We live in a chaos of electromagnetic energy. Visible, infrared and ultraviolet light courses omnidirectionally from the sun. A fraction of it bathes our planet, while some bounces off other planets, moons, comets and meteoroids. The visible light from stars up to 4000 light-years away can be seen from Earth with the naked eye. With instruments, astronomers can detect gamma rays from stars 13 billion light-years away. Radio waves from remote galaxies help Earth's official timekeepers monitor our planet's path around the sun.
Once per day, a minuscule stream of radio waves joins this cacophony, making the 13.8-minute trip from an antenna on Earth to an SUV-sized machine parked on the surface of Mars. Those short-lived waves represent our way - our only way - of communicating with Curiosity, the rover that NASA landed on Mars in August.
How, exactly, does information flow between NASA and its correspondent on Mars? Earthbound engineers exchange messages with Curiosity on a set daily schedule. Actually, "daily" isn't quite accurate. Mars takes about 37 minutes longer than Earth to complete a rotation, so astronomers refer to a Martian day as a "sol" for the sake of clarity. From here on, when I refer to a time, it's Mars time.
At approximately 10 am each sol, after the sun peeks over the Martian horizon and floods the landscape near the rover in light, NASA sends a packet of commands to Curiosity. Since a sol doesn't coincide with an Earth day, the agency can't always use the same antenna on Earth, which might not be facing Mars at the right moment. Instead, NASA uses the Deep Space Network, a system of antennae in the Mojave Desert, Spain and Australia.
The content of the instructions encoded in these radio waves depends on the sol. On many sols, the rover doesn't move a Martian centimetre. It digs into the soil, for example, or spends its time analysing the mineral contents of onboard samples it has collected.
When NASA does tell Curiosity to move, the process is deliberate. First, the engineers use imagery from the rover itself and from orbiters passing overhead to create a three-dimensional model of the surroundings. It's critically important to ensure that, wherever they direct the machine to go, it won't face any hazards. (Spirit's six-year tour of Mars ended in 2009 when that rover got stuck in a sand pit.)
When NASA is convinced a destination is safe, it transmits a set of co-ordinates for where the rover should go. NASA will also include a suggested path, but the rover has autonomy to make changes if necessary.
A set of commands also tells Curiosity when it should listen for a new set of instructions. There are contingency plans, so the rover is prepared if a transmission is delayed or missed for some reason. In the event that no instructions come for several sols, the rover takes protective actions. It is programmed to stop conducting scientific missions, stays put and listens for communications at predetermined times.
More important to the average Mars enthusiast than this daily to-do list is the information travelling from Curiosity to Earth. Those dazzling photographs that the rover takes need a little help to get here.
"Curiosity's transmitter is about [30 centimetres] in diameter and uses less power than the light bulb in your refrigerator," says Chad Edward, the chief telecommunications engineer for NASA's Mars Exploration Program.
To get its messages to Earth, Curiosity first sends information to a pair of orbiters, Odyssey and Reconnaissance, that were sent in 2001 and 2005, respectively, to analyse Mars from a distance and are constantly circling the planet. (The Mars Express orbiter, operated by the European Space Agency, is also available if necessary.) The antennae on the orbiters are more than 1300 times as powerful as the antenna on Curiosity. The rover waits for the orbiters to pass overhead to ship its messages, usually around 3 pm and again at 3 am.
"Some of the composite panoramas that the rover has sent to Earth comprise a few hundred megabits of data," says Edward. "Curiosity would take weeks to send that much data. Using the relays, we can have it in a day."
Since most of us live in a world where our laptops can lose WiFi signals if we walk out the front door and our mobile phones drop calls if we stray too far from a tower, it may seem incredible that NASA can control a robot millions of kilometres away. Curiosity, however, is relatively close by space communications standards. Voyager 1 is the most distant human-made object in the universe. It's about three times the distance to Pluto, and, with some antennae upgraded in the 1980s, we're still able to receive information from the spacecraft.
Talk about roaming. Maybe NASA should take over telecommunications here on Earth. Just a thought.