A fridge-bulb is still talking from the stars
Voyager 1’s transmitter radiates about 22 watts. From interstellar space, a 70-metre dish on Earth still locks onto it.
The high-gain antenna, a dish 3.7 metres across, aims those watts at Earth. Spread over more than twenty billion kilometres, what arrives at a Deep Space Network antenna is on the order of an attowatt — a billionth of a billionth of a watt. The receivers are cooled so their own heat does not drown the signal. The bit rate, once tens of kilobits a second, is now a slow drip.
The electricity comes from plutonium-238, decaying in a thermocouple with no moving parts. The isotope has a half-life of about 88 years, and the thermocouples age too, so the budget falls by a few watts a year. Instruments have been turned off in a planned retreat. The carrier has not.
Caveats
- The exact received power depends on distance, which dish is listening, and which Voyager. Both spacecraft are in this regime. The 22-watt figure is the radio, not the whole power budget.
- A fridge-bulb comparison without the dish would be a lie. The antenna concentrates the watts into a narrow beam. Geometry is doing as much work as the transmitter.
Sources
- 1Ludwig R., Taylor J.. Voyager Telecommunications. JPL DESCANSO Design and Performance Summary Series, 2002.