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CosmosUp | September 22, 2023

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Asteroid 3753 Cruithne – The “Second Moon” of Earth?

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Asteroid 3753 Cruithne – The “Second Moon” of Earth?

Astronomers have long been fascinated by the movements of Asteroid 3753 Cruithne. This rocky object, which is approximately five kilometers in diameter, has been dubbed by some as the “second moon” of Earth due to its peculiar orbit around our planet.

Cruithne was discovered on October 10, 1986, by Duncan Waldron of the University of Arizona Spacewatch Project. At first, it was thought to be a near-Earth asteroid, but further observations revealed that its orbit is not like any other asteroid known at the time. In fact, it doesn’t orbit Earth at all!

Instead, Cruithne follows a horseshoe-shaped path around our planet. Its orbit takes it ahead of Earth, then it falls behind, and the cycle repeats every 770 years. This trajectory is known as a quasi-satellite orbit, which means that the asteroid’s motion relative to Earth is complex, but it is not actually orbiting our planet.

Due to this unusual orbit, some have claimed that Cruithne is a second moon of Earth. However, this is not accurate. The asteroid’s path around our planet is unstable, and it is not bound to Earth’s gravity. It is more accurate to say that Cruithne shares Earth’s orbit around the Sun, rather than being a moon of our planet.

This simply means that Cruithne doesn’t loop around the Earth in a nice ellipse in the same way as the moon, or indeed the artificial satellites we loft into orbit. Instead, Cruithne scuttles around the inner solar system in what’s called a “horseshoe” orbit.

To help understand why it’s called a horseshoe orbit, let’s imagine we’re looking down at the solar system, rotating at the same rate as the Earth goes round the sun. From our viewpoint, the Earth looks stationary. A body on a simple horseshoe orbit around the Earth moves toward it, then turns round and moves away. Once it’s moved so far away it’s approaching Earth from the other side, it turns around and moves away again.

Duncan Forgan, a researcher at the University of St Andrews, in Fife, Scotland, explains.

Cruithne Orbit around the Sun

Cruithne’s Unconventional Orbit around the Sun

Horseshoe orbits are a relatively common phenomenon among moons in the solar system. Saturn, for example, has a couple of moons that follow this type of orbit.

However, what sets Asteroid 3753 Cruithne apart is the way it moves and oscillates within its horseshoe orbit. When observing Cruithne’s path through the solar system, it traces a chaotic ring around Earth’s orbit, swinging so widely that it even passes near the orbits of both Venus and Mars.

While it takes Cruithne approximately one year to orbit the Sun, it takes nearly 800 years for it to complete this intricate, ring-shaped orbit around Earth. This unique and convoluted trajectory makes Cruithne a captivating subject of study for astronomers, who are intrigued by its unusual movements and complex orbit.

3753 Cruithne: Our Mysterious Second Moon – What Secrets Does It Hold?

What is it like on Cruithne, our second moon? The answer remains elusive, as the moon is only about five kilometers across. To put that in perspective, it’s similar in size to comet 67P/Churyumov-Gerasimenko, which is currently being explored by the Rosetta orbiter and Philae lander. The surface gravity of 67P is so weak that walking at a quick pace could send you floating off into space. It was crucial for Philae to use its harpoons to tether itself to the surface, and their failure resulted in the lander bouncing far away from its intended landing site.

At present, Cruithne appears as nothing more than a few blurry pixels on an image. It sits in the middle range for non-planetary objects in the solar system, and any human or machine explorer would face similar challenges as Rosetta and Philae did on 67P.

However, if Cruithne were to collide with Earth, it would be an extinction-level event, much like the one believed to have occurred at the end of the Cretaceous period. Thankfully, there is no need to panic – the asteroid’s orbit is tilted out of the plane of the solar system, and simulations have shown that while it can come quite close to us, it’s highly unlikely to hit us. The predicted closest encounter will occur in about 2,750 years.

But that’s not the end of the story for our mysterious second moon. In around 8,000 years, it is expected to have a close encounter with Venus, which could fling it out of harm’s way and out of our Terran family altogether. The idea of losing our spare moon is a fascinating prospect, and one that will undoubtedly continue to captivate astronomers and space enthusiasts for years to come.

Cruithne come close to Venus

If 3753 Cruithne were to come into close proximity with Venus, the gravitational pull of the planet could potentially alter the asteroid’s trajectory. This could cause Cruithne to either be flung out of the inner solar system altogether or sent on a new orbit that brings it closer to the Earth.

Despite not being a moon, Cruithne still holds a special place in the hearts of astronomers. Its orbit is a perfect example of the complexity and beauty of the universe. The asteroid is also of scientific interest because it is relatively close to Earth and can be studied in detail by telescopes.

Cruithne may not be a moon of Earth, but its presence in our solar system is a reminder of the wonders of the universe. As we continue to study the asteroid, we may learn more about the early history of our solar system and the role that these rocky objects played in the formation of planets like Earth.

In the future, asteroid 3753 Cruithne could play a crucial role in humanity’s exploration and exploitation of space. With its relatively small size and manageable gravitational pull, it could serve as a testing ground for landing humans on asteroids and even mining them for rare-earth metals that are vital for our technological advancements.

But perhaps the most profound lesson we can learn from Cruithne is that the solar system is not static and everlasting. The asteroid’s unusual orbit, which swings it wide around the Earth’s orbit and brings it into the neighborhood of Venus and Mars, reminds us of the dynamic and ever-changing nature of the universe. It is a humbling reminder that we are just a small part of a much larger cosmic drama that is still unfolding, and that our place in it is both fragile and fleeting.

By studying asteroids like Cruithne and pushing the boundaries of human exploration and knowledge, we can gain a deeper understanding of our place in the universe and the challenges and opportunities that lie ahead. Who knows what other mysteries and discoveries await us as we continue to journey through the cosmos? One thing is certain – the future is full of exciting possibilities, and asteroids like Cruithne are just the beginning.

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