[amsat-bb] BY70-1

[Scott]

Very interesting stuff, Phil.

Brings to mind a couple of questions on the subject of a decaying orbit...

#1, is there some more-or-less constant altitude where an object is 
considered to have stopped orbiting and started re-entering the atmosphere, 
or does it vary with mass of the object, speed, etc.

#2, in the case of a spacecraft with radio TX capability, should we expect 
it to stop transmitting at some point prior to actual re-entry (for some 
electrical or RF reason) or do objects normally keep transmitting until they 
fail structurally due to heat & mechanical break-up?

Thanks!

-Scott,  K4KDR
Montpelier, VA  USA


==============================================

-----Original Message----- 
From: Phil Karn
Sent: Friday, February 24, 2017 12:35 AM
To: amsat-bb at amsat.org
Subject: Re: [amsat-bb] BY70-1

On 12/30/16 05:14, Nico Janssen wrote:
>
> By the way, the two SuperView satellites are now using their own
> propulsion system to increase their altitude, preventing an early decay.
> As BY70-1 does not have any propulsion, it is stuck in its low orbit.

Thanks for this explanation. I was wondering why the Superview
satellites were in stable near-circular orbits at ~520 km if the launch
vehicle malfunctioned.

I've grabbed all the historical elements sets from space-track.org for
both Superview spacecraft and for BY70-1. There are quite a few. I want
to look at BY70-1's change in specific orbital energy over time to
estimate the power being dissipated around the spacecraft as it decayed.

The specific orbital energy is the sum of the potential and kinetic
specific energy at any given time. It's constant in any 2-body orbit in
the absence of drag and thrust: negative for a closed orbit (circular,
elliptical) and positive for a hyperbolic (escape) trajectory. It's
exactly 0 for a parabolic escape trajectory. The specific orbital energy
in joules per kilogram is

E = -mu/(2*a)

where mu is the earth's gravitational parameter (3.986004418e14 m^3/s^2)
and 'a' is the semimajor axis in meters. The semimajor axis can be
computed from the mean motion as

rt =  86400 / (MM*2*pi)
a = cube_root(mu*rt^2)

where MM is the mean motion in revolutions per day (from the TLE set)
and mu is again the earth's gravitational parameter. The intermediate
variable rt is the time in seconds it takes for the mean anomaly to
increase by 1 radian, i.e, the time to complete 1/(2*pi) of an orbit.

--Phil