The Juno probe to Jupiter has fired its engine to refine its trajectory to bring it back to Earth, not permanently, just for a gravity assist.
Maneuvering the spin-stabilized Juno probe is rather different from maneuvering the three-axis stabilized craft that NASA is now accustomed too.
Juno relies on the property of conservation of angular momentum to maintain its orientation in space. Physical laws dictate that a spinning object will resist changes to the direction of its spin axis. By evenly distributing Juno’s mass around its axis of symmetry, the probe is able to exploit this physical phenomenon to stabilize itself. However, this makes maneuvering the spacecraft somewhat challenging.
Before Juno can fire its main engine, it must be pointed in the correct direction. This means changing the direction of its spin axis, which the craft will naturally resist. And since the craft is spinning, it cannot fire its thruster continuously or they will impart momentum in the wrong direction. So the Juno team commands the 12 small 4.5 N hydrazine monopropellant thrusters to fire once per rotation to make sure that the momentum is changed in the correct direction. But some of the energy of the thrusters will be wasted by pushing the spacecraft into a mode called nutation. This means the spin axis will trace a small circle in space. Juno will measure the nutation remaining after its directional thruster firings and fire its thrusters again to correct this undesired motion.
But pointing the spacecraft is not the only challenge. The helium tanks must be heated to make them ready to pressurize the main propellant tanks. Juno uses hydrazine for fuel and nitrogen tetroxide for oxidizer for its 635 N main engine. The 6 fuel and oxidizer tanks are arranged in a circle about the spin axis to balance the propellant mass, which actually outweighs the solid components of the spacecraft.
Then the spacecraft’s spin rate must be increased from 2 rotations per minute (rpm) to about 5 rpm.
And Juno’s telemetry system must be switched from a full, digital report mode to a “tones” mode. In this telemetry mode, Juno only transmits a continuous tone to report its status; one tone may mean “everything is fine,” another might mean “I am firing the engine,” and another might mean “there has been an error.” This is necessary because the spacecraft’s high gain antenna will no longer be pointed at Earth, and the low gain antenna has a much lower data rate.
This list somewhat understates the complexity of readying Juno for an engine firing, but it gives the general flow. Close monitoring from its controllers is necessary for a successful maneuver.
The burn on August 30 and the one today, September 4, will set Juno up to fly past Earth for a gravity assist. In this kind of maneuver, the spacecraft will “steal” some of the Earth’s orbital momentum. Since the Earth’s mass is enormous, and Juno’s is insignificant by comparison, Earth will lose only a tiny amount of velocity, while Juno will gain a 7 km/s boost, which is enough to send it on its way to Jupiter.
Juno will also make observations of the Earth during the flyby, with a few instruments active to study the Earth’s magnetic field.
Also during the flyby, Juno will experience the last of two eclipses it will endure during its flight. Juno will pass through Earth’s shadow for about 20 minutes, cutting it off from the Sun that provides its power. But Juno will not be helpless; it has lithium-ion batteries rated at 55 amp-hours. The previous eclipse was experienced shortly after launch, and lasted 26 minutes. But neither of these eclipses are as stressful as Jupiter orbit insertion (JOI), when the spacecraft will be pointed partially away from the Sun for nearly 3 hours.
Juno’s primary mission at Jupiter is to study the planet’s powerful magnetic field and intense radiation belts. To endure this harsh environment, Juno’s electronics have been sealed away in shielded boxes.
Juno’s engine firings will set up its encounter with Earth in October of 2013, and then its rendezvous with Jupiter in July of 2016.