GCAT: General Catalog of Artificial Space Objects

Jonathan C. McDowell


Solar System Model

GCAT Release 1.1.6 (2020 Sep 25) | Data Update 2020 Nov 26

Solar System Description Files

At any given time, an artificial space object is considered to be orbiting a particular astronomical body. In the General Catalog the body is identified by a string identifier (e.g. "Earth"). The bodies and their properties are described in two files, the Worlds file and the Spin file.

Preamble: The Hill Sphere

In general, a space object travels under the gravitational influence of all the massive worlds in the system. However, in most cases the effects of all but one body can be neglected or treated as small perturbations. We say that the object is `in orbit about' that one body. (The orbit is not necessarily bound; it may be a hyperbola rather than an ellipse). I introduce the concept of the Hill sphere of body A with respect to body B.

Let the mass of body A be M, the (smaller) mass of body B be m, and the distance between A and B be R. Then B's gravity dominates (in this approximation) when the distance from the object to B is less than

r(Hill) = R (m/3M)(1/3)

For example, the radius of the Sun-Earth Hill sphere is about 1.5 million km centered on the Earth. The radius of the Earth-Moon Hill sphere is about 66,000 km centered on the Moon, and therefore is entirely within the Sun-Earth sphere.

Here is a map of the Earth-Moon system (with the Moon's position shown on the arbitrary date of 1968 Dec 21), showing the sizes of the two Hill spheres. The map is in geocentric solar ecliptic coordinates in which the Earth-Sun line is fixed and is the X-axis.

The locations for the SEL1 (Sun-Earth Lagrange 1) and SEL2 (Sun-Earth Lagrange 2) points (see below) are also shown. The magenta circle around the Earth indicates the EL1:4 `deep space boundary' beyond which I record space objects in the Deep Space Catalog instead of the main Earth satellite catalog.

Worlds in the Solar System

The General Catalog includes data on spacecraft in the vicinity of bodies throughout the Solar System (and, like the Voyagers, on their way to the great beyond).

We consider four kinds of body:

Objects maintaining position near Lagrange points are recorded in the General Catalog as `orbiting' those points; in all other cases, objects are considered to orbit the body whose Hill sphere they are within. (In the case of nested Hill spheres, we take the innermost sphere the object is within).

Dynamical Reference Points

First, let's discuss barycenters.

Now let's talk about Lagrange points, which are closely related to the Hill sphere discussed in the previous section. When body B orbits body A, an object orbiting both will be subject to the effects of the gravity of A and B and to the acceleration due to the angular momentum it has with respect to both. Lagrange showed that the net effective forces balance at five points which are the solution of a quintic equation, the Lagrange quintic. We refer to these points as the L1 to L5 points of the A-B system. The Lagrange points for the specific and special A-B cases of Sun-Earth and Earth-Moon are referred to as SEL1 to SEL5 and EML1 to EML5.