Earth’s second moon – Asteroid 3753 Cruithne

The near-Earth asteroid 3753 Cruithne is now known to be our planet’s second moon. Something quite significant about this asteroid is that it shares the Earth’s orbit. Though due to its motion, its orbit remains stable and there is no possibility of colliding with our planet.

A paper by Paul Wiegert, Kim Innanen, and Seppo Mikkola, published in the British-based science journal Nature on June 12, 1997,  gave significant details. Cruithne’s path is much more complicated than simple satellite motion.

This Youtube video gives every significant detail

Orbit

This asteroid’s orbit makes predictions difficult on timescales longer than its Lyapunov time (~150 yr). The asteroid has a high probability of passing close to Venus and/or Mars on 10^4 yr timescales, pointing to a dynamical age much shorter than that of the solar system.

It orbits the Sun mainly between the orbits of Mars and Jupiter. Its name and numbers are allocated to it by the Minor Planet Center (MPC), part of the Smithsonian Astrophysical Observatory. Cruithne is an Aten asteroid, which means that it crosses the Earth’s path. It is a potentially hazardous asteroid, indicating that there is some risk of colliding. The chances are tiny though. They are named after the first of their class to be identified, Aten.

Some asteroids are dedicated by the discoverers to people or named after mythology such as Arawn and Sedna and will be recognized by the MPC as such. Those not named will be assigned the year and letters. Usually, it will just be referred to as Cruithne.

Cruithne is at a balance point where its average orbital period around the Sun is the same as Earth – one year. Cruithne’s orbit takes it inwards towards the orbit of Mercury, and outside the orbit of Mars. Its fast motion when it is close to Mercury’s orbit is compensated by its slow motion when it lies beyond the orbit of Mars. Earth maintains its orbit by “shepherding” it. At a slow point, our planet gives it a pull and speeds it up. At a fast point, the Earth’s gravity slows it down.

Because the asteroid and the Earth do not go around the Sun in the same amount of time every year, (the asteroid is currently going around slightly faster than the Earth), the position of the bean-shaped loop relative to the Earth varies over time.

Asteroid’s facts

Cruithne is about three miles in diameter (5 km) and was discovered by D. Waldron in 1986. It cannot be seen by the naked eye. The odd orbit was discovered in 1997. Astronomers working at Queen Mary and Westfield College in London came up with mathematical models to describe its path. Two other asteroids have been discovered that travel similarly to Cruithne, 1998 UP1 and 2000 PH5.

Its name links to one of the earliest Celtic tribes known to have inhabited Britain and Ireland. They may have been the first Celtic tribe to migrate from the mainland, or they may have been descendants of prehistoric tribes indigenous to the British Isles since the stone age.

It has an alternative name which is 1986 too. Its absolute magnitude is 15.53 which is the brightness of the object. A higher absolute magnitude means that the object is faint whereas a very low number means it is very bright.

The albedo of the object is 1. The Albedo is the amount of radiation that is reflected into space by the object.

Sources:

physics.uwo.ca

ui.adsabs.harvard.edu

universeguide.com

wwu.edu