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 Joseph-Louis Lagrange.

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.

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 in 2285.

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 Venus.

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.

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The National Aeronautics and Space Administration (NASA) has selected the USNO's Full-sky Astrometric Mapping Explorer (FAME) satellite to be funded for launch in 2004. FAME is an optical space telescope designed to determine the positions, distances, motions, brightness, and colors of stars in our galactic neighborhood. It will observe and determine the positions of stars brighter than 15th magnitude, which is about 40 million stars.

"FAME will provide a rich and unprecedented database for a wide range of studies in stellar astrophysics," says Dr. P. Kenneth Seidelmann, the Director of Astrometry at the Naval Observatory and the Chairman of the FAME science team. "It will be the most accurate astrometric catalog in history."

Astrometry, the science of determining positions of stars, is the oldest branch of astronomy. Astrometric measurements not only determine the positions of stars on the sky, but also the distances to stars by measuring their parallaxes. The parallax is the apparent change in a star's position due to the Earth's revolution around the Sun over the course of a year. "Astrometric observations are fundamental measurements that are the foundation of almost all of astrophysics," said Sean Urban, a USNO astronomer.

FAME will be able to detect giant planets larger than twice the mass of Jupiter orbiting neighboring stars. By measuring the positions of stars over time, FAME will be able to detect the "wobbling" of stars due to companion objects such as other stars, brown dwarfs, and giant planets. By directly measuring the distances to a special class of stars called Cepheids, FAME will improve our knowledge of distances to galaxies and our understanding of the size of the Universe. Cepheids are currently used for this purpose, however distances to the Cepheids themselves are not known precisely; FAME will solve this problem. FAME will also be able to determine the amount of dark matter in the disk of our Milky Way galaxy by observing its gravitational influence on stellar motions. "FAME will give us the ability to study the variability of a large number of Sun-like stars, enabling us to put the Sun's activity level in the context of other similar stars," says Dr. Scott Horner of the U.S. Naval Observatory, "This will indicate whether solar variability may change on long time scales, with possible implications for climate change on Earth."

FAME's innovative design uses a solar sail to utilize the pressure from sunlight to change the orientation of the spacecraft in order to scan the entire sky. The FAME telescope looks in two directions at once to achieve its high accuracy. It rotates with a period of 40 minutes.

The FAME team is lead by Dr. Kenneth J. Johnston, the Scientific Director of the U.S. Naval Observatory in Washington, DC. The FAME project is a collaborative effort of the U.S. Naval Observatory, the Naval Research Laboratory (Washington, DC), Lockheed Martin Missiles and Space Advanced Technology Center (Palo Alto, CA), and Smithsonian Astrophysical Observatory (Cambridge, MA). The FAME project has a total mission cost to NASA of $162 million, with additional support provided by the Navy to extend the duration of the operation of the FAME satellite.

The Explorer program, run by NASA's Office of Space Science and managed by NASA's Goddard Space Flight Center (Greenbelt, MD), provides frequent opportunities for scientific investigations from space at relatively low cost. FAME is a medium-class Explorer (MIDEX), which are the largest of the Explorer program missions. Teams of astronomers, space physicists, and engineers compete in a two-phase process to receive funding from NASA. In the first phase, 5 of the 35 proposals submitted were selected based on their scientific merit. After more detailed concept studies were completed on these five missions -- evaluating each mission's cost, management, technical plans, small business involvement, and educational outreach -- NASA selected two of the five missions to receive funding for construction and launch.

The U.S. Naval Observatory, founded in 1830, is responsible for providing the Navy, the Department of Defense, and the public with astrometric and timing data. Its mission includes maintaining the Master Clock for the United States, providing precise time, measuring the Earth's rotation, determining the positions and motions of the Earth, Sun, Moon, planets, stars, and other celestial objects, and providing astronomical data. The data from FAME will fulfill many needs of DoD, other government agencies, and the public at large for accurate astrometric data for the next two decades. The accurate star positions from FAME will enable autonomous space navigation systems to determine the positions of future satellites with accuracies better than 1 meter. Precise timing information and astrometry (star positions) are the foundation of navigation.

Oct 7 - Further observations have led asteroid-watchers to cross another potential threat off their list. Earlier this week, astronomers said there was a very slight chance that Asteroid 1999 RM45 could hit Earth in the year 2042 or 2050 - but even that risk has now been eliminated.

The extremely faint asteroid was first spotted by the Lincoln Near Earth Asteroid Research (LINEAR) project on Sept. 14. Based on six days' worth of initial observations, astronomers at the University of Pisa calculated potential orbits that might have put the asteroid on an collision course with Earth in 2042 or 2050.

At its worst, the estimated risk of collision - less than one chance in 100 million - was far lower than the probability of a previously undetected 0.6-mile-wide (1-kilometer-wide) comet or asteroid hitting the earth. NASA has estimated that risk at between one in 100,000 and one in a million.

But the university's asteroid-watching team wanted to reduce the risk associated with this particular asteroid to zero, and so on Monday astronomer Steven Chesley put out an urgent call for follow-up observations.

Chesley was worried that the asteroid, thought to measure about a third of a mile or a half of a kilometer in diameter, might fade from sight before enough observations were made to eliminate the possibility of a collision in the foreseeable future.

It didn't take long to get the required observations: Asteroid 1999 RM45 was tracked by the Siding Spring Observatory in Australia on Tuesday and Wednesday, and on the basis of those measurements, the University of Pisa team removed the object from its list of potentially hazardous objects. Over the next 75 years, the asteroid's closest approach to Earth is now projected to come in the year 2021, with a healthy miss distance of 6.9 million miles.

Boulder, Colorado, October 7, 1999 -- An international team of astronomers has discovered a moon orbiting the asteroid (45)Eugenia. The pictures, taken with the Canada-France-Hawaii Telescope (CFHT) on Mauna Kea, Hawaii, are the first images of an asteroidal satellite taken from Earth. The team's findings will be reported in the October 7 issue of Nature.

Previous attempts to photograph such satellites, using both ground-based telescopes and the Hubble Space Telescope, found no satellites. The only other such picture came from an interplanetary spacecraft, Galileo, when it discovered the small moon, now known as Dactyl, around asteroid (243)Ida in 1993. The observations could only be accomplished because of a new technique, called adaptive optics, that reduces the blurring caused by the Earth's atmosphere.

A surprising result of this discovery is the very low density of the primary asteroid -- only about 20 percent denser than water. Most asteroids appear dark and were thought to be composed primarily of rock, which is about three times denser than water. "A picture is emerging that some asteroids are real lightweights," said Dr. William Merline, leader of the team, and a senior research scientist at the Boulder office of San Antonio-based Southwest Research Institute (SwRI). A recent flyby of the NEAR spacecraft confirmed that another asteroid, (253)Mathilde, also has a low density. "Either these objects are highly porous rubble-piles of rock, or they are mostly water ice," said Dr. Clark Chapman, another team member, also from SwRI.

The presence of a moon allows scientists to determine the mass of an asteroid because of the effect of the primary asteroid's gravity on its small moon. The size of most asteroids is known from standard astronomical studies. If both the mass and the size are known, researchers can learn the asteroid's density. The density then gives a clue to the asteroid's makeup -- either in terms of composition or structure.

"If these asteroids are rubble-piles, it tells us about the severity of collisions in the asteroid belt and its subsequent evolution. If the objects are largely ice, covered with a dark-coating, then these objects may be remnants of burned-out comets and will further our understanding of the connection between comets and asteroids," said Dr. Christophe Dumas of the Jet Propulsion Lab in Pasadena.

"It is almost certain that the satellite was formed by a collision," said Merline. "As we know from the formation of our own moon and the craters on planetary surfaces, collisions played a large role in the formation of our solar system. Satellites of asteroids give us a window into these collisions, and help us understand how and why our solar system looks like it does."

The light from stars and other celestial objects is distorted by the atmosphere, much as water distorts our view of an underwater object. The new technique, pioneered at the University of Hawaii by team member Dr. Francois Roddier, analyzes the distortions and corrects the light beam by means of what is essentially a "fun-house mirror" back into its previous, undistorted form. "CFHT's exceptional site, telescope, and adaptive optics now allow us to see far sharper detail through the Earth's atmosphere. In many cases we can now compete with the clarity of space-based telescopes," said Roddier. The instrument used was built by the CFHT Corporation.

Previously, faint and close satellites would have been lost in the glare of the primary asteroid. "It is similar to taking a photo of a candle located 400 km away and then discovering a firefly (that is 300 times fainter) flying within two meters of the flame," said Dr. Laird Close, a participant from the European Southern Observatory (ESO) in Germany.

The results are the first from a program to search for satellites around nearly 200 asteroids. "If more satellites are found, it will revolutionize our understanding of the makeup of asteroids," said Merline.

"Except for a few of the very largest asteroids, this is the only way that asteroid densities can be determined other than by spacecraft flybys," according to Close.

Eugenia orbits the sun in the main asteroid belt, a collection of thousands of asteroids that exists between the orbits of Mars and Jupiter. Asteroids are thought to be bodies that never formed a planet; the gravity of the giant planet Jupiter may have stirred up the bodies enough that they collided with each other at fast speeds, perhaps either fragmenting or forming satellites, rather than colliding gently, adhering, and gradually building up a planet.

Researchers estimate that the diameter of the satellite is about 13 kilometers. Eugenia's diameter is about 215 kilometers. The researchers have determined that the satellite has a circular orbit about 1,190 km away from Eugenia. It orbits about once every five days.

While awaiting assignment of a permanent name, the satellite has been given provisional designation, by the International Astronomical Union, of S/1998(45)1, the first satellite of asteroid (45) that was discovered during 1998.

This work was funded by NASA and the U.S. National Science Foundation. A portion of the image processing and data analysis was carried out using facilities at the ESO. CFHT is funded by the National Research Council (NRC) of Canada, the Centre National de la Recherche Scientifique (CNRS) of France, and the University of Hawaii.

Other team members and affiliations are Dr. Francois Menard, CFHT; Dr. David Slater, SwRI headquarters in San Antonio; Dr. Gilles Duvert, Laboratoire d'Astrophysique in Grenoble, France; Dr. Chris Shelton, W.M. Keck Observatory, Hawaii; and Dr. Tom Morgan, NASA Headquarters, Washington, D.C.

An international team of astronomers using a ground-based telescope has discovered a moon orbiting the asteroid (45)Eugenia. The pictures are the first of an asteroidal satellite taken from Earth, and the second ever taken; the Galileo spacecraft previously discovered a moon around the asteroid Ida.

Details about the satellite (first announced in March on IAU Circular 7129) have been published in Nature by William J. Merline (Southwest Research Institute) and his colleagues. They explain how they found the moon by using an adaptive-optics system on the Canada-France-Hawaii telescope. The satellite -- designated S/1998 (45) 1 -- is about 13 kilometers in diameter and orbits Eugenia in 4.7 days at a distance of 1,190 km. Eugenia itself is about 215 km in diameter. Taking these values, the astronomers determined that Eugenia has a density of 1.2 grams per cubic centimeter, suggesting that the body may be a "rubble pile" or having an interior consisting mostly of water ice.