Destination Lutetia ... 10 July 2010

Episode 1 : Shape ?

On July 10, 2010, the European spacecraft Rosetta will fly the asteroid 21 Lutetia at a distance of about 3000 km. At this moment, Lutetia is at a distance from Earth of 450 million km. It is located in the Main-Belt between Mars and Jupiter which is thought to be constituted by the building blocks of a planet prevented from forming by the mighty planet Jupiter. This small object whose size is estimated at a hundred kilometers owes its name to the Latin name of Paris where it was discovered in 1852. Seen from Earth, it appears as small as a one euro coin placed at a distance of 11km. Its telescopic observation is consequently very challenging. It requires the use of larger Earth-based telescopes if one wishes to know better the object, its shape, its own rotation before its close approach with Rosetta. This has been achieved by an international team led by Benoit Carry of Paris Observatory. With a two-year work on the 10-m telescope of the W.M. Keck Observatory and on the 8m VLT (ESO), it has been possible to reconstruct the three-dimensional shape of 21 Lutetia as you can see in the following movie (Fig.1).

Episode 2: Surface composition ?

Lutetia has been intensively studied for many years. One of the curiosities of this object lies in its surface composition. In the 80th, Lutetia was classified as a M-type asteroid, that is, a body roughly metallic. Lutetia was imagined as a large piece of metal, a possible remnant of a larger differentiated parent body.

However, other more recent spectroscopic observations suggest instead that Lutetia has in fact a composition similar to that of meteorites called "carbonaceous chondrites" (Fig. 2, Nedelcu et al, A & A, 2007). Moreover, the slight difference in spectral slope shows evidence of inhomogeneous surface, which highlights a distinct mineralogy in some regions.

These meteorites, collected and analyzed on Earth, contain many chondrules (from the Greek chondron whis means granule), which are tiny spheres of silicate a few millimeters in diameter inserted in a ferro-silicate matrix. These chondrules are the result of the rapid crystallization of silicate liquid drops during condensation of the solar nebula that gave birth to the solar system 4.56 billion years ago. It is therefore the oldest rocks in the solar system. Evidence for this elemental composition very similar to that of the Sun

Carbonaceous chondrites are characterized by their high content in carbon and volatiles (rare gases) compared to other types of chondrites. Carbon (up to 5% by mass) can be found in different forms: carbonate, silicon grains, diamonds, grains of graphite and organic matter.

 
 
 
 
 

Episode 3: Rotation curve or light curve

From the knowledge of the shape of Lutetia and the spatial orientation of its axis of rotation, it is possible to reconstruct the light curve. This light curve is but the temporal variation of its brightness during its rotation. The pear-shaped asteroid spins in the space around an imaginary direction as does the Earth.

The light is solar light scattered by the thin layer of dust on the surface. In this light there are also shadows, projected by the asteroid facing Sun while we are looking it aside.

This light curve is somehow the signature of the asteroid, which is renewed indefinitely every time a revolution has been completed. However this lightcurve will change as well, depending on the aspect of the asteroid as seen by a terrestrial observer.

Figure 4 shows the composite light curve of Lutetia obtained from photometric observations made at Makes observatory (Reunion Island) with a small telescope of 35cm (C14). The observations were collected during the nights of 16 and 17 May 2010. Each photometric measurement results from the registration of an image of the field of Lutetia in which the brightness of Lutetia was measured relative to the one of a same reference star. The plain curve is calculated from the digital 3D shape model (see Fig.1), the knowledge of the orientation of the spin axis in space and the period of proper rotation of the asteroid (P = 8.1683h). Figure 5 visualizes the aspect and rotation of Lutetia at the same time observations were made. The phase angle is almost 20 degrees. This is the angle that characterizes the angular range with which the terrestrial observer deviated from the asteroid-Sun direction. In comparison with the Moon, the phase angle is zero at the full moon, that is to say the Sun and an Earth-observer see the Moon in the same direction.

 
 
The image of Figure 6 was made on May 22, 2010 at 16:41 UTC with an exposure time of 48mn. Lutetia is then at a distance of 2.46 AU (astronomical unit which is the Sun-Earth distance) from Earth and has an apparent visual magnitude of 12.5. Note the rapid motion of the asteroid, leaving a small scratch on the M95 galaxy (barred spiral galaxy in Leo) located in the background at a distance of 38 million light-years.

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