Earth’s other moons

April 4, 2012, University of Hawaii

( -- Earth usually has more than one moon, according to a team of astronomers from the University of Helsinki, the Paris Observatory and the University of Hawaii at Manoa.

Our 2,000-mile-diameter Moon, so beloved by poets, artists and romantics, has been orbiting Earth for over 4 billion years. Its much smaller cousins, dubbed “minimoons,” are thought to be only a few feet across and to usually orbit our planet for less than a year before resuming their previous lives as asteroids orbiting the Sun.

Mikael Granvik (formerly at UH Manoa and now at Helsinki), Jeremie Vaubaillon () and Robert Jedicke (UH Manoa) calculated the probability that at any given time Earth has more than one moon. They used a supercomputer to simulate the passage of 10 million asteroids past Earth. They then tracked the trajectories of the 18,000 objects that were captured by Earth’s gravity.

They concluded that at any given time there should be at least one asteroid with a diameter of at least one meter orbiting Earth. Of course, there may also be many smaller objects orbiting Earth, too.

According to the simulation, most asteroids that are captured by Earth’s gravity would not orbit Earth in neat circles. Instead, they would follow complicated, twisting paths. This is because a minimoon would not be tightly held by Earth’s gravity, so it would be tugged into a crazy path by the combined gravity of Earth, the and the Sun. A minimoon would remain captured by Earth until one of those tugs breaks the pull of Earth’s gravity, and the Sun once again takes control of the object’s trajectory. While the typical minimoon would orbit Earth for about nine months, some of them could orbit our planet for decades.

“This was one of the largest and longest computations I’ve ever done,” said Vaubaillon. “If you were to try to do this on your home computer, it would take about six years.” 

In 2006, the University of Arizona’s Catalina Sky Survey discovered a minimoon about the size of a car. Known by the unimaginative designation 2006 RH120, it orbited Earth for less than a year after its discovery, then resumed orbiting the Sun.

 “Minimoons are scientifically extremely interesting,” said Jedicke. “A minimoon could someday be brought back to , giving us a low-cost way to examine a sample of material that has not changed much since the beginning of our solar system over 4.6 billion years ago.”

The team’s paper, “The population of natural Earth satellites,” appears in the March issue of the journal Icarus.

The team used the Jade supercomputer at the National Computer Center for Higher Education (Centre Informatique National de l’Enseignement Supérieur, or CINES) at Montpelier, France.

This work was supported in part by the National Aeronautics and Space Administration under Grant No. NNX08AR22G issued through the Planetary Science Division of the NASA Science Mission Directorate. Any opinions, findings, and conclusions or recommendations expressed in this article are those of the author(s), and do not necessarily reflect the views of the National Aeronautics and Space Administration.

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3 / 5 (4) Apr 04, 2012
...And the occasional mini-moon might be broken into its constituent elements using solar energy, to provide space stations with various raw materials, without the need to bring them up from the Earth. Unfortunately, few would contain metals, but oxygen and silica would be useful.

And for a "skyhook"/sling -even using extant materials, like kevlar or spectra, the mass of small asteroids could be used as counterweights for accelerating spacecraft without using reaction mass.
1.9 / 5 (8) Apr 04, 2012
Or they could be shuttled into L4 or 5 and used to make colonies. There is heaps more stuff out there than on Earth.
I mean Space is Big. Really, really Big.
Hitler looked in the wrong direction for Lebensraum. He should have looked Up.
Just like we should look Up.
Hasn't anyone read the "Limits to Growth" report? If we don't look Up we are toast. I mean as in history. You know, . . Extinct.
Never mind.
DoDos. I can't stand Dodos. Hurry up and go extinct so that my kind can have a bit of elbow room.
not rated yet Apr 04, 2012
Sadly, the full article is $ 32, so we can't read more...
5 / 5 (4) Apr 04, 2012
A *free* preprint of the Icarus paper can be found here:
5 / 5 (1) Apr 04, 2012
This category of asteroid seems like a natural for a rendevous mission. Low Energy, I think. The orbit may also provide a natural turn around.

And we might capture (low energy transfer) it into a resonance orbit. Resource material.
1 / 5 (1) Apr 05, 2012
Neat article. Just one observation... The obvious 'dub' for these little pieces of rock is Miims.
not rated yet Apr 06, 2012
not very different from Mars 2 moons, it being noted by credible cosmo... that Gayanmede is an artificial satilite, and a base / space station
not rated yet Apr 06, 2012
Can we tag a ride on them?
3.5 / 5 (2) Apr 10, 2012
Apr 04, 2012
The bodies at the earth's lagrange points are also earth satellites, orbiting the earth once per year.[/quote]

They are not earth satellites, they don't orbit the earth. L4 and L5 are located 60 degrees before and behind the earth on the same orbital plane as the earth/moon around the sun. There is only one known object in an Earth LaGrange point, called 2010 TK7 it is only 300m long. There may be others but we havn't detected them yet.
1 / 5 (1) Apr 10, 2012
@IzitTech ... It depends on your point of view. If you could observe from a point above (or below) the solar system it would 'look' like an object at L4 or L5 orbits the Earth once every time they go around the Sun. For example, (from your point of view) if L4 was to the right of the Earth when it is on one side of the Sun, it would be to the left of the Earth on the other side of the Sun (after 6 months). Therefore the object has appeared to travel half way round the Earth.
3 / 5 (2) Apr 11, 2012
@IzitTech ... This can be demonstrated by poking a stick through a tennis ball at one end of the stick and a smaller ball at the other end. Hold the tennis ball in one hand and move and twist your hand to simulate an asteroid leading a planet around an orbit. Note that when you have completed one orbit the smaller ball has made a full 360 deg. circuit of the tennis ball, and your hand!
not rated yet Apr 11, 2012
The L4 and L5 LaGrange points are positions with "null" gravity preceding and following the earth on its orbit around the sun which is created by the gravity of the Sun/(Earth/Moon). 60 degrees in front and behind. An object caught in these areas doesn't change it's position relative to Earth. They don't move between the earth and the sun at any time. An example would be to take a Frisbee and paint a mark on the circumference that represents Earth. Make a mark 60 degrees around the circumference in front (L4) of the Earth position and another mark 60 degrees on the other side (L5) of the Earth position. Make a hole (the Sun) at the center of the Frisbee and poke a stick through it. If you spin the disk anti clockwise, representing the orbit of the earth around the Sun, objects in L4 and L5 don't orbit the Earth, they orbit the Sun. They are not satellites of Earth.
not rated yet Apr 11, 2012
The reference to "null" gravity in my previous post isn't entirely accurate. There is still gravity there but the Sun or the Earth/Moon doesn't exert an influence greater than the other, in effect they cancel each other out.
3 / 5 (2) Apr 11, 2012
@IzitTech .. I think you are missing the point. We know that the L4, L5 points are orbiting the Sun while 'locked' into the Earth's orbit. But they *also* make a 360 deg. circuit around the Earth once during the Earth year. (The proof is in my comments above.) ... In a similar way, the Moon rotates 360 degrees once during its orbit around the Earth.
1 / 5 (2) Apr 12, 2012
If a moon cannot be seen in the naked eye rising and setting it's not a moon in the cultural sense.

Calling these temporary visitors, which are space rocks at best "moons" is ridiculous.

There only real importance lies in the potential for hitting the planet one day.
not rated yet Apr 12, 2012

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