Voyager radios communicate from beyond

When two unmanned spacecraft, Voyager 1 and 2, were launched in 1977, there were probably few who believed they would still be sending back useful information 35 years later, from 32 billion kilometres away, with Australia still playing a part in receiving and processing this information.

The two identical probes, weighing 722 kg, were originally sent to look at the planetary systems of Jupiter and Saturn, but the journey also included finding and studying the boundaries of the solar system, which includes the Kuiper belt, the heliosphere and interstellar space.

However, the craft have continued their mission and will eventually finish up deep in space orbiting forever until they hit something or something hits them and they are destroyed.

Both craft have flown through Jupiter’s dangerous radiation environment and have survived the extreme cold of being so far from the sun. Both have visited Uranus and Neptune sending back information and images that left scientists gasping. And both have become the first objects to leave the solar system.

Voyager 1 has three-axis stabilisation gyroscopes and referencing instruments including a sub sensor/compass star tracker that ensures the radio antenna is pointed towards Earth. The radio system was designed to be capable of operation over an extremely long range, beyond the boundary of the solar system, and to operate for many years.

At the heart of the system is a 3.7 m diameter parabolic high gain antenna, designed to communicate with one or more of the three deep space network stations on Earth. Operating in the S-band (13 cm wavelength) and X-band (3.6 cm wavelength), information at up to 115.2 Kb was possible when the craft was at Jupiter distance from the Earth. As the craft moved outwards, so the speed of data transfer slowed.

In addition, two 10 m long whip antennas gather information on planetary radio astronomy and plasma waves. At 13 m long an Astromast triaxial beam holds two low-field magnetometers. A wide and narrow angle camera gathers images while two television-type cameras gather high- and low-resolution images.

To power the equipment, three radioisotope thermoelectric generators, each containing 24 pressed Plutonium-238 oxide spheres, generate heat equivalent to about 157 W at 30 VDC of usable power, the remainder being dissipated as waste heat.  At launch, there were about 470 W available but the output is declining as Plutonium-238 has a half-life of 88 years and is losing about 4 W a year as it decays.

As the power goes down, some of the instruments have been switched off.  Eventually the gyroscopes will no longer be controllable from Earth and Voyager will be left to the whims of outer space winds. However, there is a ‘fail safe’ system that will allow the craft to send back information even when Earth loses complete control.  Total power loss is still some years ahead and it is expected there will be sufficient power to operate some of the equipment until at least 2025.

When the craft is unable to communicate directly with the deep space stations, a digital recorder can store up to 62,500 Kb of information for transmission at a later time. Even with radio waves travelling in a straight line at 300,000 m per second, it takes something approaching 20 hours for signals to reach Earth from the craft.

The Australian Tidbinbilla antenna near Canberra is one of the three worldwide deep space network stations that are placed about 120° apart around the Earth.  The other two stations are in California and Spain. The antennas are usually cooled to about 18K to reduce noise and clarify the tiny received signals.

At the moment, Voyager is sending back information on the million miles an hour wind that blows out from the sun. This indicates that the craft is still inside the massive bubble the sun creates around itself called the heliosphere. Beyond this region is interstellar space.

It says a lot for the workmanship and technology of the 1970s for these devices to be still working in conditions that would challenge today’s most basic equipment. The Voyagers have become the first man-made objects to ever travel this far and their journey is not over yet.