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

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-----Original Message----- From: Phil Karn Sent: Friday, February 24, 2017 12:35 AM To: [email protected] 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