While scanning the sky, ESA’s Gaia mission maps not only the stellar content of our Galaxy, but also the Solar System’s asteroid population (see http://sci.esa.int/gaia/58562-challenges-closer-to-home-gaia-s-asteroids/).
This visualisation shows the detection by Gaia of more than 13 000 asteroids as the spacecraft (represented by the pale blue dot) scanned the sky between August 2014 and May 2016. This is a subset of the total number of asteroids seen by Gaia – the sample shown here is of bright asteroids that have been detected more than ten times during the period. The corresponding data for the position on the sky for each detection of every asteroid – the epoch astrometry – will be included in Gaia’s Data Release 2, the second intermediate data release from the mission, expected in April 2018 (see https://www.cosmos.esa.int/web/gaia/release).
Gaia sweeps the sky with two fields of view, separated by an angle of 106.5 degrees (see http://sci.esa.int/gaia/58214-gaia-scanning-the-sky/). Each of these sweeps takes six hours, the time it takes for Gaia to make one rotation on its spin axis. The spin axis also undergoes a slow precession, so that the slice of sky captured by the two fields of view shifts slightly for each rotation. In this way a map of the distribution of stars and asteroids is built up.
Not all regions of the sky are accessible to Gaia at any one time. In particular, as indicated in the opening sequence, there are two large cones of exclusion (with boundaries indicated by the dashed lines) where Gaia never observes (see http://sci.esa.int/gaia/58710-asteroid-search-region/). One cone includes the direction of the Sun (represented by the large white dot), and the other is directly opposite. As Gaia orbits the Sun, these cones sweep across new regions of the sky and the previously excluded regions can be observed. Eventually the entire sky is covered.
In this visualisation, asteroids detected during a six-hour scan are marked yellow, turning blue when the next scan begins. As time progresses the impressive accumulation of detected asteroids can be clearly seen and the main asteroid belt, located between Mars and Jupiter, emerges.
Asteroids can be detected more than once, as they may be detected during several scans. Most of the asteroids depicted here are detected ten or more times in the period covered by this visualisation. In the time between detections, a given asteroid moves through the Solar System and is detected at a different position each time.
Although none of the asteroids depicted here are newly discovered, mapping known asteroids in this way is very valuable. By doing so, the mission plays an important role in contributing to our understanding of the physical and dynamical characteristics of asteroid populations. In particular, the precise measurement of each position provided by Gaia helps astronomers to refine their models of asteroid orbits.
The time stamp in the upper left of the animation shows the progression of time; the counter shows the accumulated number of asteroids detected at that time. As time passes, the sample approaches completeness and the rate of detection decreases.
The view on the left is a projection of the asteroid positions on the ecliptic plane – the plane of Earth’s orbit around the Sun. On the right, the view is side-on. Both views are from the perspective of an observer following Gaia along its path around the Sun and thus Gaia appears to remain in one place.
The transition at the beginning of the animation from the Gaia ecliptic pole scanning law (EPSL), which was used for the first 28 days of commissioning, to the nominal scanning law (NSL) which was used subsequently (see https://www.cosmos.esa.int/web/gaia/iow_20140602), can be discerned from the emerging pattern of detections.
The animation was made by P. Tanga, Observatoire de la Côte d’Azur, France.
Video downloads available at http://sci.esa.int/gaia/59821-selected-asteroids-detected-by-gaia-between-august-2014-and-may-2016
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