Born and raised in France, Franck's passion for planetary astronomy developed over the years of his graduate school while traveling and living in Latin America (Mexico and Chile), Europe, and USA. He is particularly interested in developing and using ground-based telescopes equipped with Adaptive Optics systems to study and monitor phenomena in our Solar System.

The largest recently built telescopes are equipped with primary mirrors 8 to 10 metres in diameter, but surprisingly they do not provide images with quality better than a 20 cm telescope. The blurring effect caused by the Earth's atmosphere limits the image quality, and the only way to overcome this is to correct in real time using a complex instrument called Adaptive Optics (AO). The first AO systems were tested on 4m-class telescopes in Chile and the US in the 1990s, and today this technique is commonly available to take advantage of big telescopes. Franck has been using these AO systems to study the diversity of our Solar System, focusing his activity on monitoring volcanic activity on Io, a moon of Jupiter, searching for multiple asteroid systems, and studying Jupiter's atmosphere.

Our Solar System through the eyes of Adaptive Optics

Our Solar System, composed of the Sun, eight major planets, five dwarf planets, and millions of comets and asteroids, is characterised by a large diversity of targets with a broad scale of size and shape but also of interactions and phenomena. Each planet and their satellites are unique due to the siege of variable factors such as volcanic activity or collisions between planets and asteroids. To understand our Solar System and its evolution, we need to monitor it in detail. That's why the contribution of ground-based telescopes equipped with Adaptive Optics, or AO for short, which provide quality "as if the telescope was in space" has been increasing over the past 15 years.

Adaptive Optics on large telescopes

The charming twinkling of the stars that you may have noticed on a clear night is in fact the product of distortions caused by the Earth's atmosphere which bends and deforms the light coming from these stars. If you look through a telescope, you will see that this atmospheric distortion is unpredictable, blurring your view. In an attempt to limit this effect, astronomers built telescopes at high altitudes and far away from cities, like on the top of the dormant volcano Mauna Kea in Hawaii, or in the dry desert of Atacama in Chile.

Even when the blurring of images was reduced it remained significant and astronomers had to wait for the development of advanced computer calculation and technology to find a solution.

AO is a system which corrects in real time the effect of atmospheric turbulence. Analysis of the distortions is made and a correction is produced by deforming the telescope's mirror hundred of time per second using motors called actuators. Since 2001, the Keck II telescope has been equipped with an AO system. The first AO system on the Very Large Telescope in Chile was offered to the European community in 2003. AO systems are nowadays commonly used by astronomers in all fields, from extra-galactic, galactic to Solar System astronomy. They compete directly in image quality with the Hubble Space Telescope and other space-based observatories!

The most energetic eruption seen as it happened

On 20 February 2001, we pointed the Keck II telescope equipped with AO toward Io, a moon of Jupiter known for its volcanic activity. The exquisite resolution achieved using this 10 metre telescope revealed incredible detail on the surface, such as dark calderas and craters of volcanic origin. Two days later we re-observed the same side of Io and discovered a new a bright eruption which was visible because of the high temperature of its magma, its area and the type of volcanism (fire fountains). We later realised that this eruption was the most energetic ever witnessed, either on Io or the Earth! It covered an area of 1900 square kilometers, larger than the entire city of London.

Today, ground-based telescopes equipped with AO systems are the best tools for monitoring volcanic activity on Io. Io always surprises us, it is definitely one of the most interesting targets to observe from the ground. Our ultimate goal is to understand the nature and evolution of this exotic volcanism.

The existence of Multiple Asteroid Systems revealed with Adaptive Optics

In 2005, we announced the discovery of two tiny moons in orbit around 87 Sylvia, a 300km diameter asteroid located in the main belt. This system was the first asteroid trio discovered. Because 87 Sylvia was named after Rhea Sylvia, the mythical mother of the founders of Rome, we proposed to call the moons Romulus (18km) and Remus (7km). As AO systems became more reliable and efficient, it was possible to spot smaller and closer satellites around large asteroids. In 2007 we discovered that 45 Eugenia also possesses two satellites of 7 and 6km diameter. Two smaller satellites with a diameter of 5 and 3km were detected around 216 Kleopatra, an interesting elongated asteroid known to have a metallic composition. More recently, in 2009 our group announced that two tiny moons (less than 4km) orbit around 93 Minerva, a large 145km diameter asteroid.

More than just a curiosity, these moons have enabled us to determine the mass and density of the asteroids, revealing that the rock turns out to be extraordinarily porous, with up to 60 percent of its interior composed of empty space. Several scenarios have been proposed to explain the existence of these asteroids. We suggested that they were formed when two large asteroids smacked into each other and broke apart. Most of the fragments from the breakup reassembled into a loose agglomeration only held together by gravity. The satellites are probably the leftover debris of this catastrophic disruption.

Monitoring the satellite orbits over long time scales could give us accurate insights about these asteroids, such as the distribution of material in the interior and their surface properties, without having to develop expensive space missions.

The MAD future of Adaptive Optics

An important limit of Adaptive Optics systems is the fact that the corrections can only be done for small patches of the sky. This means that objects which appear large as seen from the Earth, like the giant planets Jupiter and Saturn, could not be observed using the full potential of AO systems. In 2007, the European Southern Observatory (ESO) developed the first prototype of the next generation Adaptive Optics system called Multi-Conjugate Adaptive Optics (MCAO). The ESO MCAO system uses information provided by three references, instead of the one used in conventional AO systems, to reconstruct a 3D estimate of the atmospheric turbulence above the telescope and thus provides a correction on a large field of view.

In 2008, we used this MCAO system to image Jupiter using as reference two moons located on both sides of the planet. This technique allowed us to observe Jupiter for almost two hours, a record duration since even the Hubble Space Telescope cannot observe Jupiter for more than 50 minutes. The image quality is twice as good as that provided by Hubble, not only because of MCAO but also because ESO's Very Large Telescope is much bigger. We even made a discovery: a major alteration in the brightness of the equatorial haze, which lies in a 16,000km-wide belt over Jupiter's equator.

AO systems are now more reliable than ever before. A dedicated MCAO system should be available in 2011 on the Gemini South telescope, allowing astronomers to regularly monitor the atmospheric activity of Jupiter and Saturn. The next generation of AO systems are being built and should provide perfect image quality and observations in the visible light. They are designed to allow astronomers to study exoplanets, planets in orbit around other stars. Finally, coupled with interferometry, a technique based on combining the light of several telescopes, AO systems will provide resolution equivalent to the image quality of a virtual telescope with a primary mirror hundreds of metres across. There is no doubt that with AO systems, many more discoveries will be made!.

This feature article was written as part of the Cosmic Diary Cornerstone project for the International Year of Astronomy 2009. To find out more, check out www.cosmicdiary.org and www.astronomy2009.org.