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<p>Space is a big place indeed. Couple of points:</p>
<ul>
<li>Kessler syndrome requires objects after a collision to remain
(at least temporarily) in a sustainable orbit around Earth. That
is, any fragments of a collision must continue to travel post
collision with a velocity and at a trajectory that allows them
to stay in an orbit around Earth of sorts. It's therefore almost
a prerequisite for Kessler syndrome that the objects
participating in the collision be in an Earth orbit before the
collision. If they aren't, then most of the fragments will
either end up down here or somewhere of no consequence. By
definition, asteroids aren't in Earth orbits.<br>
</li>
<li>Any mission to defend against an asteroid would likely require
intervention by impact / attachment / etc. many millions of
miles from Earth, not in low earth orbit as some of our
contributors here seem to assume. By the time your asteroid has
reached the heights where most of our satellites orbit, it'd be
way too late. Asteroids of consequence are likely to have a mass
orders of magnitude higher than anything we can send their way,
so any mission would need to bank on making a small difference
(by crash, persistent push, or ...) on the object's trajectory
early enough to make sure it or its fragments give us a wide
berth.</li>
</ul>
<p>"Don't look up" is great cinema, but you're very unlikely to get
any naked eye visual warning of an asteroid impact that would
allow you to see your nemesis for any great length of time. Just
ask the dinosaurs: They didn't have any mobile devices and social
media to distract their attention, probably did look up now and
then, and for all we know didn't see it coming either. <br>
</p>
<div class="moz-cite-prefix">On 4/11/2022 4:48 pm, Bruce Perens via
Starlink wrote:<br>
</div>
<blockquote type="cite" cite="mid:CAK2MWOsfhfLjNV97akGFJfUyyYirZPp4vqPTuvahZnwFODTg-A@mail.gmail.com">
<div dir="ltr">
<div dir="ltr"><br>
</div>
<br>
<div class="gmail_quote">
<div dir="ltr" class="gmail_attr">On Thu, Nov 3, 2022 at 5:52
PM Dave Taht <<a href="mailto:dave.taht@gmail.com" moz-do-not-send="true" class="moz-txt-link-freetext">dave.taht@gmail.com</a>>
wrote:</div>
<blockquote class="gmail_quote" style="margin:0px 0px 0px
0.8ex;border-left:1px solid
rgb(204,204,204);padding-left:1ex">
Space is a big place, and I'm pretty sure the orbit, impact,
and debris could be tracked.<br>
</blockquote>
<div><br>
</div>
<div>Actually, no. The Space Shuttle got a very concerning
bulls-eye in its front cockpit window a few decades ago from
a tiny paint chip. The speed of two objects in
counter-rotating orbits when they hit imparts a truly large
amount of energy. And there are now so many such things that
there is a significant risk to suited astronauts on EVAs.</div>
<div><br>
</div>
<div>NORAD will not actually tell us how small an object it
can track, nor how many, this being something potentially of
interest to enemies. The Satellite Catalog that they publish
covers objects of 10 cm diameter and larger, a 1U PocketQube
satellite is 5x5x5 cm plus antennas that bring it to 10 cm,
and the early ephemerides published by NORAD for such
objects can be inaccurate. We aren't allowed to launch
anything smaller.</div>
<div><br>
</div>
<div>We also are now required to provide a position-changing
ability to avoid collisions, and active re-entry at the end
of the life of a satellite. This is mainly about the
potential for Kessler Syndrome.</div>
<div><br>
</div>
<div>The 60 years of thinking that orbital space is so big
that we don't have to concern ourselves with debris are
definitely over.</div>
<div><br>
</div>
<blockquote class="gmail_quote" style="margin:0px 0px 0px
0.8ex;border-left:1px solid
rgb(204,204,204);padding-left:1ex">
What would you do with a starship that after launch, due to
lost tiles, or other problems is certain to burn up on
re-entry? Why not test getting out of orbit?<br>
</blockquote>
<div><br>
</div>
<div>Put it somewhere that you can use the habitable volume.
Starship potentially has a larger habitable volume than ISS.
That is <i>without</i> converting the tanks.</div>
<div><br>
</div>
<div>Otherwise, if you have the delta-V to get there, there is
a junkyard orbit above geosynchronous. Things will stay
there for a really long time. The other option is a
controlled re-entry with a known termination in the middle
of an ocean.</div>
<div><br>
</div>
<div>China drops entire stages on farmers fields and rural
roads in their own country quite often, but this is not
thought well of by others.</div>
<div><br>
</div>
<blockquote class="gmail_quote" style="margin:0px 0px 0px
0.8ex;border-left:1px solid
rgb(204,204,204);padding-left:1ex">
That takes all the fun out of it. Impact is so much easier.
Our knowledge of the solar system is only skin deep.<br>
</blockquote>
<div><br>
</div>
<div>People are even starting to get annoyed about stuff that
hits the moon, although this doesn't create orbital debris
unless the energy is really huge.</div>
<div><br>
</div>
<div> Thanks</div>
<div><br>
</div>
<div> Bruce</div>
</div>
</div>
<br>
<fieldset class="moz-mime-attachment-header"></fieldset>
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</blockquote>
<pre class="moz-signature" cols="72">--
****************************************************************
Dr. Ulrich Speidel
School of Computer Science
Room 303S.594 (City Campus)
The University of Auckland
<a class="moz-txt-link-abbreviated" href="mailto:u.speidel@auckland.ac.nz">u.speidel@auckland.ac.nz</a>
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