It is now official, humankind has put its toe into interstellar space as Voyager 1 crossed into the heliopause nearly a year ago. This milestone has been hotly debated in the scientific literature as heavyweight teams presented evidence pro and con. It was only in the last few weeks, however, all parties have agreed on what the data from the probe mean. And they agree that it is now past the interface between the solar wind and the great beyond.
Dr. Gary Flandro, then of the Jet Propulsion Lab in Pasadena and now of the University of Tennessee, discovered in the late 1960s that a special alignment of the outer planets would make possible a Grand Tour of our solar system. The two Voyager spacecraft were built on a now inconceivable three year schedule and launched in 1977 to take a look at Jupiter and Saturn. From there, they have just kept going.
Equally incredible is that debates on the state of Voyager are fueled by real data from a 35 year old probe with aging and failing instruments now 11.7 billion miles (~19 billion kilometers) from the Earth. This is 125 times the already large distance between the Earth and the Sun. Size, weight, and electrical power are always at a premium on spacecraft and there is never enough to go around. So, engineers expend enormous care in making sure that everything that does get on board counts. Voyager’s radio transmitter put out about 25 Watts of energy when new. If this energy were spread in all directions, it would not permit communication over such vast distances.
Fortunately, Voyager is equipped with a high gain antenna that collects and focusses this energy into a relatively narrow beam. The antenna’s gain of ~45 dB (decibels) means that the transmitter’s impact is multiplied by about 32,000. The focussed beam is equivalent to a 790 kilo Watt unit radiating in all directions. Some of this energy travels 19 billion kilometers and reaches Earth where a 70-meter diameter antenna (more below) picks it up.
We can now estimate how much power, P, a perfect 70-m antenna collects from Voyager under ideal conditions:
This kind of calculation with approximations, assumptions, and rough values is routine in technical fields to get a feel for numbers, to develop intuition, and for sanity checks. It is higher in the food chain than a scientific wild-ass guess. This particular crude one is in-line with a far more sophisticated calculation by Dr. Mike Willis. The answer, 10^-18W, is a decimal point followed by 17 zeros and then a 1. It is a millionth of a millionth of a millionth of a Watt or one attowatt. Detecting such a signal in the presence of ubiquitous noise borders on miraculous.
The miracle is performed by the Deep Space Network, a collection of sophisticated antennas near Madrid, Canberra, and in our Mojave Desert. These antennas work individually when one is facing Voyager’s direction. They also work together in a method called arraying to simulate the effect of an antenna nearly the size of the Earth. JPL manages DSN for NASA and the methods make for fascinating reading.
Let’s look at one small piece of the puzzle – taking a signal that’s a millionth of a millionth of a millionth of a Watt and boosting it to a level that can be used. For decades, the right tool has been the maser, the much lesser-known predecessor of the laser. Once the antenna scoops up and refocusses the radio frequency energy from Voyager (or another distant probe such as Cassini around Saturn), it is presented through a system of waveguides (think rectangular metal pipes) to a crystal of ruby held at a couple of degrees above absolute zero, in a modest magnetic field, and bathed in microwave radiation. The crystal thus prepared takes in the attowatt of power from the antenna and faithfully boosts it by a factor ten thousand to one hundred thousand without adding too much noise. The resulting signal can then be handled by other circuitry using more well-known semiconductor devices. A ruby maser amplifier figured prominently in the discovery of the cosmic background radiation that established our present understanding of the origin of our Universe. A ruby crystal was also central to the first demonstration of laser action.