Minor Planets & Near Earth Objects
During the course of its mission, Gaia will map all sources brighter than visual magnitude ~20. Among these will be thousands of Solar System objects, primarily main belt asteroids circling the Sun between the orbits of Mars and Jupiter. With its ability to detect faint and fast-moving objects, it is expected that Gaia will also detect several thousand Near-Earth Objects (NEOs), which are thought to be comets and asteroids that have been nudged by the gravitational attraction of nearby planets into orbits that allow them to enter the Earth's neighbourhood.
Favourable observing conditions
Gaia will operate from the second Lagrange point L2 of the Sun-Earth system and will be able to observe down to an angular distance of 45° from the Sun. This allows for observations of objects in the asteroid blind spot between the Sun and Earth and to discover small bodies orbiting the Sun inside the Earth's orbit - a region that is virtually unobservable from the Earth. Objects that are in more exotic orbits are also potentially discovered by Gaia, during observations of the sky regions far from the ecliptic.
Gaia will accurately measure the positions and velocities of asteroids over the five years of the mission, leading to a determination of their orbits with an unprecedented precision.
The tiny gravitational pull experienced by asteroids during close approaches between two bodies alters their path. This small deviation will be recorded in the Gaia astrometric measurements, from which the object's mass can be derived. The expected number of asteroid masses to be determined this way is ~100, as compared to the approximately 20 known today.
Gaia will perform multi-epoch photometric observations of the Solar System's minor bodies. These observations will reveal their surface properties and composition by the amount of light that the minor bodies reflect at particular wavelengths.
A refined classification of the population of minor bodies will emerge from the giant data base, revealing the kinship between asteroids, NEOs, and meteorites. In addition, the variation of physical parameters with the distance to the Sun will also be studied.
Associated with each Sun-planet system are five Lagrange points where a third (small) object can remain and corotate with the planet about the Sun. Small Solar System bodies librating in orbits stable over the age of the Solar System around the L4 and L5 Lagrange points (respectively leading and trailing by 60° in the planet's orbit) are referred to as Trojans.
Apart from a few of these type of objects recently discovered in orbit with Neptune, and some as yet to be confirmed for Mars, the best known and studied groups are the Jupiter Trojans comprising some 2000 objects. Gaia will help shed light on several aspects of these Trojans.
Orbit and location
One of the outstanding issues for the Trojans is their formation history: were they formed in the same region where they are found today, or were they formed elsewhere and subsequently trapped in their current orbit? As such, can they be considered a class of objects in their own right with distinctive physical properties, or are they a subset of asteroids? Gaia's precise astrometric measurements will improve the accuracy of derived orbits.
Using photometry, good estimates can be derived for rotation periods, spin axis orientations and overall shapes for many of the Trojans. For the largest members (diameter> ~100 km) Gaia will be able to determine their sizes.
Gaia will measure the Trojan's spectral-reflectance to derive their chemical composition. This can be compared to other small bodies throughout the Solar System, including asteroids (from Earth-orbit crossing and main-belt asteroids to the Centaurs beyond Jupiter), trans-Neptunian objects and comets. The comparison will provide information on the original gradient in composition of the planetesimals in the early phase of the Solar System.
L4 versus L5
The Gaia data will allow for investigating possible differences between the two clouds of Trojans for all of their observed characteristics. There are hints that for example their distribution over orbit inclinations are slightly different.
Trans-Neptunian Objects & Centaurs
Planetoids (small Solar System bodies) with their perihelia between the orbits of Jupiter and Neptune (~5.2 - 30 AU) are referred to as Centaurs. Bodies orbiting the Sun beyond the orbit of Neptune are called Trans Neptunian Objects (TNO), which also include the Kuiper belt objects that orbit between 30-50 AU.
Both TNOs and Centaurs appear very faint due their large distances from the Sun and Earth. Currently, hundreds of small bodies in the outer Solar System have been detected, the majority of which have a brightness below Gaia's survey limit of 20th magnitude. Still it is expected that Gaia will detect ~50 small bodies in the outer Solar System, most of which will be Centaurs, or TNOs with a high orbit inclination.
Whole sky survey
Gaia's advantage in the search for TNOs and Centaurs over ground based observations, is the fact that it will survey the whole sky. Ground based observations are limited to a narrow band around the ecliptic and also avoid the regions of the ecliptic that cross the galactic plane. These limitations prevent the detection of small bodies in orbits with high inclinations and those that are currently in the plane of the Milky way, but not so for Gaia.
Sizes and Albedos
The largest of the Centaurs will be resolved by Gaia. This allows direct measurements of their sizes, from which their albedos can be derived.
Binaries and Mass
Roughly 10-20% of the detected objects will be of binary nature. Gaia will be able to detect the binarity and even to determine the orbit of the binary, providing a direct measurement of the mass.
Oort Cloud Perturbations
Far beyond the outer planets, between a few 1000 AU and 150 000 AU (~2.4 lightyear) out from the Sun, the Solar System is generally thought to be enveloped by a large cloud of cometary objects known as the Oort cloud. For comparison, the semi-major axis of Pluto's orbit is just under 40 AU.
The Oort cloud has not been observed directly, but its existence is derived from observations of long-period comets (orbital period > 200 years, mostly near parabolic orbits). Models show that in order for long-period comets to still exist today, they need to be replenished, as they would otherwise have been depleted on a timescale much shorter than the lifetime of the Solar System (~4500 million years).
Characteristics of the long-period comets, as derived from the parts of their orbits closer to the Sun where they have been observed, lead to knowledge of the shape and location of their source:
- the majority of long-period comets have an aphelion distance of ~20 000 - 150 000 AU
- the distribution of the orbit inclinations is largely isotropic
- the distribution of the directions of their perihelion points is largely isotropic
The last two observations imply that there is no preferred direction from which these long-period comets come and hence the distribution of the source of these comets is also isotropic: a spherical cloud.
There are, however, some non-random groupings of cometary orbits, and it has been suggested that these are a result of recent stellar passages. Close or even penetrating passages of nearby stars through the Oort cloud can deflect comets into the inner Solar System, thereby initiating Earth-crossing cometary showers and possible Earth impacts.
Studying the link between past known impact events on Earth and cometary showers will be possible with the data set obtained by Gaia: a complete and accurate census of the distribution and motions of the stars in the solar neighbourhood that allows for determining the frequency of close encounters with the Oort Cloud.
Gaia will detect nearly all of the local star systems within 50 parsec (~160 lightyear) from the Sun - compared to the 20% detected by Hipparcos - with an astrometric accuracy that allows for the stellar positions to be traced back far in time and geologic history.
The encounters predicted by using Gaia data are expected to establish whether the currently observed number of comets is an average number, or represents a period of enhancement. These results will have implications for the size of the population of objects in the Oort Cloud.