Star Spy: Close encounter
Saturday morning, about 8.30am, our planet will be visited by an intruder from beyond our region of the solar system.
In between the inner rocky planets, from Mercury through Mars, and the outer gas and ice planets, Jupiter through Neptune, is the Main Asteroid Belt.
It is mostly rocky debris of almost any size up to and including dwarf planet Ceres, all in orbit around the Sun.
Some of these asteroids have lost their way and entered the realm of the inner solar system where they can potentially cross orbits with the planets. Usually the planet will be elsewhere along its orbit, but occasionally not.
2012 DA14, as our visitor is affectionately called, is one such rogue asteroid.
We keep very good track of these near-Earth objects (NEOs), just in case one wants to cross our orbit at a place that will be occupied by us at the time.
2012 DA14 will come very close to doing just that on Saturday morning.
At the distance of 27,700km, it will pass not just inside the orbit of our Moon, but inside the orbit of many of our manmade satellites.
Nasa reports: "Since regular sky surveys began in the 1990s, we’ve never seen an object this big get so close to Earth."
Space is big and mostly empty, even inside the solar system, but it wasn’t always so.
Collisions were common in the beginning as the rocky planets formed from rocks and dust clumping together with more and more, rocks and dust. It was a bumpy ride.
At the size of half a football field, 2012 DA14 could have made a nice addition to a growing planet, and it would be interesting to observe it crash into another planet, but you wouldn’t want it to come down in your neighbourhood on Earth.
It is expected to pass close to Indonesia at 8.45am. In the unlikely event that you spot it with just your eyes, it means that astronomers have misjudged its passage. Otherwise, Google it and you may find observatories that are broadcasting its passage live.
A comet, the 6th to bear the name Lemmon for Mt Lemmon in Arizona where it was discovered, is also making a trip through the inner solar system.
Comet C/2012 F6 Lemmon, to be precise, is a long-period comet which means it has travelled a great distance to get here and it will be anywhere from 200 to a million years before it returns again to the Sun.
It is now visible to the naked eye, but only just. And it is a moving target. You have to know precisely where to look for it on a given night. It is in our southern sky, in the general region of the South Celestial Pole.
It has been observed to have a green colour and to be developing a tail that is visible with a telescope as you can see in Fraser’s images.
The tail consists of water and other ices that are vaporised by radiation from the Sun as the comet’s journey carries it closer and closer to the centre of the solar system.
These vapours also carry dust and other small particles from the nucleus or head of the comet out into the tail.
These bits are left behind and can be encountered by Earth as we move through our orbit around the Sun, and seen as shooting or falling stars as they burn up in our atmosphere.
In 1994 comet Shoemaker-Levy was seen by observers on Earth to break apart on its way out of the inner solar system, and the pieces crashed into Jupiter leaving a trail of dark spots across the planet’s dense atmosphere.
It is thought that the many collisions with these icy objects early in the formation of the solar system may have been responsible for depositing the water we find on Mars and the Moon, and in abundance in our own oceans.