Fascinating 'second moon' to pass by Earth

October sees the clocks go back an hour to GMT or Universal Time on the 27th of the month.  Unlike many people who do not look forward to this time of year of darker, longer nights, for astronomers this is a good thing; there are more hours of darkness, and they begin earlier in the evening so that general observations can be made within a more comfortable timeframe.  Of course we still have to wait for specific events if they are to be seen. The darker Autumn sky therefore gives us the chance to pick out the summer sky constellations and objects against a darker background. The increase in contrast that this provides means that objects can be seen more clearly and for longer.  The Autumn constellations can also be seen at their best. 

The Media boys and girls are at it again. After insisting on using the term “supermoon” to describe normal, regularly occurring events, and claiming that the star T Coronae will appear like a second Sun in the sky, when it will, actually, just about reach naked-eye visibility (at the time of writing the nova event is still yet to happen), the current headlines in the science press are that the Earth will soon gain a second Moon.

This news concerns the recently discovered asteroid 2024 PT5 which belongs to a class of asteroids known to make close approaches to the Earth. Typically such “close” approaches are of a mere one million to two million miles, that is 4-8 times farther away than the Moon which lies at an average distance of 250 000 (quarter of a million) miles.  There have been one or two observed passes in recent years that have come closer.

A Moon is a rocky or icy body that moves in a fixed path (orbit) around a larger body under the influence of that bodies gravitational field. What will happen to 2024 PT5 is that it will be influenced by the Earth’s gravitational field into a “horseshoe” shaped trajectory that will fling it back into interplanetary space some 57 days later.  This type of trajectory, usually called a “slingshot” is used by NASA and other space agencies wishing to send probes to the outer planets and was first employed by the Pioneer 11 space probe in 1974 when it used the gravity assist of Jupiter to alter its course toward Saturn.

2924 PT5 will only be under the influence of Earth’s gravitational field for 57 days and will not even complete a single full orbit around the Earth. Therefore it cannot be considered as a “Moon” but is merely a passing asteroid, albeit one of particular interest.

The asteroid is a relatively tiny fragment left over from the formation of the Solar System and is far too small to be visible to the naked-eye, binoculars or small telescopes. Some dedicated amateurs who have equipment set up to track and locate Near Earth Asteroids may be lucky enough to find it as a miniscule track on an image of a star-field because they know what they are looking for but, otherwise it is down to the large telescopes in professional observatories to keep an eye on this for us.

The giant planets, especially Jupiter and Saturn with their much greater and stronger gravitational fields  influence asteroids like this all the time. It is clear that many of the smaller Moons of the giant planets are gravitationally captured asteroids which have been pulled into fixed orbits around the planet.  Whilst the International Astronomical Union has made an (arbitrary) decision on how big something has to be in order to be a planet, it has made no such comparable decision about the size of moons, so the smallest speck of dust in a fixed orbit can be considered a Moon.