Juno keeps in touch with Earth via a sophisticated two-way radio and antennas that link to NASA’s Deep Space Network. Because the spacecraft will travel so far from Earth, it can’t be steered in real time by mission controllers. A signal sent from Earth would take about 45 minutes to reach Jupiter, and it would take another 45 minutes to receive the spacecraft’s reply. This is why Juno (and other spacecraft that operate at great distances) must be able to carry out tasks automatically, following lists of instructions from Earth, and reporting its current status and the data gathered by its science instruments. For example, if Juno encounters a problem and enters the standby condition known as safe mode, the spacecraft will automatically reposition itself so that the solar arrays are pointed at the sun, and then wait for instructions from Earth.

The spacecraft has five antennas, including the largest and primary communication antenna, known as the high-gain antenna. Four other antennas can be used as backups, or when the main antenna is pointed away from Earth, for certain science operations and navigation maneuvers.

HIGH-GAIN ANTENNA (HGA) 
The high-gain antenna (named for the gain achieved though its narrowly focused beam, sits on top of the electronics vault. This 2.5-meter (8-foot) -wide, saucer-shaped radio antenna is Juno’s main communications link with Earth. It will be the primary antenna used during Juno’s time at Jupiter. The HGA has the strongest signal of the spacecraft’s five antennas, which enables Juno to transmit data at a much higher rate than the others. The antenna will transmit less power than a 40-watt light bulb, yet this will be sufficient to carry all the results from the scientific instruments, plus information about the spacecraft’s health and safety. Juno’s HGA is protected from heat produced by the sun’s harsh light with insulating blankets when it is in the inner solar system.
In addition to its communications role, the HGA also functions as part of Juno’s Gravity Science system. Like a laser, the antenna requires extremely accurate pointing because it sends and receives radio signals in the form of a tight beam.

MEDIUM-GAIN ANTENNA (MGA) 
The medium-gain antenna is one of two small antennas located next to Juno’s high-gain antenna. It has a wider field of view than the HGA so it doesn’t have to be pointed quite as precisely at the Earth to transmit and receive signals, but its signal is not as powerful. The MGA will be used during the cruise phase of the mission at times when it would be difficult to keep the high-gain antenna locked onto Earth while pointing the solar arrays more or less toward the sun.
The MGA is the antenna that the spacecraft will most often switch to if it enters safe mode because of a problem.
 
LOW-GAIN ANTENNAS (LGA) 
The two low-gain antennas have a wider field of view than the MGA, meaning they require less precise pointing to communicate with Earth. They have relatively weak signal strengths, so they can send data only at low rates. The fore LGA, on top of the spacecraft, is pointed in the same direction as the HGA and MGA; the aft LGA is on the bottom of the spacecraft and points in the opposite direction. Juno may use this antenna instead of the MGA sometimes if it enters safe mode.

TOROIDAL LOW-GAIN ANTENNA (TLGA) 
The toroidal low-gain antenna fills in the gap between the low-gain antennas by sending its signal out of the sides of the spacecraft. This antenna comes in handy during maneuvers when the other antennas must be pointed far away from Earth. Juno will rely on its TLGA during a couple of critical moments: the Deep Space Maneuvers, which adjust the spacecraft’s path on the way to Jupiter, and the Jupiter Orbit Insertion burn that will be executed upon arrival at the giant planet.

To keep in touch with Juno, ground controllers use the giant radio antennas of NASA’s Deep Space Network, or DSN. The DSN antennas are 70-meter and 34-meter (230-foot and 112-foot) -wide dishes located at three sites around the world: California, USA; Madrid, Spain; and Canberra, Australia. The DSN sites are positioned around the globe so that, as Earth rotates, the entire solar system is in view of at least one of the antennas. The DSN and Juno’s High Gain Antenna also work in concert to serve as the main components of Juno’s Gravity Science investigation.