DEPLOYING AND TESTING JUNO

In space, there’s nothing to anchor an object in place, so a spacecraft will tend to rotate when left alone. After Juno separates from its rocket, it’s set to start spinning like a top, and it continues to whirl in a stable and predictable way for the rest of its life. With nothing to hinder Juno’s rotation, mission controllers can be confident that the spacecraft’s antennas, solar arrays, and science instruments are pointing in the right direction.

To adjust its orientation or to change how fast it spins, Juno can fire any one of its 12 thrusters. The spacecraft becomes more stable when it spins faster, and extra stability is most important when Juno enters Jupiter’s orbit. Juno can also stabilize its spin by slightly tilting its three solar panels – much like how outstretched arms help you remain steady on a balance beam.

One of the most important parts of a spacecraft engineer’s job is to anticipate and solve potential problems. The craft needs to be smart enough to monitor itself and be pre-programmed to respond to any glitches or unexpected trouble. Some of Juno’s key components have a backup system, ensuring that it can survive and continue its mission even if a critical system breaks down. For instance, if one of the batteries fails, Juno’s programmed to switch to the backup.

Juno’s equipped with sensors that keep an eye on pressure in the fuel lines, power from the solar arrays, the temperature of the compartment that houses the computer’s brain, and other factors important to the spacecraft’s health. The engineers have determined ahead of time what Juno must do to preserve its well-being if the numbers go beyond their expected range – if the computer gets too hot, for example.

And if the spacecraft encounters a situation it’s not prepared to handle, it’s programmed to phone home, going into what engineers call a “safe mode.” In Juno’s safe mode, it turns its solar panels to face the sun, making sure that it has enough power to communicate with mission controllers, who can then work to get it back on track.

Once Juno reaches a point just beyond the orbit of Mars, it will fire its main engines to perform a deep-space maneuver. This move gives Juno the needed boost toward Jupiter – otherwise, the spacecraft would circle the inner solar system forever. Juno’s adjusted trajectory will intersect Earth’s orbit, and the spacecraft will zoom right by Earth.

Three solar panels extend outward from Juno’s hexagonal body, spanning more than 20 meters (66 feet) – longer than a bus. The spacecraft is about 4 meters (around 14 feet) tall, higher than the top of the backboard on a basketball hoop. The solar arrays themselves are huge, having dimensions of 2.65 meters by 8.9 meters (about 8.5 feet by 29 feet). The three arrays have a combined area that’s over 70 square meters (over 760 square feet) – that’s more area than 27 full-size mattresses. But Jupiter is so far from the Sun that, despite their size, the arrays will only produce enough power for about four light bulbs.

Once Juno is safely spinning away from the last rocket stage, the solar arrays immediately unfold. To fit within the 5.4-meter (17.7-foot) wide nose of the rocket – called the payload fairing – the nearly 9-meter (29- foot) long arrays fold against Juno’s body like an accordion. The arrays deploy while Juno cruises around Earth, shrouded in the planet’s shadow. Until the arrays are open and functioning, Juno will be powered entirely by its batteries.

The magnetometer boom, which juts out from one of the solar panels, holds the instruments that will measure Jupiter’s magnetic field.

View Instruments Section

Arranged to stabilize Juno’s spin, the solar arrays are also positioned to allow the science instruments to have an unobstructed view. The other large components – a radiation-shielded electronics vault and a saucer-shaped main antenna – are stacked in the center of Juno’s forward deck, which also helps stabilize rotation. Engineers also had to design the spacecraft’s body to fit the largest of six microwave radiometer antennas on its side.