RE: ISS bit dimmer than predicted

Ted Molczan (
Sun, 20 Jun 1999 21:24:40 -0400

Mark Hanning-Lee of Pasadena CA, wrote:

> Hi I saw ISS pass almost overhead on Thursday night, culminating close
> to the predicted time ... 6/19 4:44:35 UTC.
> A nice pass with fairly constant magnitude (partly because the phase
> didn't change dramatically). However it was less bright than SkyMap's
> predicted -1.3. It never quite reached as bright as Arcturus. So for
> this pass at least, the standard magnitude in use was perhaps too
> bright.

Earlier today, I used some of Russell Eberst's observations to estimate
ISS's std Mv, which I put at about 2.5 (1000 km, half phase).

Using that value, I obtained a maximum brightness of 0.2 Mv for the pass you
observed. That happens to be the magnitude of Arcturus. Your report that it
never quite reached the brightness of Arcturus, seems well within the
expected uncertainty, so you might want to use std mag = 2.5.

Ted Molczan