Juno visits her husband

Nieuws | de redactie
15 januari 2016 | As fabulous Rosetta/Philae around and on comet Chury and Pluto-visitor New Horizons come to an end, the next great adventure is near. Early July spaceship Juno will be near its goal: the enormous planet Jupiter – Juno’s Roman husband – and its clouds of gas, huge spots and many secrets.

July 4, 2016 is arrival day for the Juno mission, the first sent expressly to study the largest planet in the solar system since NASA’s Galileo mission in the 1990s. Humans have been studying Jupiter for hundreds of years, yet many basic questions about the gas world remain: How did it form? What is its internal structure? Exactly how does it generate its vast magnetic field? What can it tell us about the formation of other planets inside and outside our solar system?

Visiting the largest planet

Juno’s primary goal is to reveal the story of the formation and evolution of the planet Jupiter. Using long-proven technologies on a spinning spacecraft placed in an elliptical polar orbit, Juno will observe Jupiter’s gravity and magnetic fields, atmospheric dynamics and composition, and the coupling between the interior, atmosphere and magnetosphere that determines the planet’s properties and drives its evolution. An understanding of the origin and evolution of Jupiter, as the archetype of giant planets, can provide the knowledge needed to help us understand the origin of our solar system and planetary systems around other stars.

Jupiter is by far the largest planet in the solar system. The Juno spacecraft will, for the first time, see below Jupiter’s dense cover of clouds. This is why the mission was named after the Roman goddess, who was Jupiter’s wife, and who could also see through clouds. NASA’s Galileo mission made an important earlier voyage to Jupiter. One of its jobs was to drop a probe into Jupiter’s atmosphere. The data returned from that probe showed us that Jupiter’s composition was different than scientists thought, indicating that our theories of planetary formation were wrong.

Scientific goals

Juno uses a spinning, solar-powered spacecraft in a highly elliptical polar orbit that avoids most of Jupiter’s high-radiation regions. The designs of the individual instruments are straightforward and the mission did not require the development of any new technologies.

The principal goal of NASA’s Juno mission is to understand the origin and evolution of Jupiter. Underneath its dense cloud cover, Jupiter safeguards secrets to the fundamental processes and conditions that governed our solar system during its formation. As our primary example of a giant planet, Jupiter can also provide critical knowledge for understanding the planetary systems being discovered around other stars.

With its suite of science instruments, Juno will investigate the existence of a possible solid planetary core, map Jupiter’s intense magnetic field, measure the amount of water and ammonia in the deep atmosphere, and observe the planet’s auroras.

How do giant planets form?

Juno will let us take a giant step forward in our understanding of how giant planets form and the role these titans played in putting together the rest of the solar system. Theories about solar system formation all begin with the collapse of a giant cloud of gas and dust, or nebula, most of which formed the infant sun, our star. Like the sun, Jupiter is mostly hydrogen and helium, so it must have formed early, capturing most of the material left after our star came to be. How this happened, however, is unclear. Did a massive planetary core form first and capture all that gas gravitationally, or did an unstable region collapse inside the nebula, triggering the planet’s formation? Differences between these scenarios are profound.

Even more importantly, the composition and role of icy planetesimals, or small protoplanets, in planetary formation hangs in the balance – and with them, the origin of Earth and other terrestrial planets. Icy planetesimals likely were the carriers of materials like water and carbon compounds that are the fundamental building blocks of life.

Water and ammonia far away

Unlike Earth, Jupiter’s giant mass allowed it to hold onto its original composition, providing us with a way of tracing our solar system’s history. Juno will measure the amount of water and ammonia in Jupiter’s atmosphere and help determine if the planet has a core of heavy elements, constraining models on the origin of this giant planet and thereby the solar system. By mapping Jupiter’s gravitational and magnetic fields, Juno will reveal the planet’s interior structure and measure the mass of the core.

How deep Jupiter’s colorful zones, belts and other features penetrate is one of the most outstanding fundamental questions about the giant planet. Juno will determine the global structure and motions of the planet’s atmosphere below the cloud tops for the first time, mapping variations in the atmosphere’s composition, temperature, clouds and patterns of movement down to unprecedented depths.

Auroras near the poles

Deep in Jupiter’s atmosphere, under great pressure, hydrogen gas is squeezed into a fluid known as metallic hydrogen. At these enormous pressures, the hydrogen acts like an electrically conducting metal, which is believed to be the source of the planet’s intense magnetic field. This powerful magnetic environment creates the brightest auroras in our solar system, as charged particles precipitate down into the planet’s atmosphere.

Juno will directly sample the charged particles and magnetic fields near Jupiter’s poles for the first time, while simultaneously observing the auroras in ultraviolet light produced by the extraordinary amounts of energy crashing into the polar regions. These investigations will greatly improve our understanding of this remarkable phenomenon, and also of similar magnetic objects, like young stars with their own planetary systems.

Spinning through space

For Juno, like NASA’s earlier Pioneer spacecraft, spinning makes the spacecraft’s pointing extremely stable and easy to control. Just after launch, and before its solar arrays are deployed, Juno will be spun-up by rocket motors on its still-attached second-stage rocket booster. Juno’s planned spin rate varies during the mission: 1 RPM for cruise, 2 RPM for science operations and 5 RPM for main engine maneuvers.

To simplify and decrease weight, all instruments are fixed. While in orbit at Jupiter, the spinning spacecraft will sweep the fields of view of its instruments through space once for each rotation. At two rotations per minute, the instruments’ fields of view sweep across Jupiter about 400 times in the two hours it takes Juno to fly from pole to pole.

Power generation is provided by three solar arrays consisting of 11 solar panels and one MAG boom. Two 55 amp-hour lithium-ion batteries provide power when Juno is off-sun or in eclipse, and are tolerant of the Jupiter radiation environment. The power modes during science orbits are sized for either data collection during an orbit emphasizing microwave radiometry or gravity science.

Solar innovations

Jupiter’s orbit is five times farther from the sun than Earth’s, so the giant planet receives 25 times less sunlight than Earth. Juno will be the first solar-powered spacecraft designed to operate at such a great distance from the sun, thus the surface area of solar panels required to generate adequate power is quite large. Juno benefits from advances in solar cell design with modern cells that are 50 percent more efficient and radiation-tolerant than silicon cells available for space missions 20 years ago. The mission’s power needs are modest. Juno has energy-efficient science instruments.

Solar power is possible on Juno due to the energyefficient instruments and spacecraft, a mission design that can avoid Jupiter’s shadow and a polar orbit that minimizes the total radiation. The spacecraft’s three solar panels extend outward from Juno’s hexagonal body, giving the overall spacecraft a span of more than 66 feet (20 meters). The solar panels will remain in sunlight continuously from launch through end of mission, except for a few minutes during the Earth flyby. Before deployment in space, the solar panels are folded into four-hinged segments so the spacecraft can fit into the launch vehicle’s payload fairing.

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