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Date: 7 Nov 92 23:59:37 GMT
From: Henry Spencer <>
Subject: Hubble's mirror  or Really Costar.

In article <> (_Floor_) writes:
>] As for an annoying problem, the South Atlantic Anomaly and flapping of
>] the solar arrays give a lot more headaches around here...
>What is the South Atlantic Anomaly?

This is perhaps a candidate for the FAQ list...

Most people know that the Earth's magnetic poles aren't located at its
geographic poles, but it's less well-known that Earth's magnetic center
is not at its geographic center either.  This means that the Van Allen
belts, which follow the magnetic field, are slightly off-center, i.e.
they are slightly closer to the Earth on one side and slightly further
away on the other side.  The area where they are closest is in the South
Atlantic.  In that area, known as the South Atlantic Anomaly, the Van Allen
belts come down low enough that radiation intensity in low Earth orbit
is noticeably greater there.  This is troublesome both to astronauts and
to sensitive electronics; for example, it's common for astronomy missions
to suspend operations during passage through the SAA because the radiation
causes unacceptably high noise levels in their sensors.

In HST's case, some of its computer hardware is more radiation-sensitive
than it was supposed to be, and this causes trouble during SAA passage.
MS-DOS is the OS/360 of the 1980s.      | Henry Spencer @ U of Toronto Zoology
              -Hal W. Hardenbergh (1985)|  utzoo!henry

From: (Henry Spencer)
Subject: Re: Amusing Space Shuttle Computer Power Reference
Date: Tue, 16 Jun 1998 15:21:54 GMT

In article <6m5t6k$ll5$>,
Michael Alan Schaeffer <> wrote:
>>Admittedly, any spacecraft whose orbit goes through the
>>South Atlantic Anomaly had better be prepared to detect and correct
>>occasional memory errors, but that's no big deal.
>	What is the South Atlantic Anomaly?  I'm guessing it's an area
>of higher than normnal radiation, somewhere over the south atlantic.

Correct.  The Earth's magnetic field is somewhat asymmetrical, and the Van
Allen belts therefore are asymmetrical too.  The SAA is an area where the
inner belt is unusually close to Earth's surface, close enough that LEO
satellites see significantly elevated radiation levels there.  In fact,
most of the radiation dose accumulated in LEO comes from SAA passages.

The practical consequences are that memory errors and other electronics
glitches are very likely to occur there -- I've seen crude SAA maps drawn
using data on satellite memory errors! -- and sensitive detectors, e.g.
in telescope instruments, see a lot of noise, in some cases to the point
where they have to be shut down to protect them (this is an issue for
Hubble, I believe).
Being the last man on the Moon is a |  Henry Spencer
very dubious honor. -- Gene Cernan  |      (aka

From: (Henry Spencer)
Subject: Re: Origins of South Altantic Anomaly
Date: Mon, 1 May 2000 16:34:37 GMT

In article <8ec7vf$uee$>,  <> wrote:
>I'm researching an anecdote that I have assumed was true for quite some
>time.  I have been told that the South Atlantic Anomaly may have been
>caused by the early (too low) detonation during an atmospheric test of
>a nuclear weapon.

No.  The SAA arises from the shape of Earth's magnetic field, which could
not have changed suddenly without attracting a lot of attention.  While
the SAA itself was not known until satellites started flying, Earth's
magnetic field was mapped quite precisely at ground level much earlier,
for navigation and scientific purposes.  (It is also quite implausible
that something as -- relatively!! -- tiny as a nuclear explosion could
have changed Earth's magnetic field semi-permanently.)

(For those wondering what this is all about...  The SAA is an area above
the South Atlantic where radiation levels in low orbit are unusually high.
Its cause is fully understood:  not only is Earth's magnetic field tilted
with respect to Earth's axis, it is also somewhat off-center, and hence so
are the Van Allen belts.  The SAA is the area where the inner Van Allen
belt comes down lowest.)

This anecdote may be a confused version of something that did happen:  the
Starfish high-altitude nuclear test in 1962 did substantially increase the
density of high-energy electrons in the inner Van Allen belt (including
the SAA), and this was hard on satellites that were up at the time or
shortly after.  The effect was temporary, but it decayed slowly:  those
electrons dominated natural ones in parts of the belt for five years and
were detectable (with careful data analysis) for 15-20 years.
"Be careful not to step                 |  Henry Spencer
in the Microsoft."  -- John Denker      |      (aka

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