It’s getting crowded up there
IF you go out on a clear evening as the twilight is giving way to blackness, and gaze upwards, it is almost certain that you will see quite a few points of light steadily crossing the sky.
You will be watching several of the thousands of Earthorbiting artificial satellites, and you may have noticed the same in the early morning sky before the twilight becomes too bright.
I say “artificial” because Earth has a natural satellite: the Moon. We use the word “satellite” to describe both natural and artificial objects.
We often refer, for example, to satellites of other planets.
The easiest of those to see are the four brightest satellites of the planet Jupiter. Their existence was noted by the famous Galileo Galilei, who observed them through his primitive telescope in the early 17th century. They can be seen using a good pair of binoculars.
More than 8000 artificial satellites have been sent into orbit around Earth since the very first one, Sputnik 1, was launched in 1957.
However, they are not all active, and some have burned up on re-entry.
The total number of orbiting objects is far greater than simply the number of satellites that have been launched.
For example, some are fragments of rockets.
There are close to 20,000 objects being tracked that are 10cm or more across, and there are many more untracked ones of that size.
In comparison, a cricket ball is just over 7cm in diameter. That’s not to speak of countless other small pieces of debris — even small flecks of paint were known to leave pits in the windows of the space shuttles.
Of course, as we gaze up to the night sky, we don’t see the smaller orbiting objects, but there are plenty up there to watch.
Probably the most famous one is the International Space Station, which can be very bright indeed as it crosses the sky.
At the Launceston Planetarium we are sometimes asked why there are certain periods of time each night when satellites are more likely to be seen.
As mentioned above, these are the periods well away from midnight, in the evening or early morning hours.
The reason for this is that we see satellites only because sunlight is shining on them, and not on us.
Because they typically orbit several hundred kilometres up, the Sun may be below the horizon for us, but not for a satellite. You may have noticed an effect similar to this when you watch an aeroplane passing over shortly after sunset — the plane looks much more prominent than usual against the blue sky background because the sunlight is falling onto the aeroplane and not on you.
You can gain an even better appreciation of this by watching a satellite fading into darkness over a period of a few seconds, as the Sun sets for the satellite.
Even though it can be fun to watch for satellites, the increasing number of them in orbit poses a problem for astronomers.
This is especially true of the numerous Starlink satellites launched by SpaceX, the company headed by Elon Musk.
Beginning in 2019, hundreds of these have been launched, and there will be very many more.
The problem is so great that it has become necessary to find ways of darkening them.
One, called DarkSat, was painted to reduce the amount of reflection, but it was not a great success. Now, SpaceX is planning to install dark “visors” on satellites launched in the future, with the first trial one going up very shortly among the next batch.
Perhaps you have seen some of these Starlink satellites, appearing as strings of lights crossing the sky along very similar paths. At their brightest, they are a little brighter than the star Ginan, the fifth brightest star in the Southern Cross.
If it is clear tonight, you have a fine chance of seeing many of them passing very high in our Tasmanian sky, especially between about 6.10pm and 6.25pm. However, as you watch them, please spare a thought for the astronomers!