## Wednesday, December 05, 2012

### Radio Signals in Space

I've discussed at length the myth that radio signals always travel at the speed of light. Post here. But I thought it would be an interesting exercise to determine how long it would take radio signals to reach various points in our galaxy . If you get the urge to check my math be aware that many sources cite distances in AU, (Astronomical Unit) which is equal to 149,597,870,700 meters, or about 92,955,807 miles. The basis for that unit is the distance of the Earth from the Sun which is random and pretty useless. More here.

The length of time needed to send radio signals depends on the straight-line distance between the two objects. I will determine that number with the simple equation T = D/C. The variable T is time, D is the distance and C is the speed of light (about 300,000 km/s). Because these are all moving targets and various planets are closer and further from Earth at different parts of their orbit these will be round numbers. For distance I will take an average of the apogee and perigee and then compute these based on that number.I am also omitting Centaurs from this list because their erratic orbits make my math meaningless.

The Sun
It would take 8 minutes for a radio signal from the Earth to reach the sun. Incidentally that's also the amount of time it takes for light from the sun to reach the Earth to traverse the same 93 million mile stretch.

Mercury
5 minutes doesn't sound like long but that's slower than a Youtube video on a shared DSL connection.

Venus
The one-way trip takes about 4.5 Minutes, longer than it takes to microwave dinner. It's distance can be anywhere from 26 to 160 million miles so it's time overlaps with Mars.

Earth
It's not zero, it's a minimum of 250 milliseconds for a radio signal to make the round trip from the surface to a geostationary satellite in orbit back to the surface. The Clark orbit is 22,236 mi above sea level so that's still quite fast.

The Moon
Ham radio enthusiasts routinely bounce signals off the Moon just for fun. The round trip takes about 3 seconds.

Mars
At about 33.9 million miles signals take about 21 minutes to arrive. Notably at it's shortest distance that whittles down to 4.5 minutes, which is similar to Venus. They are not equidistant, but the distance varies with orbit enough that the times overlap slightly.

Ceres (dwarf planet)
Ceres is a a rock–ice body  about 590 miles in diameter making up a third of the mass of the main asteroid belt. The one-way trip takes under 40 minutes, but it's still faster than some delivery pizza.

Jupiter
At about 480 million miles away the trip takes about 45 minutes. Famous for it's Jovian radio bursts, you can actually tune in to Jupiter. More here.

Saturn
It takes about 80 minutes for a radio signal to travel either to or from Saturn.  Yes, Saturn emits radio signals too. They have even been monitored by the Cassini spacecraft. The radio waves are connected to the auroras near the poles of the planet similar to those on Earth. You can hear a clip here.

Uranus
It is the last of the planets you can see with the naked eye. A radio signal takes around 155 Minutes to make the trip to Uranus.  I once had a delivery pizza take that long and I got a free 2-liter for my trouble.

Neptune
The most distant true planet, takes a whopping 250 minutes to receive radio signals from Earth.

Pluto (minor planet)
Though recently demoted, it is still a part of our solar system and also our second most massive dwarf planet. A radio signal would travel 3 billion miles and take over 4 hours to get here.

Sedna (Trans-Neptunian object)
It's not alone out there. Around the 8 billion mile mark you will also find dwarf planets Makemake, Haumea and others with less memorable names. With erratic orbits, radio signals take anywhere from 6-7 hours to reach this region.

Eris (dwarf planet)
We are way out now. Eris is almost 3 times as distant at Pluto at over 9 billion miles away. Radio signals take about 7.5 hours to get here.

Voyager Satellite
Presently on the edge of our Solar System, 11 billion miles away our signals now take over 17 hours to reach it. This satellite is now over 30 years old and still operating despite having less computing power or memory than my phone. It's still sending and receiving data in the  8 GHz range and will for years to come.