Cruithne: Earth's Second Moon!?
The Solar Trojans of Earth London - October, 26 1999 - (SPACEDAILY)
We will never see it but the Earth has at least one other natural
satellite. In discovering several new types of orbital motion, a team
of British scientists has shown that the gravitational forces of our
planet and of the Sun allow our planet to capture passing asteroids.
One of them is named 'Cruithne', and can be considered -- at least for
the next 5000 years -- as 'Earth's second Moon'.
The work of coorbital dynamics by a team from Queen Mary and
Westfield College in London was published 27 September in the US
publication 'Physical Review Letters'. Fathi Namouni, Apostolos
Christou and Carl Murray have taken even further the discoveries of
The 18th century French mathematician gave his name to the five special
points of equilibrium between the gravitational forces of a planet like
our Earth and those of the Sun. The 'Lagrangian points' -- also known
as libration points -- demonstrate the so-called 'three-body problem'
when a planet and its Sun can catch a third companion (see diagram).
The first point L1 is situated on a line between the planet and its
Sun. SOHO, the ESA-NASA Solar and Heliospheric Observatory is the first
spacecraft to exploit such a position. It is currently orbiting the
inner L1 position 1.5 million km from Earth using this vantage point to
study the Sun. L2 is on the same line but on the outer side from Earth.
The L3 point is precisely on the other side of the Sun. L4 and L5 are
at the summit of two equilateral triangles with a common base being the
line between the Earth and the Sun. Joseph-Louis Lagrange had already
shown that objects turning around L4 and L5 could easily stay there.
This configuration applies to other planets of the solar system. Indeed
Jupiter has hundreds of Trojan asteroids and Mars has at least two.
Although Saturn itself has none, its own moons Tethys and Dione
maintain Trojan asteroid satellites at Lagrangian points.
The orbits of these third bodies are exotic. The Trojan asteroids
describe a 'tadpole-shaped' pattern around the L4 and L5 points. Even
more peculiar is the 'horseshoe orbit' in which the third body turns
around the three points of equilibrium, L3, L4 and L5.
Cruithne is such an object. Discovered in 1997, it is a 5-km diameter
asteroid that takes 770 years to complete its horseshoe orbit. Thus
every 385 years it comes to its closest point to Earth, some 15 million
kilometres. Last time was in 1900, next -- if you can wait -- will be
The British team integrated Cruithne's parameters into their
mathematical models, deducing that it can remain in its present state
for 5,000 years before leaving. They have even calculated that 'Earth's
second moon' is likely to be a second-comer having been trapped in a
similar orbit some time in the past 100,000 years.
"Cruithne is a case example, proof that our work is not just abstract
calculations," says Carl Murray. "The mathematical model that we have
developed has been able, not only to predict several new types of
previously unsuspected motion, but has it has subsequently been
confirmed by investigating numerically the orbits of real solar system
objects. Nature has already provided examples of every kind of orbit
that the theory can provide."
Examining existing catalogues of near-Earth objects to see whether
there were any other similar cases, the Queen Mary and Westfield
College team have discovered four: three concerning Earth and one for
The main significance of the work is that it provides a complete
classification of coorbital motions. It could lead to a greater
understanding of other asteroids, including their likelihood of hitting
Earth and of how the planets were formed. Space mission planners could
devise new gravitational tricks for their space probes. Murray himself
is one of the European members of the Imaging Science Subsystem team
on the Cassini orbiter part of the Cassini-Huygens mission.
The team also shows that the forces of attraction in the three-body
problem are also present in other domains of science -- such as
chemistry where, for instance, two electrons of an atom of helium
display a similar 'ménage à trois' around their nucleus.