Russell McMahon wrote:
>=20
> > >Incidentally, why are there so many Mars missions arriving there
> > >all at once?  Is there some astronomical-geometry that favours i=
t?
> >
> > Probably because mars has been the closest to earth for some 28,0=
00 years,
> > so the travel time is shortest, and there is probably an advantag=
e in
> signal
> > strengths as well.
>=20
> The minimum energy path between two bodies in Keplerian circular or=
bits is
> named a Hohman transfer. It is an ellipse which is tangential to bo=
th orbits
> where it touches the source and target. This alignment with earth a=
nd mars
> occurs variably. The present closest approach for yonks condition p=
robably
> makes it especially attractive.
>=20
> Good explanation of Hohman transfer here (note the server ! :-)  )
>=20
>         http://web.mit.edu/12.000/www/teams/9/trajectory/hohman.htm=
l
>=20
> Named afaik for a NASA scientist who first thought of it.

Not quite.
There was no NASA in 1925 if this Stanford quiz is to be believed.
However, there does appear to have been a Hofman at NASA, hence the c=
onfusion
with Hohmann.

http://quizbowl.stanford.edu/archive/spencer01/SS2.htm
9. Using the vis-viva equation, one can easily find the delta-v=20
needed to enter one; let the second term be the arithmetic mean=20
of the lower and higher orbits' radii. They require only two delta-v'=
s, as
at both endpoints they are tangent to the initial and final orbits. F=
TP,=20
name these elliptical paths,=20
first worked out by their namesake discoverer in 1925,=20
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
the most fuel-efficient transfer orbit between
one circular path and another.
ANSWER: Hohmann transfer orbit


http://www.astro.amu.edu.pl/~breiter/lectures/astrody/Hohmann.pdf

Walter Hohmann (1880=961945) was a professional
engineer who eventually became the city architect of
Essen, Germany. In 1925 he published his masterpiece,
Die Erreichbarkeit der Himmelsk=F6rper
(The Attainability of Celestial Bodies), in which he demonstrated
that the interplanetary trajectory requiring the
least expenditure of energy is an ellipse tangent to the
orbits of both the departure and the arrival planets.
The =93Hohmann transfer ellipse=94 has endured, but his
investigations in interplanetary mission design go far
beyond that result and represent a milestone in the
development of space travel.

etc.

Robert

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