[amsat-bb] Re: Life Expectancy ARISat-1

Phil Karn karn at philkarn.net
Sun Aug 7 15:05:52 PDT 2011

On 8/7/11 3:40 AM, Nico Janssen wrote:

> Actually the TLEs give you the values for the drag (First Time
> Derivative of the Mean Motion) and for the ballistic coefficient
> (BSTAR drag term). Note that in the TLEs the definition of the
> ballistic coefficient is the inverse of Phil's definition, i.e.
> the higher the BSTAR value, the higher the drag.

I did know that, but I didn't mention it because I thought the satellite
is probably too new for these values to be especially accurate, and in
the past they've always seemed rather noisy. But if they do settle down
to meaningful average values then they should be useful in predicting
lifetime, yes.

> When comparing the latest TLE sets for the ISS and for ARISSat-1,
> it appears that the drag for the satellite is roughly twice as
> high as for the ISS. But since the values for the drag vary a lot
> from one TLE set to the next, it helps to use average values over
> a number of TLE sets.

Twice as high - now that's a very interesting data point. What are those
average values?

I know that the ISS performs some drag-avoidance maneuvers with its
solar arrays, such as a "night glider" mode where they're turned
parallel with the velocity vector at night to reduce drag when they're
not doing anything anyway. I've also noticed that rarely do they all
track the sun even in the daytime. Perhaps they're off-pointed to reduce
drag when maximum power is not needed.

I've always thought the ISS should do something more with all its trash
than stuffing it into cargo ships and sending them back into the
atmosphere. Imagine a big electrically powered slingshot that fires the
trash against the velocity vector, giving a forward impulse to the ISS
while simultaneously deorbiting the trash...

Phil, KA9Q

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