Our Solar System
Observing Solar System objects will be possible for Gaia, although presenting a specific challenge because of their significant proper motions. They promise a rich scientific reward, with three areas of Solar System research which are expected to yield important results: minor planets, trans-Neptunian objects and perturbations of the Oort Cloud.
Minor Planets
Minor planets, most of which orbit in the asteroid belt between the orbits of Mars and Jupiter, are expected to be representative of the original population of planetesimals that formed in that region of the Solar System. They should provide important information about the gradient of mineralogical composition of the early planetesimals as a function of heliocentric distance.
Any study of the origin and evolution of the Solar System should investigate the main physical properties of minor planets and asteroids including masses, densities, sizes, shapes, and taxonomic classes, all as a function of their location in the Solar System. Present estimates are that Gaia will detect between 10 000 to 100 000 objects, compared with about 20 000 currently known.
To determine asteroid masses one must be able to measure the tiny gravitational perturbations experienced by these objects under the conditions of mutual close approach. At present asteroid masses have only been obtained for about 10 objects. Simulations have shown that Gaia should be able to provide accurate mass determinations for approximately 100 asteroids during its mission lifetime.
Asteroid densities can be obtained when the mass and shape of each object has been determined. Most asteroids are too small for their sizes to be directly measured with ground-based instruments. A few sizes have been determined using alternative techniques such as stellar occultation but these methods cannot provide large numbers of measurements for many objects. Gaia should be able to directly measure the diameters of more than 1000 asteroids. These measurements are essential for understanding the general process of collisional evolution of the asteroid belt objects.
Direct measurements of sizes, combined with simultaneous measurements of apparent magnitude at visible wavelengths, will provide the surface albedo for about 1000 objects. Albedo is directly related to the mineral characteristics of the surface layers. Asteroid albedos range from 0.03 to 0.5, with most of the higher albedo objects located in the inner part of the asteroid belt and lower albedo objects found mostly in the outer regions.
Gaia photometry will provide color-indices for the sample of observed asteroids covering a wide range of sizes. These measurements may form the basis for taxonomic classification for these objects as a function of size. This is important for studies of the origin of ordinary chondrites and for the effects of surface weathering due to Solar wind, cosmic rays and micro-impacts.
Preliminary simulations show that for the known asteroids the predicted ephemeris errors based on the Gaia observations alone, for up to 100 years after the end of the mission, are a factor of 30 better than the predicted ephemeris errors corresponding to the whole set of past, present and future ground-based measurements.
Trans-Neptunian Objects
Ground-based surveys in the past few years have discovered over 100 icy bodies beyond Neptune, members of a population called the Kuiper Belt. They are related to a wide range of outer Solar System bodies, such as the short-period comets, the Neptunian satellite Triton, and the Pluto-Charon system. The connection between Kuiper Belt objects (KBOs) and so many different Solar System bodies hints at a common origin in the outer Solar System. The Kuiper Belt is also our closest link to the circumstellar discs found around other main sequence stars, and an understanding of the physical processes operative in the Belt (now and in its early days) will mark a key step forward in understanding the problem of planetary formation. The known Kuiper Belt objects possess a hugely non-uniform distribution of orbital elements, falling into three distinct dynamical classes: (1) the Resonant KBOs which lie in or near the mean motion resonances of Neptune; (2) the Classical KBOs which have modest eccentricities but some of which have considerable inclinations; (3) the Scattered KBOs which are characterized by large, highly eccentric and highly inclined orbits.
Dynamical studies suggest that the Kuiper Belt most likely formed in situ in the trans-Neptunian region, with its current spatial distribution resulting from dynamical erosion by the planets.
Although the Kuiper Belt is the subject of intensive ground-based observations, some questions can only be answered by an all-sky survey such as Gaia. The Gaia sample will be unique because it will represent the complete sample of KBOs to V=20 mag, free of all directional bias, and it will contain all the brightest KBOs, which are the best targets for physical studies.
To date the only binary pair detected in the Kuiper Belt is the Pluto-Charon system. Is this system unique or are such binary systems common? Gaia should be able to address the question of how many Pluto-Charon type systems there are in the Kuiper Belt by virtue of its excellent astrometric capability. Gaia may also detect other Pluto like objects. Accretion models of the Kuiper Belt predict the formation of 1-10 Plutos. There is also unaccounted for mass in the Solar System which could be in the form of Pluto-like objects.
Oort Cloud
Our Solar System is surrounded by a spherical cloud of comets known as the Oort Cloud. The study of the possible perturbations of this cloud of comets by passing stars has important implications for our understanding of the Solar System. Gaia is ideally suited to contribute to these studies since it will provide complete and extensive data on the three-dimensional stellar space velocities in the solar neighbourhood. These data will allow us to investigate stellar encounters (past and future) with the Solar System.