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D - Factary


Deep Space Network (DSN)


A NASA network of communication dishes used to communicate with spacecraft beyond Earth's orbit.


Deep Space Network (DSN)



The DSN consists of three deep-space radio dishes placed approximately 120 degrees apart around the world: at Goldstone, in California's Mojave Desert, near Madrid in Spain and near Canberra in Australia. Placing them like that allows constant observation of spacecraft as the Earth rotates and helps to make the DSN the largest and most sensitive scientific telecommunications system in the world.




The amount of mass per unit volume or to put it more simply, how much stuff is in a given space. In the SI system density has units of kg/m3.


The density of water is 1000 kg/m3, while the density of air near the ground is about 1.2 kg/m3. That means that the mass of the air in a box with 1 metre long sides is 1.2 kg - not the "hardly anything" most people would guess - and it will weigh 12 newtons!



Astronomers sometimes also use a different kind of density called number density. This describes the number of particles per cubic centimetre. This has the units cm-3 (per cubic centimetre). The number density in the solar atmosphere is typically 1010 cm-3 while in the Earth's atmosphere it is about 1019 cm-3. In other words, near the ground, there are 1019 atoms or molecules in every cubic centimetre of the Earth's atmosphere. That is a billion times more dense (in number terms) than some parts of the Sun's atmosphere we study.


In 'normal density' (i.e. mass per unit volume) the Sun's atmosphere is one hundred billion times less dense than the Earth's atmosphere.


BrainiacYou may also come across something called relative density. This is where everything is compared with the density of pure water. In this way, water has a relative density of 1.0. Anything with a density smaller than this will float and anything denser will sink. Interestingly enough when astronomers first calculated the densities of the planets in the Solar System they discovered that Saturn has a relative density of 0.7 - it would float in a big lake of water!


Differential Rotation


Is the change in rotation rate with latitude. To put it another way, it’s when an object’s rotation rate depends where on its surface you measure it. Solid bodies like the Earth do not show this effect (one spin of the Earth is the same whether it is measured in Australia or the UK). However the giant planets, such as Jupiter and Saturn, and the Sun do have differential rotation. This is because they are composed of gas and plasma. Material at low latitudes (near their equators) rotates faster than at high latitudes (near the poles). For the Sun, low latitudes rotate once every 26 days, but higher latitudes take over 30 days to rotate once.




Diffraction of wavesThis is the spreading out of waves when they pass an obstacle or through a gap. Diffraction occurs the most when the gap is of similar size to the wavelength of the wave. Diffraction can occur with any wave: water waves, light, sound, radio waves etc.


The diffraction of X-rays can be used to study the structure of crystals. This is because the wavelength of the X-rays is similar in size to the 'gaps' between the crystal’s molecules.


When parallel waves approach a gap in an obstacle, they are diffracted through the gap. A great example of this is shown in this picture. The waves are coming from the bottom right of the image and notice how they are diffracted through the gap between the islands.


Diffraction grating


Image of a CDSee also, spectrum and diffraction.


A device for producing a spectrum. Instead of using a prism to spread out the wavelengths, a series of scratches on a glass surface can be used. This makes use of the effect known as diffraction. The same effect can be seen on any CD where the tracks act as the 'scratches'.




The visible surface of the Sun (or any heavenly body) projected against the sky. It can also refer to any flat, round object, like the disk of gas and dust that forms around some stars.


Image of Christian DopplerDoppler, Christian (1803-1853)


Doppler was an Austrian-born mathematician, physicist and astronomer who first put forward the idea which describes how the frequency of waves from a source change with the source’s velocity relative to an observer. This is now known as the Doppler principle or Doppler effect. For much of his life Doppler suffered ill-health and never really settled in one place. However, there was a certain genius to the man who had the foresight to predict the following about his discovery:


“It is almost to be accepted with certainty that this will in the not too distant future offer astronomers a welcome means to determine the movements and distances of such stars which, because of their unmeasurable distances from us and the consequent smallness of the parallactic angles, until this moment hardly presented the hope of such measurements and determinations.”


In other words he could see that in the future astronomers would use the Doppler effect to measure the velocities and distances of far away stars. Exactly what is done today!


Doppler Shift


The change in the wavelength of radiation received from a source because of its motion along the line of sight to the observer. A Doppler shift in the spectrum of an astronomical object is commonly known as a redshift when the shift is towards longer wavelengths (the object is moving away) and as a blueshift when the shift is towards shorter wavelengths (the object is approaching). Doppler shifts occur in sound waves as well as in electromagnetic radiation.



Be careful: you might read about the redshift of very distant galaxies. It is a similar shift of the spectrum to longer wavelengths, but is not caused by the Doppler effect. This redshift is caused by the expansion of the universe, but that, as they say, is another story!



Here's a good example of scientists not always getting things right and it involves the Doppler effect. On the recent Cassini-Huygens space mission to Titan (one of Saturn's moons) scientists discovered a problem which they hadn't taken account of. It was planned that the main spacecraft would drop a robotic probe into Titan's atmosphere so it could find out what the conditions down there were like. Unfortunately in the planning it was forgotten that after the two parts of the spacecraft separated, they would have a large relative velocity and so the frequency of the radiation (radio waves) they were supposed to use to talk to each other would be changed by the Doppler effect. Because of this change they wouldn't be able to tune in to the correct wavelength to hear each other's messages. Well nobody's perfect! Fortunately this error was discovered in time and the engineers have now figured out a change to the spacecraft orbit which will keep the Doppler shift small enough not to affect the communications.




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