# RE: Number of Observable Satellites

From: Ted Molczan (molczan@rogers.com)
Date: Thu May 23 2002 - 07:04:44 EDT

• Next message: Tony Beresford: "Re: Number of Observable Satellites"

```David Dodge asked:

> If someone could provide me with a guesstimate of how many
> satellites the average person can see from a mag 4 sky, an
> idea of how many satellites reach 6 mag, and how many
> satellites there are currently in orbit, I would be most appreciative.

There are more than 8600 catalogued objects currently in orbit.

Orbital data is published for 8355 objects, including 105 for which
hobbyists are the sole source of orbital elements.

Data sufficient to estimate visual magnitude is available for 8,160
objects, which can be broken down in order of reliability:

1. Standard visual magnitudes derived from visual observations are
available for 861 objects. Russell Eberst has produced the vast majority
of the observations used in the derivations.

2. Standard visual magnitudes derived from published satellite
dimensions are available for an additional 1,178 objects.

3. Standard visual magnitudes can be derived from published RCS (radar
cross-section) data for an additional 6,121 objects. Mike McCants
maintains a database of RCS values, from which he extracts median values
suitable for roughly estimating visual magnitude.

For the purpose of estimating the number of objects visible to the
unaided eye, I first estimated the visual magnitude of all objects at
perigee and apogee distance, assuming a phase angle of 90 deg

Next, I selected the subset of objects that differ by no more than 1
magnitude between apogee and perigee. This is a way of filtering out
numerous objects in highly elliptical orbits that spend most of their
time at apogee, where they are telescopic objects. Many of those objects
have their perigee around 63 S latitude, far from major populated areas,
so it makes sense to exclude them.

Next, I sorted the remaining data by visual magnitude at perigee.

Since satellites typically vary in brightness by at least 1 magnitude
relative to their standard magnitude, I included objects 1 magnitude
fainter than the stated limits:

399 objects reach magnitude 4.0 or brighter at perigee. Standard
magnitudes derived from visual observations are available for 320 of
these objects.

1640 objects reach magnitude 6.0 or brighter at perigee. Standard
magnitudes derived from visual observations are available for 796 of
these objects.

Ted Molczan

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