Gaia DR2 Solar System Objects

The Gaia satellite is currently observing the entire sky in order to measure the positions and movements of stars very precisely. As a byproduct, occasionally, asteroids fly through the telescope's field of view. Instead of throwing away the data (those fly-by's are basically "noise" to the telescope's main mission), the Gaia team is releasing their observations of asteroids and other solar system bodies in the Gaia Solar System Objects catalogue.

In the most Gaia data release (DR2), the Gaia team released the data for ~14,000 asteroids and other solar system bodies (except planets; they're too bright!). For each asteroid, the data release includes measurements of the asteroid's brightness ("magnitude" in astronomy-slang) as a function of time. These time series are very irregularly sampled, because the time an asteroid will be observed at depends on the combination of that asteroid's path and the telescope's observation pattern. The fact that we see asteroids at all stems from the fact that they reflect light from the sun. How much light they reflect (and how bright an asteroid appears to a telescope) depends on the relative position of sun, asteroid and the telescope, and also on the size, shape and composition of the asteroid itself. The brightness changes with time for two reasons: (1) because the asteroid moves with respect to the sun and our telescope, and (2) because the asteroid spins around its own axis, and because it is irregularly shaped, we'll see different parts of it.

This data set is great, but it doesn't tell us everything! The asteroids for which the Gaia team released data are all well-known: that means, we already know something about their size and path through the solar system. This means that with the current data release, we can prepare tools and methods for when Gaia releases the data set for all the asteroids we have never seen before!

In order to help us make sense of what's going on in this data set, I combined it with some previous knowledge we have about this system. Data about solar system objects is collected by the Minor Planet Center, to which many telescopes (including Gaia) report their detections. The MPC then computes things like paths, and how elliptical the orbit is. I added their data to the Gaia DR2 data. In addition, the Sloan Digital Sky Survey has observed hundreds of thousands of asteroids, many in wavelengths other than Gaia used for their measurements. Looking at the same asteroid in different wavelengths is interesting because different materials reflect sunlight differently well in the various wavelengths, so measuring the brightness in different wavelengths can help us understand what the asteroid is made of.

How to use this visualization

Here's what you can see in the data visualization below. On the upper left is the sky position of the asteroid. The wavy structure you see in there is the ecliptic of the solar system: that is, the plane in which most of the solar system bodies (including the planets like the Earth) lie. You can click-and-drag to move aroun this plot, or scroll to zoom in or out. On the upper right, I plotted a histogram of the number of data points for each asteroid. Asteroids with more data points allow you to do better analyses, so you can make an interval selection in the right plot, and it will show you all data points on the left with a total number of measurements in the range you selected.

The plots in the second row are a bit more physically motivated. Asteroids live in different places in our solar system. Near-Earth Objects (NEOs) are asteroids that come close to the earth (and might potentially be dangerous to us), but there's not that many of those. The vast majority of asteroids live in the Asteroid Belt, and are thus aptly called Main-Belt Asteroids (MBAs). Jupiter has its own little group of asteroids called Jupiter Trojans. All these different types of asteroids have different properties. They might not all be aligned with the plane of our solar system (which you can see in the plot labelled inclination). Asteroids might lie on close to circular orbits (like our Earth), or they might be highly eccentric, with orbits that are stretched out in a particular direction (this is what's shown in eccentricity). The semi-major axis is a measure of distance between the sun and the asteroid.

The plot in the last row shows you a time series of all observations taken for a given asteroids by the Gaia telescope. Note that by default, it'll show nothing. In order to make a time series show up, you have to click on one of the points in the upper left plot to pick an asteroid to look at. As soon as you do, its time series will appear. You can click-and-drag in this panel in order to move around, and you can scroll to zoom in and out.

Happy exploring!