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Earth's moons
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Title: Detecting Earth's Temporarily-Captured Natural Satellites - Minimoons
Author: Bryce Bolin, Robert Jedicke, Mikael Granvik, Peter Brown, Ellen Howell, Michael C. Nolan, Peter Jenniskens, Monique Chyba, Geoff Patterson, Richard Wainscoat

We present a study on the discoverability of temporarily captured orbiters (TCOs) by present day or near-term anticipated ground-based and space-based facilities. TCOs (Granvik et al. 2012) are potential targets for spacecraft rendezvous or human exploration (Chyba et al. 2014) and provide an opportunity to study the population of the smallest asteroids in the solar system. We find that present day ground-based optical surveys such as Pan-STARRS and ATLAS can discover the largest TCOs over years of operation. A targeted survey conducted with the Subaru telescope can discover TCOs in the 0.5 m to 1.0 m diameter size range in about 5 nights of observing. Furthermore, we discuss the application of space-based infrared surveys, such as NEOWISE, and ground-based meteor detection systems such as CAMS, CAMO and ASGARD in discovering TCOs. These systems can detect TCOs but at a uninteresting rate. Finally, we discuss the application of bi-static radar at Arecibo and Green Bank to discover TCOs. Our radar simulations are strongly dependent on the rotation rate distribution of the smallest asteroids but with an optimistic distribution we find that these systems have > 80% chance of detecting a > 10 cm diameter TCO in about 40 h of operation.

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Earth minimoons
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 Simulations Show Mini-Moons Orbiting Earth
 
Our 2,000-mile-diameter Moon 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 (Paris Observatory) 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.

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RE: Earth moons
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Earth Has Other Moons, Astronomers Say

Scientists studying satellites orbiting the planet have come to an astounding conclusion: Earth has multiple moons at any given time, the MIT Technology Review reported.
Mikael Granvik, along with colleagues at the University of Hawaii, first discovered a mysterious body orbiting the Earth in 2006. The object -- or RH120 as it was known -- turned out to be a tiny asteroid just a few meters across. Moreover, it was a natural satellite just like our moon.

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Natural Earth satellites
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Title: The population of natural Earth satellites
Authors: Mikael Granvik, Jeremie Vaubaillon, Robert Jedicke

We have for the first time calculated the population characteristics of the Earth's irregular natural satellites (NES) that are temporarily captured from the near-Earth-object (NEO) population. The steady-state NES size-frequency and residence-time distributions were determined under the dynamical influence of all the massive bodies in the solar system (but mainly the Sun, Earth, and Moon) for NEOs of negligible mass. To this end, we compute the NES capture probability from the NEO population as a function of the latter's heliocentric orbital elements and combine those results with the current best estimates for the NEO size-frequency and orbital distribution. At any given time there should be at least one NES of 1-meter diameter orbiting the Earth. The average temporarily-captured orbiter (TCO; an object that makes at least one revolution around the Earth in a co-rotating coordinate system) completes (2.88 ±0.82)
ev around the Earth during a capture event that lasts (286 ±18)\days. We find a small preference for capture events starting in either January or July. Our results are consistent with the single known natural TCO, 2006 RH_{120}, a few-meter diameter object that was captured for about a year starting in June 2006. We estimate that about 0.1% of all meteors impacting the Earth were TCOs.

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RE: Earth moons
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Earth may once have had two moons

A new theory suggests the Earth once had a small second moon that perished in a slow motion collision with its "big sister".
Researchers suggest the collision may explain the mysterious mountains on the far side of our Moon.
The scientists say the relatively slow speed of the crash was crucial in adding material to the rarely-seen lunar hemisphere.

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Early Earth may have had two moons

Earth once had two moons, which merged in a slow-motion collision that took several hours to complete, researchers propose in Nature today.
Both satellites would have formed from debris that was ejected when a Mars-size protoplanet smacked into Earth late in its formation period. Whereas traditional theory states that the infant Moon rapidly swept up any rivals or gravitationally ejected them into interstellar space, the new theory suggests that one body survived, parked in a gravitationally stable point in the EarthMoon system.
Several such 'Lagrangian' points exist, but the two most stable are in the Moon's orbit, 60° in front or 60° behind.

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