The solar system, is all the bodies gravitationally linked to the Sun: the eight planets, their natural satellites (usually called "moons"), the dwarf planets, and the billions of small bodies (asteroids, frozen objects, comets, meteoroids, interplanetary dusts, etc.)

In a simplified way, the solar system is divided between the Sun, four inner telluric planets, an asteroid belt composed of small rocky bodies, four external gazeous giants and a second belt called Kuiper belt, composed of frozen objects. Beyond this belt a disk of scattered objects can be found, the heliopause and, according to the theory advanced by Jan Oort, the Oort cloud.

The main elements composing the solar system are thus:

The eight planets (Mercury, Venus, the Earth, Mars, Jupiter, Saturn, Uranus and Neptune by increasing distance from the Sun) have each very specific characteristics.
Seven great size satellites: the Moon, the 4 Jupiter's galilean satellites (IO, Europe, Ganymede, Callisto), Titan Saturn's satellite and Trito Neptune's satellite which sizes are greater than Pluto's, and for two of them (Ganymede and Titan), greater than Mercury's. They are world in themselves, for which certain, like Io or Titan, have very peculiar characteristics.
The dwarf planets, of which Pluto, Eris and Ceres. According to the official definition of August 24th, 2006, dwarf planets are bodies in orbit around the Sun, with a sufficient mass to be nearly spherical, but has not cleared the neighbourhood around their orbit.
To these big size bodies, the small solar system bodies can be added: small satellites, asteroids and comets.

All those described bodies haven't travelled a lot in the solar system, keeping orbits near the ones where they were created, even if recent studies seam to propose migrations of giant planets such as Neptune to explain some observations. It isn't such a case for comets (little objects of a few kilometers diameter), very probably formed in the external solar system, but which, ejected far from the Sun, have past most of their history very far from the Sun. This remoteness allows them to keep big quantities of volatile components (ice such as water ice, carbonic ice, ...), which, when the comet is diving in direction of the Sun, by the passing of a nearby star, give birth to a several millions kilometers long gas and plasma tail.
The study of the solar system contitutive bodies progressed very inequally. Four main steps can be distinguished:

the exploration by rapid fly-bys,
the detailed study by remote sensing from orbit,
the in situ analyses,
the samples return.

The scientific issues that are present in the solar system study can be linked to some great themes such as the solar system origin, the life origin and compared planetology.

The solar system exploration main steps can be presented as a geographical approach as well as a chronological one.

First of all, the Moon was the first objective of the planetary exploration and the three phases (exploration, detailed study and samples return) happened swiftly from 1959 to 1973. But manned flights to the Moon upseted the chronology by bringing back samples even before finishing the detailed mapping or the in situ analyses.

The Moon is by far the most known body in the solar system. About 400 kg samples have been collected, in which several 2 to 3 meters deep drill samples. Those samples enabled to show that the Moon was formed at the same time than the Earth by a giant impact with a body which size was about Mars one, but its evolution stopped very early due to its small dimension.

Thus, its exploration presents significant gaps because the missions main purpose was the preparation of inhabited flight and the Moon's detailed observation has for a long time covered only about 20% of its surface mainly in the equatorial zone. This gap was at last filled with the Clementine mission in 1994 and Lunar Prospector in 1998.

Recently, international interest in the Moon exploration is back with the arrival of new protagonists such as Japan, India and China.

The forthcoming projects are focalized on well defined areas on the Moon, the first is South Aitken Basin where the Moons mantle could outcrop, and then the polar craters that never see the Sun and could trap great quantities of water ice, or even the young craters such as Copernic or Tycho. The precursor mission for these dedicated projects is LUNAR RECONNAISSANCE ORBITER fo the NASA which concentrated in 2009 on the volatiles trapping at the Moon's poles.

During the next decade, a majority of projects will use astromobiles dedicated to such or such area. Theses projects are generally coupled to technological developments, which goals are 1/ a strategy of sample return from regions unexplored by APOLLO and LUNA missions, 2/ A comeback of inhabited missions to the Moon. At last, there is the problem of the internal structure of the Moon which is still poorly known due to the lack of seismic data. Several projects envision to drop off a network of seismic stations.

The telluric planets constitute the inner solar system: MERCURE, VENUS, EARTH, MARS.

Mercury is the planet nearer to the Sun, its diameter is comprise between the Moon's and Mars'. Paradoxically, Mercury is a very high cost target on the energetic level as Jupiter's even if it is located 8 times nearer to the Earth.

For a long time only one mission had visited Mercury (the american Mariner 10 mission, between 1974 and 1975). Three successive fly-by enabled to performe several measurements (temperatures, magnetic field,...) and to record thousands images of the planet covering nearly half its surface showing a mean of 1 to 2 km details.
Only thirty years later, in 2004, a new mission towards Mercury has been launched: the American probe MESSENGER. Several fly-bys of the planet have been scheduled before the in orbit insertion in 2011.

The european BEPICOLOMBO mission should fill this gap by a detailed study of Mercury. BEPICOLOMBO, nowadays in development phase, will supply a large set of coordinated observations of the planet and its environment. The launch is planned for 2014, with in-orbit insertion in 2020.

VENUS is a planet which dimension, mass and density can be compared to the Earth's. The observation of the similarities and differences in the evolution of those two so similar bodies is of great interest in compared planetology.

A major difference which complicates Venus study, is its extremely dense atmosphere constituted of carbonic gas and a very thick cloud cover.
This renders difficult its surface observations from orbit. Two methods are still available: to land probes down to the surface or to use microwaves (radar system).

The first probes observations revealed that the greenhouse effect due to carbonic gas keep very high surface temperatures of about 450°C, which combined with the high pressure, contitutes a difficult environment for automatic systems.

Another major difference with the Earth is the very great rotation period (243 days retrograde). This low rotation speed is probably due to its thick atmosphere and to tide forces which contributed to slow down the planet rotation.

Spatial probes sent by Russians and americans enabled to determine the thermal profile and the atmosphere composition function of the altitude, to determine the upper rocks chemical composition and to obtain the planet's global map with a resolution of a few hundred meters. The VENUS EXPRESS european mission, to be launched by the end of 2005, is an orbital mission around Venus dedicated to the observation of the atmosphere of this planet. It supplies a detailled knowledge of the high atmosphere as well as information on the CO and H2O content of the atmosphere under the clouds.

The nature of the balance, or imbalance, between rock surface and atmosphere, remains badly understood. The understanding of Venus climate evolution and stability requires now the in situ characterization of the surface rocks and of the atmosphere in contact with these rocks, as well as the gas exchange flux with the atmosphere.
Due to the similarities and the remarquables disparities between the Earth and Venus, its exploration is a major goal for the modern planetology.

Mars is by several aspects closer to our planet, by its mean temperature, but also by processes and geological forms existence similar to those observed on Earth (volcanoes, faults, canyons).

Two american missions enabled to obtain a global cartography of the martian surface (Mariner 9) and to realize a more detailed analyse of the ground by landing automatic laboratories (Viking).

No trace of living organisms (in the terrestrial meaning) have been found. The martian ground doesn't seem very favourable. It is bathed in Ultra Violet rays, and a heavy chemical activity renders it extremely toxic.

Since then other american missions have been exploring Mars, from its orbit (Mars Global Surveyor in 1996, Mars Odyssey in 2001, Mars Reconnaissance Orbiter in 2005), as well as its surface (Mars Pathfinder in 1996, Mars Exploration Rover in 2003, Phoenix in 2007).

After the russian mission MARS-96 failure to which were associated numerous european and french scientists, ESA decided to realize for 2003 a mission to Mars planet (MARS-EXPRESS). This mission enabled the boarding of the main french and european instruments developped for MARS-96.

Since the begining of 2004, Mars Express spacecraft have been studying the atmosphere of Mars, the structure of the planet and its geology. MARS EXPRESS has shown that Mars has a recent volcanic activity (<100 Ma). Even reduced, this activity is similar to a sleeping planet activity, but not yet stopped, without understanding of its evolution. On the contrary, the magnetic field, which is induced by the core, is now stopped, but left magnetized grounds dating from a primeval activity period. This magnetic field is a fundamental factor in the atmosphere evolution through time, due to its capability to protect from the solar wind. Thus, internal dynamics and magnetic field are very badly known despite their importance for the coupling of internal layers with the past and present external layers (degassing, escape). The fundamental lacunes on Mars internal structure (core size, mantle mineralogy, crust thickness, etc.) must be filled to bring constraints to the internal dynamics models.
On the other hand, one of the main discoveries of Mars Express mission concerns the presence of hydrated minerals (phyllosilicates or silts), potentiel witnes of the stable existence of large quantities of liquid water during the early history of Mars.

Nowadays CNES participates to several future exploration missions of Mars such as Mars Science Laboratory (MSL), a Nasa mission, or EXOMARS, an Esa mission. Roskosmos' PHOBOS GRUNT mission main objective is to collect and bring back samples of Mars moon, Phobos, on Earth. Other missions are also scheduled in a near or far future, such as a "NETLANDER type mission" (geophysics network) and Mars Sample Return.
 

The giant planets, Jupiter, Saturne, Uranus and Neptune present several common points:

They constitute the external solar system and their characteristics reflect the conditions prevailing in this region at the moment of their condensation from the primitive solar nebula. Pluto represents a special case and could be an ancient asteroid from Kuiper belt which trajectory could have been swerved by Neptune during a near gravitational encounter.
from 15 to 300 times more massives than the Earth, they are essentially constituted of gas (hydrogen and helium) surrounding a silicate and ice rocky core with a size greater than the Earth.
they possess a serie of satellites and small size particles rings, more or less developped.

Their exploration pose two specific problems linked to their great distance from the Sun:

the energy necessary to reach them is important and requires heavy launchers in direct trajectory, or gravitational assistancy complex strategies.
the missions durations become rapidly very long. For example, a direct trajectory from Earth to Uranus has a typical duration of about 10 years !

The study of the external solar system is thus less advanced than the inner planets one.

The VOYAGER american mission took advantage of an exceptionally favourable configuration that happens about every 180 years.
The four giant planets were then nearly aligned enabling their successive fly-by. The VOYAGER 2 probe could thus, by successive rebounds, visited Jupiter, Saturne, Uranus and then Neptune. The VOYAGER 1 probe visited Jupiter then Saturne and then concentrated on a Titan close fly-by, the biggest Saturn's satellite.

This mission is the basis of our present knowledge of the external solar system. The very turbulent aspect of Jupiter and Saturn atmospheres was revealed, the Jupiter's great red spot appearing as a giant cyclonic system. The satellites systems revealed an extraordinary diversity, which enable to compare them to mini solar systems.

The CASSINI-HUYGENS mission associating ESA and NASA, was launched in october 1997 with great success. This mission's goal is the study of the Saturne system and particularly its satellite Titan. CASSINI is an orbiter developped by NASA which reached the vincinity of Saturne in July 2004. In December 2004, CASSINI dropped HUYGENS probe (developped by ESA) in direction of Titan to study its atmosphere during its landing. On January 2005, 14th, HUYGENS realized its descent through the atmosphere until its landing 2h32m later.

The mission objectives can be summed up as:

Detailed study of the atmosphere, the rings and Saturne's magnetosphere,
Close study of saturne's satellites, Titan atmosphere characterization.

Cassini mission unveilled titan's surface which appears extraordinarily complex. The moon shows numerous lakes and fluvial networks, proof of a real methane and ethane cycle, probably still active, as well as numerous and varied tectonic and volcanic structures. One of Titan's main attraction is its wealth in organic compounds detected in its atmosphere and on its surface. The complex organic chemistry taking place in the atmosphere, the lakes, on the surface or in the hypothetical sub-surface ocean is still badly known.

Physical characteristics of the planets

Orbital characteristics of the planets

This term was adopted on August 24th, 2006 by the International Astronomical Union. In June 2007, three bodies have been officially recognized as dwarf planets, Eris, Pluto and Ceres (decreasing size order). According to Michael E. Brown, discoverer of Eris, about fifty other bodies could soon be added to this designation, then hundred of others in the future.

The next Nasa mission, DAWN, launched in 2007, should be inserted into orbit around the dwarf planet Ceres in 2015.

The small solar system bodies represent from far the more numerous family (over 900 comets and more than 100,000 asteroids have been observed).
They are generally considered as the last survivors of the "planetoids", intermediate phase of the planets accretion: similars to the comets for the external solar system, to the asteroids for the telluric planets. The major interest of those objects is thus the information that they can provide about the formation and the first phases of the solar system evolution.

The study of those objects is very far behind the planetary systems one. It's the return of the Halley comet (in 1986) with it exceptional scientific as well as mediatic interest that leaded to the first exploratory missions. Five probes have been launched in which the European probe GIOTTO.

The images obtained by GIOTTO revealed a very dark surface, with a reflectance near 4%. On the other hand the gas and dust emission which constitutes the cometary activity originated in a few regions covering less than 10% of the surface. The essential of the surface is thus covered by a heatresistant crust.
The chemical analysis of the dusts shown a very important proportion of organic componant very superior to rich carbon meteorits one.

ROSETTA (ESA mission) aims at realizing a rendez-vous with Churuymov Gerasimenko comet to study its nucleus and its activity when nearing the Sun. During its cruise, the probe will fly over several asteroids and will collect data.

Very few asteroids have been closely observed by a spatial probe.
Those bodies which size ranges from a few kilometers to nearly 1000 km, orbit between Mars and Jupiter. Another set, named Kuiper Belt, orbits beyond Neptune.

The main information that are available come from two sources: the observations from the Earth and the laboratories analysis of meteorites.

Asteroids thus appear extraordinarly varied: from very dark objects, redish, probably similar to cometary nucleus very lowly active, to rocky or metallic bodies.

To this day, a number of space missions have realised fly-by of asteroids (951 Gaspra, 243 Ida/Dactyl, 253 Mathilde, 9969 Braille, 5535 Anne Frank, Stein) or of comets (1P/Halley, 19P/Borelly, 81P/Wild 2, 9P/Tempel 1). Only two missions have been specifically dedicated to the study of geocruiser asteroids (Near Earth Objects, NEO) : NEAR-Shoemaker (NASA) and HAYABUSA (JAXA). The next Nasa mission, DAWN, launched in 2007, should be inserted into orbit around the asteroid 4 Vesta (in 2011) before reaching the dwarf planet Ceres (in 2015).