Re: (meteorobs) phantom-orbits

At 05:08 PM 2/19/00 -0800, Joshua <staff@meteors.com> wrote:
>Greetings all,
>My friend is certain that he read an article in Wired magazine in which
>the architect of a prominent space probe said that the craft would be
>placed in a so-called "phantom-orbit": an orbit around no mass, only

What they are talking about is halo orbits around the L1 and L2 Lagrangian

Lagrangian points are points of gravitational stability in any two-body
orbiting system: Earth-Sun, Earth-Moon, or Sun-Jupiter, for example.  There
are five Lagrangian points,  L1, L2 and L3 are all on a line drawn through
the centers of the two bodies, and are unstable, meaning objects orbiting
there are easily perturbed out of their orbits; L4 and L5 lead and follow
by 60 degrees in one objects' orbit.  An example is the Trojan asteroids
which are found in the orbit of Jupiter, 60 degrees ahead and 60 degrees
behind Jupiter.

To picture the dynamics of a Lagranigian point, take the L1 point.  Here is
a rough diagram that I hope works:

Sun O                              L1 X    o Earth

An object in orbit around another needs to move fast enough to counter the
acceleration force of gravity trying to pull it into its primary.  The
closer an orbit is to the primary, the faster an orbiting object needs to
move in order to remain in orbit.  For example, the Earth moves around the
Sun at about 30 km/sec, while Venus orbits at about 35 km/sec; Mercury
whizzes around at nearly 48 km/sec.  At the L1 point, however, an orbiting
object is subject to gravitational acceleration from both the Sun and the
Earth.  The net force is the sum of the two vectors:

Sun O           <---------------- L1 X -->  o Earth

which results in an object at the L1 point being able to move more slowly
in its orbit than an object at that distance from the sun would have to
absent the Earth.  Since it is pulled toward the Earth at the same time it
is pulled toward the Sun, an object at the L1 point can maintain the same
relative position to the Earth as it orbits, rather than racing around and
constantly 'lapping' the Earth inside the Earth's orbit.

The L2 point, outside the Earth's orbit, is subject to the combined
gravitational force of Earth and Sun, and so an object there can move
*faster* than its distance from the Sun would ordinarily dictate, and again
maintain the same position relative to the Earth.

As I mentioned before, the first three Lagrangian points are unstable, and
an object can be easily perturbed out of them.  However, a spacecraft can
maintain an orbit there with relatively low fuel expenditure.

A number of spacecraft have used the L1 point to study the sun and the
sun-earth environment; ISEE-3 was the first.  It was subsequently
maneuvered to the  L2 point to observe the Earth's magnetic tail, and then
maneuvered to intercept Comet Giacobini-Zinner; during the the G-Z mission
its designation was changed to ICE.  Currently in a halo orbit around L1 is
SOHO; a nice diagram of its orbit can be found at
http://sohowww.nascom.nasadot gov/gif/halo_orbit.gif .

Bear in mind that an object in a halo orbit is not really "orbiting" the L1
point, but is still in orbit around the Sun.  What it is doing is
essentially surfing the gravitational gradients in space to our advantage.
All the rules of physics still apply!

What's the advantage?  Ease of communications, for one thing; a satellite
at L1 will never go behind the sun.  It also can view the sun constantly.
The L2 point is onder consideration for placement of a large IR space
telescope, in order to be far from the Earth, and avoid the thermal effects
of an Earth orbit.

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