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Subject: Re: Hubble Retirement
From: Henry Spencer <>
Date: Sun, 2 Mar 1997 21:36:21 GMT

In article <5f8e8m$>,
Brynn Rogers <> wrote:

>>is for NGST to reside at the L2 Lagrangian point, which is quite distant.
>L2, isn't that the same distance out as the moon? (or am I thinking of L5)?

No, farther -- it's the Sun-Earth L2 point, not the Earth-Moon L2 point,
that is NGST's nominal location.  The S-E L2 point is something like
1.5 million km out, several times the distance of the Moon.  It's still
partly in Earth's shadow, a nice location for an infrared telescope that
wants to keep cool.

>What do we have in mind to boost it there? [shuttle plus ????]

I haven't seen the details on NGST, but I would bet money that it's
planned for an Atlas launch.  The shuttle is out of fashion with the
unmanned people, and Titan is Officially Too Expensive for most things.

>Are there any man made satellites at the Lagrange points?

None at the Earth-Moon Lagrange points, and there never have been any.
SOHO is in a halo orbit around the Sun-Earth L1 point, as its predecessor
ISEE-3 was too.

>               /-\              __
> L2           Earth     L3     Moon        L4
>               \_/              --

Nope, not quite right.  The three in-line points are either:

	L3		Earth		L1	Moon	L2
	L1		Earth		L2	Moon	L3

depending on whether you believe the space people or the astronomers.
Quite how this disagreement on numbering arose is not clear to me, but
it is now fairly standard in space literature that L1 is the in-between
point and L2 is the far-side-of-smaller-body point.

The Trojan (60-degree) points are L4 (leading) and L5 (trailing).
Committees do harm merely by existing.             |       Henry Spencer
                           -- Freeman Dyson        |

Newsgroups: alt.sci.planetary
From: (Henry Spencer)
Subject: Re: Lagrange Points around Jupiter
Date: Mon, 7 Feb 2000 20:19:52 GMT

In article <>,
Matthew F Funke  <> wrote:
>...a space station in synchronous orbit around Europa would be kind of nifty...
>     Unfortunately, at this altitude, the space station would experience
>some 35 or 36 times the amount of gravitational force from *Jupiter* than
>it would get from Europa when it swings around to its closest approach to

That's not as significant as you might think, given that Europa is being
attracted by Jupiter's gravity too.  What matters is the *difference*
between the effect on the station and the effect on Europa, not the
absolute size of either.  (The Moon is more strongly attracted to the Sun
than to the Earth, but nevertheless remains in Earth orbit.)

However, as Frank has already pointed out, because Europa is tide-locked
to Jupiter, a synchronous orbit around necessarily involves staying in a
fixed position with respect to the Jupiter-Europa line, and the only such
orbits available are those of the Jupiter-Europa Lagrange points.  You got
the right conclusion although for the wrong reason:  Jupiter's gravity is
indeed strong enough to mess up conventional synchronous orbits.

>     Are what I need here Lagrange points?  Can anyone point me to a good
>site where it will explain to me what Lagrange points are and how to find
>them?  (I've got a pretty solid math background...

Not aware of any web sites, alas.  Books I can point you to.  (For most
any advanced technical topic, you really do have to be able to read paper,
not just computer monitors.)  Archie E. Roy's "Orbital Motion" is my
first-choice book for intermediate orbital dynamics.  (Prussing&Conway's
"Orbital Mechanics" is a better introductory book but doesn't get as far
as Lagrange points.  For advanced stuff like halo orbits, try Ehricke's
"Space Flight, vol 1:  Environment and Celestial Mechanics", but for that
you'll need a library, it's long out of print.)

>Finally, would
>Lagrange points even be stable anywhere around the Jovian system?

Depends on what you mean by "stable".  In the long term, probably not; the
L4 and L5 points are the only stable ones even in an ideal system, and
around Jupiter, there are likely to be enough disturbances from the other
Galilean satellites and Europa's slightly-elliptical orbit to mess them up
in the long run.  (People tend to think that orbits departing only
slightly from being circular ought to produce behavior departing only
slightly from that of circular orbits, but this assumption is verifiably
wrong for subtle issues like this.)  However, in the short term, or with
occasional orbit corrections to compensate for long-term deviations, they
ought to be usable.
The space program reminds me        |  Henry Spencer
of a government agency.  -Jim Baen  |      (aka

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