time and avoid the risks associated with entirely new developments, GRACE Follow-On was essentially planned as a copy of GRACE, with the same German-American team and the same instruments. That is why GRACE Follow-On also relies primarily on microwave measurements.

In addition, a future technology is being tested for the first time on GRACE Follow-On: laser interferometry. Researchers at the Albert Einstein Institute (AEI) in Hanover have been developing this technology for 20 years for the gravitational wave observatory LISA, which is intended to observe gravitational waves from astrophysical objects in space to explore the invisible, gravitational side of the universe. It will measure minute changes in distance between satellites using laser light. Many of the novel techniques required are also ideally suited for GRACE Follow-On.

A laser interferometer from Hanover

In 2008, NASA approached the AEI and proposed that they jointly develop such a laser instrument and use it on GRACE Follow-On. Thus was born the Laser Ranging Interferometer (LRI), which was planned, constructed and launched into space within eight years – an astonishingly short time for such projects.

The LRI is organized as a partnership program between NASA, the German Research Center for Geosciences (GFZ) in Potsdam and the Albert Einstein Institute, together with the German Aerospace Center (DLR) and the German space industry, funded by the German Federal Ministry of Education and Research (BMBF). NASA is supplying the laser, the resonator for frequency stabilization and the electronics for phase readout. Germany is supplying the optical system with associated electronics.

Elevation profile of the Himalayas, from remote sensing data of the Shuttle Radar Topography Mission, in the center of the right axis is Myanmar. Above, the filtered measurements of the novel Laser Ranging Instrument (LRI) during the overflight at 490 km altitude and 200 km separation. © G. Heinzel/Max Planck Institute for Gravitational Physics; GRACE-FO data: NASA, GFZ, JPL; Map data: SRTM

LISA technology in use for climate research

In May 2018, GRACE Follow-On launched on a Falcon 9 rocket from Vandenberg Air Force Base in California. Once in orbit, the satellites underwent some routine tests, and the microwave instrument started operation. Turning on the LRI’s laser and electronics was a particularly exciting moment for the AEI researchers: The tricky task was to adjust the laser beams from both satellites so that they hit the other’s receiver, which was a couple of centimeters in size, at a distance of 200 km. For this difficult task, the researchers had developed a search procedure in which the satellites spiraled the position of the laser beams. This search was successful, and the LRI started measurements on June 14, 2018. What was originally planned as a three-month experiment was still reliably delivering data two and a half years after launch. What is more, the LRI’s accuracy is orders of magnitude better than that of the microwave instrument, so that further follow-up missions will likely exclusively rely on laser technology and retire the microwave instrument.

Further Information

The German LRI contribution was financed by the Federal Ministry of Education and Research (BMBF, project 03F0654B) and the German Aerospace Center (DLR). The German Research Center for Geosciences (Deutsches GeoForschungsZentrum, GFZ) is in charge of the entire German contribution to GRACE Follow-On, which, in addition to the LRI, also includes the rocket launch and operations.

Find more information of GRACE FO on the website of the Max-Planck-Institute for Gravitational Physics. A YouTube video about the technology can be found here.

References

Klaus Abich et al. In-Orbit Performance of the GRACE Follow-on Laser Ranging Interferometer. Phys. Rev. Lett. 123 (3),031101 (2019)

B. S. Sheard, G. Heinzel, K. Danzmann, D. A. Shaddock, W. M. Klipstein und W. M. Folkner. Intersatellite laser ranging instrument for the GRACE follow-on mission. Journal of Geodesy, 86, 1083-1095 (2012)

B. D. Tapley, S. Bettadpur, J. C. Ries, P. F. Thompson und M. M. Watkins. GRACE Measurements of Mass Variability in the Earth System. Science, 305, 503-506 (2004)

Colophon

Gerhard Heinzel

Gerhard Heinzel is a physicist at the Max Planck Institute for Gravitational Physics in Hanover and leader of the space interferometry research group.

Citation

Cite this article as:
Gerhard Heinzel, “Climate research with gravitational wave technology” in: Einstein Online Band 13 (2021), 1005

Definitioner

Radar

Abkürzung des englischen "Radio Detection and Ranging", etwa: Nachweis und Abstandsbestimmung mit Hilfe von Radiowellen. Auf der Erde eingesetzt u.a. im Flug- und Schiffsverkehr; für die Allgemeine Relativitätstheorie interessant, da sich durch an Planeten reflektierte Radarsignale beispielsweise die relativistische Lichtlaufzeitverzögerung nachweisen lässt.

Laser

Abkürzung für "Light Amplification by Stimulated Emission of Radiation", zu deutsch etwa: Lichtverstärkung durch stimulierte Strahlungsaussendung. Verfahren zur Erzeugung sehr konzentrierten, starken Lichtes mit fester Frequenz, das in Form einer sehr gleichförmigen elektromagnetischen Welle ausgesendet wird. Dabei haben alle ausgesendeten Wellen jeweils zur gleichen Zeit ihre Maxima und Minima ("kohärentes Licht").

wave

In a general sense: any travelling pattern, whether or not it involves matter being transported as well. Simple examples are water-waves - wave crests and troughs travelling over a water surface, and a Mexican wave in a football stadium, with fans alternately standing up and sitting down - the pattern moves throught the stadium, not the fans themselves.

An especially simple form for a wave is a sinus wave, a regular pattern of wave crests and troughs.

time

It is a fact of life that not all events in our universe happen concurrently - instead, there is a certain order. Defining a time coordinate or defining time, the way physicists do it, is to define a prescription to associate with each event a number so as to reflect that order - if event B happens after event A, then the number associated with B should be larger than that associated with A. The first step of this definition is to construct a clock: Choose a simple process that repeats regularly. (What is "regular"? Luckily, in our universe, all elementary processes such as a swinging pendulum, the oscillations of atoms or of electronic circuits lead to the same concept of regularity.) As a second step, install a counter: A mechanism that, with every repetition of the chosen process, raises the count by one. With this definition, one can at least assign a time (the numerical value of the counter) to events happening at location of the clock. For events at different locations, an additional definition is necessary: One needs to define simultaneity. After all, the statement that some far-away event A happens at 12 o'clock is the same as saying that event A and "our clock counter shows 12:00:00" are simultaneous. The how and why of defining simultaneity - a centre-piece of Einstein's special theory of relativity - are described in the spotlight topic Defining "now". With all these preparations, physicists can, in principle, assign a time coordinate value ("a time") to any possible events, and describes how fast or how slow processes happen, compared to that time coordinate.

surface

Geometric space with two dimensions. Examples include the plane or the surface of a sphere.

space

In a strict sense: Space as we know it from everyday life: the totality of all locations in which objects can sit, with three dimensions. In a more general sense used by mathematicians, all kinds of sets of points are spaces - a line for instance, which has but a single dimension, or a two-dimensional surface, but also higher-dimensional spaces. Also, in such more general spaces, geometry can be different from the standard Euclidean geometry taught in high schools - such spaces can be curved.

planet

Planets are not-too-small companions of a star that are not stars themselves (nor ever were stars). In our solar system, the planets are, listed from the one closest to the sun to the one farthest: Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, Neptune. As of August 2006, Pluto, which used to be a proper planet, is officially a "dwarf planet". In the night sky, the distinguishing characteristic of planets is that they move around relative to the unchanging background of stars - which gave them their name, loosely translated from Greek as "wanderers".

NASA (National Aeronautics and Space Administration (NASA))

Part of the US government in charge not only of manned space missions, but also responsible for numerous highly successful satellite and probe missions. NASA is a partner in projects such as the Hubble space telescope or the gravitational wave detector LISA. NASA website

microwaves

Electromagnetic radiation with wave-lengths between a millimetre and 30 centimetres, corresponding to frequencies between some and some hundred billion oscillations per second. Looking up to the night sky, almost all energy that reaches us of the cosmic background radiation is in the form of microwaves.

AEI (Max Planck Institute for Gravitational Physics/Albert Einstein Institute)

See Albert Einstein Institute.

mass

In classical physics, mass plays a triple role. First of all, it is a measure for how easy it is to influence the motion of a body. Imagine that you're drifting in emtpy space. Drifting by are an elephant and a mouse, and you give each of them a push of equal strength. The fact that the mouse abruptly changes its path, while the elephant's course is as good as unaltered, is a sure sign that the mass (or, in the language of physics, the inertia or inertial mass) of the elephant is much greater than that of the mouse. Secondly, mass is a measure of how many atoms there are in a body, and of what type they are. All atoms of one and the same type have the same mass, and adding up all those tiny component masses, the total mass of the body results. Thirdly, in Newton's theory of gravity, mass determines how strongly a body attracts other bodies via the gravitational force, and how strongly these bodies attract it (in this sense, mass is the charge associated with the gravitational force). In special relativity, one can also define a mass that is a measure for a bodies resistance to changing its motion. However, the value of this relativistic mass depends on the relative motion of the body and the observer. The relativistic mass is the "m" in Einstein's famous E=mc² (cf. equivalence of mass and energy). The relativistic mass has a minimum for an observer that is at rest relative to the body in question. This value is the so-called rest mass of the body, and when particle physicists talk of mass, this is usually what they mean. Just as in classical physics, the rest mass is a kind of measure for how much matter the body is made up of - with one caveat: For composite bodies, the energies associated with the forces holding the body together contribute to the total mass, as well (another consequence of the equivalence of mass and energy). In general relativity, mass still plays a role as a source of gravity; however, it has been joined by physical quantities such as energy, momentum and pressure.

light

Light in the strict sense of the word is electromagnetic radiation the human eye can detect, with wave-lengths between 400 and 700 nano metres. In relativity theory and in astronomy, the word is often used in a more general sense, encompassing all kinds of electromagnetic radiation. For instance, astronomers might talk about "infrared light" or "gamma light"; in this context, light in the stricter sense is referred to as "visible light". Within classical physics, the properties of light are governed by Maxwell's equations; in quantum physics, it turns out that light is a stream of energy packets called light quanta or photons. In the context of relativistic physics, light is of great interest, and for a number of reasons. First of all, the speed of light plays a central role in both special and general relativity. Also, there are a number of interesting effects in general relativity which are associated with the propagation of light, namely deflection, the Shapiro effect and the gravitational redshift.

laser

Abbreviation for "Light Amplification by Stimulated Emission of Radiation". Technique for the production of very concentrated, strong light with a fixed frequency, which propagates as a very simple electromagnetic wave in which wave crests and wave troughs are in perfect step ("coherent light").

gravity

See gravitation

gravitational field

The totality of all gravitational influences that one or more massive objects can exert on bodies in their vicinity.

More precisely: At every location in space, the gravitational field is defined as the acceleration that a small test particle present at that location would feel due to the gravitational forces of the masses around it.

gravity (gravitation)

In classical physics: An action-at-a-distance force by which all bodies that possess mass attract each other (see Newtonian theory of gravity), synonym: gravitational force. In Einstein's general theory of relativity: The fact that matter that possesses mass, energy, pressure or similar properties distorts spacetime, and that this distortion in turn influences whatever matter might be present. An introduction to the basic ideas of general relativity is provided by the section General relativity of Elementary Einstein. More information about the nature of gravity in general relativity can be found in the spotlight text Gravity: From weightlessness to curvature.

frequency

Measure for the rapidity of an oscillation, defined as the inverse of the period of oscillation: A process that, in oscillating, repeats itself after 0.1 seconds has the frequency 1/(0.1 seconds)= 10 Hz. (The unit Hertz, abbreviated as Hz, is defined as 1 Hz = 1/second.)

For a simple wave, the frequency is given by the number of maxima going by a stationary observer in a second. Ten maxima going by per second correspond to a frequency of 10 Hz.

field

A field describes how a physical quantity is distributed in space and time. For instance, the area where electric forces act on a test particle is subject to an electric field. Or the gravitational forces which act on the mass of a test body define a gravitational field. In general a field contains energy, occupies space and can change over time.

Earth

Our very own planet in the solar system - the third planet from the sun. The earth has a mass of about 6 trillion trillion (in exponential notation, 6·10²⁴) kilograms.

charge

A measure of the strength of a force (action-at-a-distance) originating from a body, and of how susceptible it is to being influenced by other bodies via the same force. The most famous example is electric charge: Electrically charged bodies act on other electrically charged bodies via an electric force whose strength is proportional to the electric charges of the bodies involved. It is a characteristic property of charges that they are conserved; they can neither be created from nothing nor simply disappear. For instance, when a positron with electric charge +1 (in suitable units) and an electron with electric charge -1 annihilate to give electromagnetic radiation, overall charge conservation is satisfied: Before the annihilation, the sum of the electric charges was 1+(-1)=0, and afterwards, when there is only uncharged electromagnetic radiation left, it is also zero. In the context of particle physics, there are more abstract charges not directly connected with forces and interaction, but subject to similar conservation laws.