ESA Science & Technology - ESA to look for the missing link in gravity

Scientists will study space phenomena that do not seem to conform to our perceived understanding of gravity. In this way, they hope to develop a greater comprehension of the Universe.

Gravity is one of the four fundamental forces of nature. It shapes the Universe around us, allowing planets, stars and galaxies to form. However, the more scientists study gravity and its effects on celestial objects, the more mysteries they seem to uncover. One example is the so-called 'Pioneer anomaly', named after the NASA space probes Pioneer 10 and 11, on which the effect was first noticed. The anomaly was revealed when a number of spacecraft were seen to be affected by an unknown force that slowed them down. The same behaviour has now been detected on NASA's Galileo and the joint ESA-NASA Ulysses spacecraft.

Scientists have known for a long time that there appears to be 'too much' gravity in the Universe. They can observe the effects of gravitational forces at work, but the origin of these forces cannot be identified. This 'excess' of gravity is usually referred to as 'the missing mass problem', since scientists assume that only matter can create gravity. It is therefore supposed that the Universe is filled with large quantities of 'dark matter' that has yet to be detected. What if that assumption is wrong?

Some theories suggest that gravity might pull a little harder at extreme distances than had previously been considered, so the concept of dark matter may not even be necessary. Alternatively, the anomalies may be the result of a fifth force of nature: one that is very weak and only shows up in the remotest regions of space. Space is an ideal testing ground to examine the existing theories. In the apparent weightlessness of space, scientists can detect the most delicate of forces and can measure them with extreme accuracy.

Developing an ambitious series of space experiments and missions, ESA is focusing its efforts on testing Albert Einstein's Theory of General Relativity, the most advanced description of gravity ever formulated. One of the first objectives is the detection of gravitational waves. General Relativity has predicted their existence but, so far, they remain undetected. These waves should travel through space like ripples on a pond. LISA, a joint ESA-NASA mission, will be the first space mission to attempt to detect such gravitational waves. Finding them would be the ultimate test of General Relativity. A second objective, to be tested by the ESA Gaia and BepiColombo missions, will be to measure precisely how matter distorts space, searching for any deviation in the amount predicted by General Relativity. Microscope, a mission carried out in coordination with the French National Space Agency (CNES), is designed to test a concept from General Relativity called The Principle of Equivalence. According to this, objects are accelerated by gravity in the same way, independent of their mass and chemical composition. If Microscope detects a violation of this principle, it could be the clearest sign yet of a new dimension to gravity, known as quantum gravity.

Quantum gravity is a much-sought-after theory. Its purpose is to reconcile Einstein's General Relativity with quantum physics, the most advanced theory describing the fundamental forces in Nature, with the exception of gravity. Quantum gravity supposes that space is granular on the smallest of scales. In a similar way, for example, a beach appears smooth from a distance but is actually composed of individual pieces of sand. Hyper, a mission currently under study at ESA would attempt to detect the quantum granularity of space, as one of its investigations into gravity. Looking further into the future, ESA has taken the first steps in defining a mission which would examine directly the Pioneer anomaly.

With this series of missions, ESA will carry out a unique investigation into the very nature of gravity. This may well provide the next fundamental breakthrough in our understanding of the Universe.

Note to editors

LISA

The Laser Interferometer Space Antenna (LISA) is a joint mission with NASA. It is a three-spacecraft mission, designed to detect the 'ripples' in space given out by massive black holes. Such ripples are called 'gravitational waves' and are a prediction of Einstein's General Relativity. As yet, they have never been observed and LISA will be the first mission to attempt detection from space. It is scheduled for launch in 2011.

Gaia

Gaia is a mission that will conduct a census of one thousand million stars in our galaxy. It will monitor each of its target stars about 100 times over a five-year period, precisely charting their movements and changes in brightness. Gaia will be launched on a Russian Soyuz-Fregat rocket sometime in the period 2010-2012. It is expected to discover hundreds of thousands of new celestial objects, such as extrasolar planets and failed stars called brown dwarfs. Within our own Solar System, Gaia should identify tens of thousands of asteroids. During its investigations, Gaia will also be able to measure precisely how matter distorts space, so bending starlight, and will search for any deviation in the amount predicted by General Relativity.

Microscope

Microscope (MICROsatellite ` trannie Compensie pour l'Observation du Principe d'Equivalence) will test the Equivalence Principle of the general theory of relativity. According to this principle, all objects, independently of their mass and composition, acquire the same acceleration when subject to a gravitational field. Microscope will find out if this principle is correct and universal. If the principle is violated, it may reveal a new, as yet undiscovered, natural force or phenomena at work. It may give us a more complete understanding of the true nature of gravity and of the laws of nature. Scheduled for launch in 2005, Microscope is a joint mission with the French National Space Agency (CNES).

BepiColombo

BepiColombo will be a collection of three spacecraft that will provide the most complete exploration yet of Mercury, the innermost planet. The technology required for BepiColombo is being developed at present and the mission is scheduled for launch in 2011. One component of BepiColombo will map the planet, another will investigate its magnetic field and a third will land on Mercury, to study the surface. Among other investigations, BepiColombo will be able to measure precisely how matter distorts space, searching for any deviation in the amount predicted by General Relativity. With BepiColombo, Mars Express, and Venus Express, ESA is the only space agency in the world with current plans to visit each planet in the inner Solar System.

Contact details

Dr Michael Perryman
senior astrophysicist
ESA-ESTEC
The Netherlands
Tel.: +31 71 565 3615
E-mail: Michael.Perrymanesa.int

Science Programme Communication Service
ESA-ESTEC
The Netherlands
Tel.: +31 71 565 3223
E-mail: Monica.Taleviesa.int