GrahamS
Can someone explain how it is that we can receive a signal from a a tiny spacecraft stuck to a comet more than 300 million miles away, hurtling through space at 34,000mph, and yet I have to stand at the window to use my mobile in the house?
Ok, brace yourself, here comes the science:
A mobile phone, or “cell” phone as it used to be called talks to an antenna on a fixed mast. Unfortunately, that mast does not know where your phone is. Broadly speaking that isotropic (and not in reality due to various factors too complex to go into here) antenna is transmitting it’s radio waves outwards in a sphere. So, the area of that sphere , relative to the distance(r) you are away from it is 4*pi*r^2. (note the squared term).
Take a typical phone transmitter at around 10w total power output, now divide that power into the area of the transmission sphere. You can see that the total power available for the phone to pick up is tiny.
In fact, even in Line of sight of the tower, without any other blockages, reflections or absorbers, just 1km away, the power is just 80picoWatts/cm^2.
Now, luckily, although the Comet Lander is a LONG way away, we know exactly where it is, and so instead of transmitting our data out in all directions, we can use a High gain antenna, pointed directly at the space craft. And of course, the space craft knows where earth is too, so it will gimball it’s antenna to keep it pointing at us. Finally, of course, we don’t need to make our space communication antennas “pocket” sized, so we can use a massive area to capture as much of the transmission power as possible:
DeepSpaceNetwork[/url]
If you have a spare minute, you can “do the sums” for the communication with Voyager 1, which is around 12Billion miles away, and we can still talk to it!!!