Quasi-satellite

A quasi-satellite is a celestial object that remains close to a planet while both objects orbit the Sun. During its orbit around the Sun, a quasi-satellite crosses paths with the planet’s orbit. Both the quasi-satellite and the planet take the same amount of time to complete their orbit. However, the quasi-satellite’s orbit around the Sun is more eccentric, or elongated, than the planet’s orbit.

The planets of the Solar System, including Earth, have numerous quasi-satellites orbiting near them. Among Earth’s quasi-satellites are 2004 GU9, 2006 FV35, 2013 LX28, 2014 OL339, and 2016 HO3, which is also called 469219 Kamo‘oalewa and is particularly noteworthy because it has been thoroughly studied.

Background

Quasi-satellites are typically asteroids that orbit the Sun. However, because a quasi-satellite is often close to a planet, it appears to be orbiting the planet as a moon does. A quasi-satellite, therefore, appears to be a planet’s moon, being pulled into orbit by the planet’s gravity. However, unlike a moon, a quasi-satellite does not orbit the planet and instead orbits the Sun, being bound by the Sun’s gravity instead of the planet’s gravity. This is one of the main differences between a moon and a quasi-satellite.

A quasi-satellite’s orbit around the Sun and that of its nearby planet are synchronized. In this sense, a quasi-satellite and its planet share a co-orbital configuration. The configuration generally is a 1:1 orbital resonance. This means that the quasi-satellite remains close to the planet over the period of many orbits. An orbital resonance is a relationship between two objects where their orbital periods are in a ratio of small whole numbers. This permits the objects to exert a gravitational pull on each other, thus influencing how eccentric their orbits are.

Furthermore, the orbit of a quasi-satellite is like a horseshoe orbit. In this type of orbit, the object follows the shape of a “U” from the perspective of the planet. The object moves from the L4 Lagrange point to the L3 and L5 Lagrange points. It then returns to the L4 Lagrange point. An object can shift between a quasi-satellite orbit and a horseshoe orbit.

A Lagrange point is a position in space where an object tends to remain due to the gravitational pull of two larger objects. At the Lagrange point, the two-body system generates an enhanced region of attraction where a smaller object can move into. There are five Lagrange points, labeled L1 through L5. Two of the Lagrange points are stable, while three are unstable. The Lagrange points are named after Italian-French mathematician Josephy-Louis Lagrange.

Overview

The planets of the Solar System contain many quasi-satellites that orbit nearby. Quasi-satellites have been discovered near Venus, Earth, Jupiter, Saturn, Uranus, and Neptune. Research indicates that some of these quasi-satellites, particularly those near Uranus and Neptune, could stay in orbit for the entire existence of the Sun. Other findings show that Jupiter can sustain quasi-satellites for millions of years, while Saturn can sustain them only for about one hundred thousand years.

Earth has more than twenty quasi-satellites, which have orbits that are either permanent or temporary. Many of these quasi-satellites are difficult to observe because they are small and faint. Among Earth’s quasi-satellites are 2004 GU9, 2006 FV35, 2013 LX28, and 2014 OL339. However, perhaps Earth’s most notable quasi-satellite is 2016 HO3, also known as 469219 Kamo‘oalewa, which was discovered in 2016. It is Earth’s closest and smallest quasi-satellite, measuring between 40 and 100 kilometers across. Its distance from Earth ranges from 14 million to 40 million kilometers.

Both Earth and Kamo‘oalewa orbit the Sun similarly. However, like other quasi-satellites, Kamo‘oalewa’s orbit around the Sun is more eccentric than Earth’s orbit. Kamo‘oalewa’s orbit is tilted by eight degrees compared to Earth. This means that Kamo‘oalewa moves above and below Earth throughout its orbit. It also pulls ahead and falls behind Earth. At times, Kamo‘oalewa accelerates, pulling ahead of Earth. It also decelerates and falls behind Earth because its orbital path moves outside Earth’s orbit. Astronomers believe that Kamo‘oalewa began this orbit about five hundred years ago and should continue orbiting this way for the next three hundred years.

Kamo‘oalewa rotates quickly, completing a full rotation every 28 minutes. The quasi-satellite orbits the Sun every 366 days. Furthermore, its orbit is inclined to Earth’s orbit.

Kamo‘oalewa has been studied by the Large Binocular Telescope and the Lowell Discovery Telescope, which are both in Arizona. The quasi-satellite is typically studied in April, when it comes closest to Earth. Findings indicate that Kamo‘oalewa has similarities to the Moon. In fact, some researchers believe it is likely a chunk of the Moon that broke away due to an impact with an asteroid or other object. One of the main reasons for this theory is that the minerals that make up Kamo‘oalewa share similarities with the composition of the minerals that are on the surface of the Moon. Furthermore, the color of the quasi-satellite resembles the color of the Moon.

Other theories exist about the origin of Kamo‘oalewa as well. One of these theories suggests that the quasi-satellite originated from the breakup of a larger object without any contact. This theory indicates that the object ventured close to the Earth-and-Moon system and was torn apart by Earth’s gravity. Yet another theory suggests that Kamo‘oalewa was born out of a trojan (a small celestial body) or an asteroid of Earth that has yet to be discovered. Regardless of its origin, Kamo‘oalewa continues to be studied every April.

Bibliography

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