In this last article of 2022, it seems appropriate to mention two of the most exciting things in astronomy that happened this year. The first is the successful launch and deployment of the James Webb Space Telescope, the biggest optical telescope yet to be operated in space. It is producing a steady stream of stunning images. The second is the start of construction of the Square Kilometre Array, the biggest radio telescope in the world.
Anyone who has done astronomy from the ground will know why we want to put telescopes in space. A big chunk of useful observing time for ground-based telescopes is lost because there is not much that can be done during the hours of daylight, when the sky is bright blue. Some nights are cloudy, making observations impossible.
Then, on top of all this is the most frustrating issue of all, "seeing." Some nights the atmosphere is turbulent, making looking through a telescope like looking at the bottom of a rippling stream. When the seeing is bad, the images shimmer and shake, and maybe for the odd second or two, the atmosphere steadies and we see a sharp image; then the turbulence comes back. On occasion there are nights with perfect seeing.
We can improve things by putting our telescopes on top of high mountains; this puts them above a good part of the atmosphere, increasing the chance of getting sharp images for long enough to make useful observations. If we put a telescope in space, outside the atmosphere, the seeing is always perfect, there are no clouds, and with no scattering, we can see faint objects even when the sun is not blocked out by the Earth. Why then do we still put observatories on the Earth's surface?
There are two reasons. Firstly we can make bigger ground-based telescopes than we can space-based telescopes. Secondly, space-based telescopes are hard or impossible to modify after they have been launched. so they are hard to upgrade or repair, whereas it is easy to put new instruments on ground-based telescopes. If they don't work well, it is just as easy to take them off.
The James Webb Space Telescope has a 6.5 metre mirror. This is smaller than the mirrors on the leading ground-based telescopes. However, that 6.5m mirror was too big to fit into any current launch vehicle, so it had to be folded up and then unfolded in space. That this worked so well is a monument to the engineers who built it.
Even though the amounts of power radiated by cosmic radio sources are huge, they lie at enormous distances. Probing further out in space, back in time towards the beginning of the universe has driven a quest for more and more sensitive radio receivers. Over the last three decades there has been tremendous progress, so that we are now coming up against the limits imposed by Mother Nature. We might be able to double the sensitivity of our receivers, but to probe the beginnings of the universe we need around one hundred times more sensitivity. There is only one way to achieve this sensitivity, and that is to make an absolutely enormous antenna, to catch as much signal power as possible. We need about a square kilometre of signal collecting area.
We cannot build a single dish that big, but we can use modern digital signal processing techniques to make that square kilometre collector out of an array of thousands of cheap, small antennas. Since imaging detail depends on the size of the array, the antennas will be in two clusters: one in Western Australia and the other in South Africa.
After sunset, Saturn lies low in the southwest, Jupiter higher in the south and Mars even higher in the southeast. The Moon will reach First Quarter on the 29th.
Merry Christmas and a Happy New Year to All!
Ken Tapping is an astronomer with the National Research Council's Dominion Radio Astrophysical Observatory, Penticton, BC.
This article is written by or on behalf of an outsourced columnist and does not necessarily reflect the views of Castanet.