At the same time, the DSN relays Cassini its next assignment: which instruments to turn on and when, how long to use its engine or thrusters and in which direction, and the time and date that it should next point its high-gain antenna toward Earth for another chat with the DSN, of course. The spacecraft shares its scientific observations and its health information - status of power supply, propulsion system, software, instruments, temperatures, and so on. When the DSN communicates with Cassini, the pair have much to say. A narrow signal is stronger than one thatвЂ™s spread out, but also only works well if itвЂ™s pointed the right direction, and the solar system doesnвЂ™t stand still, which presents a problem. But the antennas in the DSN have just one listener at a time, so they use their bowl-like shape to focus signals into a beam, like a spotlight. Signals from powerful radio stations on Earth fade if you drive a few dozen miles from the transmitter because their signal is spread out to reach as large an audience as possible. ThatвЂ™s in the neighborhood of the wattage of a big-city FM radio station. вЂњNormally, itвЂ™s an 18,000-watt transmission going up to the spacecraft,вЂќ Doody said. Then, in a nearby building, the signal is converted into science and engineering data.Д«ut while Cassini and other spacecraft transmit only a weak signal to Earth, the DSN transmits to spacecraft not in a trickle, but a blast. When CassiniвЂ™s signal arrives at a DSN antenna, the signal bounces off the antennaвЂ™s collecting dish, whose shape focuses the signal up to the reflector (the thing supported by long arms of scaffolding above the dish), which in turn reflects the now concentrated signal down into a narrow opening in the center of the collecting dish.

ThatвЂ™s just enough electricity to power a kitchen blender, and only part of that power can be used for communication with Earth nearly 900 million miles away. Pump Up the VolumeДЄll of CassiniвЂ™s hardware - its science instruments, propulsion system, computers, radio communication system and more - share electricity from the spacecraftвЂ™s power source, which produces about 600 watts. The vast size of the DSNвЂ™s antennas, along with amplifiers cooled to near absolute zero, allow them to detect and collect weak radio signals, which is important because Cassini barely whispers. In principle, every NASA spacecraft in the solar system and beyond always has a line of sight with some part of the DSN.Д®ach DSN complex has one radio antenna that measures 70 meters (230 feet) in diameter, several that measure 34 meters (112 feet) in diameter, and one antenna measuring 26 meters (85 feet) in diameter, so that each site can communicate with several spacecraft at once. Each complex is approximately 120 degrees of longitude from the other two, providing the DSN with 360-degree coverage of outer space. The DSN has three sites around the globe - near Madrid, Spain Canberra, Australia and Goldstone, California. That concrete and steel takes the form of the monstrous, dish-shaped radio antennas of NASAвЂ™s Deep Space Network (DSN). вЂњBut theyвЂ™re no good without thousands of tons of concrete and steel,вЂќ he said. вЂњTheyвЂ™re just little cities of engineering, all in a tiny little package,вЂќ said Dave Doody, leader of the Cassini missionвЂ™s real-time operations team at NASAвЂ™s Jet Propulsion Laboratory. They have their own power supply, attitude control system, science instruments, and a communications system with a radio antenna for communicating with Earth.

But by necessity, spacecraft must also be fully self-contained. Like NASAвЂ™s other spacecraft traveling beyond Earth orbit, Cassini would be unable to share its astounding discoveries with Earth if not for the Deep Space Network, a global collection of radio antennas that listen and talk to NASAвЂ™s fleet of spacecraft exploring the solar system.ДЇrom the moment humans started launching things into space, engineers have tried to make spacecraft as light and compact as possible so that rockets can heave them far from Earth.

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