Earth’s other ‘moon’ and its crazy orbit could reveal mysteries of the solar system

Posted: February 28, 2015 by oldbrew in Celestial Mechanics

Cruithne's orbit of the Sun   [credit:]

Cruithne’s orbit of the Sun

‘One day, Cruithne could be a practice site for landing humans on asteroids’ says a report at . Why so?

‘Cruithne has an orbit that stretches from the orbit of Mercury to beyond the orbit of Mars. But remarkably, Cruithne’s period is almost exactly the same as Earth’s. This sets the table for some interesting orbital interactions.’ – quoting takes up the story:
We all know and love the moon. We’re so assured that we only have one that we don’t even give it a specific name. It is the brightest object in the night sky, and amateur astronomers take great delight in mapping its craters and seas. To date, it is the only other heavenly body with human footprints.

What you might not know is that the moon is not the Earth’s only natural satellite. As recently as 1997, we discovered that another body, 3753 Cruithne, is what’s called a quasi-orbital satellite of Earth. 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.

Cruithne’s 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.

Cruithne from a stationary Earth position

Horseshoe orbits are actually quite common for moons in the solar system. Saturn has a couple of moons in this configuration, for instance.

What’s unique about Cruithne is how it wobbles and sways along its horseshoe. If you look at Cruithne’s motion in the solar system, it makes a messy ring around Earth’s orbit, swinging so wide that it comes into the neighbourhood of both Venus and Mars. Cruithne orbits the sun about once a year, but it takes nearly 800 years to complete this messy ring shape around the Earth’s orbit.

Full report here
Cruithne and Earth seem to follow each other because of a 1:1 orbital resonance as shown in two animations here.

Wikipedia attempts to explain horseshoe orbits here.

  1. oldbrew says:

    Here’s’s version of the orbit:

  2. BoyfromTottenham says:

    Hi from Oz. Fascinating! But how big is it, and therefore what gravitational and maybe other effects might have on the Earth, and what does the effect cycle look like?

  3. oldbrew says:

    Boy: Cruithne is about 5 km. in diameter – see the ‘two animations’ link above. Not sure what you have in mind re ‘effect cycle’.

    The animation above takes 385 years in each direction i.e. 770 years in all from start to finish. The small blue blob is Earth from a fixed reference frame.

    Cruithne doesn’t get nearer to Earth than about 30 x Earth-Moon distance so it has no gravitational effect on Earth worth mentioning.

  4. I find it easier to think of Cruithne as a planetoid with an orbit more elliptical than a planet and less elliptical than an asteroid..

    Satellite of the Earth it is not.

  5. hunter says:

    That cannot be a stable orbit, can it? And if it is Earth centric what happens when it degrades? At the least this would be an interesting candidate to send a probe to. Could it be a burned out cometary core? How does something take 770 years to wobble around the inner solar system? that is truly fascinating.

  6. oldbrew says:

    hunter: it’s orbiting the Sun once a year, but the position it returns to relative to the Earth varies over 770 years. Each orbit takes it out past Mars and in towards Mercury.

    It’s possible that Venus could dislodge it from its orbit in 8000 years time, some say.
    The Daily Mail Online ran an expanded version of our feature the other day.

  7. oldbrew says:

    It’s an Aten asteroid. Planets can also have trojans e.g. Jupiter has two enormous groups of them. They are linked to one of a planet’s Lagrange points (L4 or L5) – links here.

  8. oldbrew says:

    Related Talkshop post on Toro and Sisyphus asteroids: ‘Why Phi? – Earth’s secret neighbours’

    ‘1685 Toro is an Apollo asteroid that orbits the Sun in a 5:8 resonance with Earth, and near a 5:13 resonance with Venus.’

    In other words its orbit period is the same as the Earth-Venus conjunction period.