On July 4, Americans usually celebrate Independence Day with parades, fireworks and picnics, but this year NASA is adding its own contribution to festivities as the Juno deep space probe becomes just the second spacecraft in history to orbit Jupiter. After a five-year journey, the solar-powered unmanned explorer will autonomously fire its main engine for 35 minutes as it starts a 20-month mission to study the gas giant. What will be found remains to be seen, but if history is any indicator, it's likely to be very unexpected.

When Juno reaches Jupiter on July 4, at 8:35 pm PDT (Earth Received Time) it will have traveled a distance of about 1.74 billion mi (2.8 billion km) and will be so far from Earth that a radio signal will take 48 minutes to reach us. After the spacecraft completes its burn, it will settle into a highly elliptical polar orbit that will eventually send Juno within 2,900 mi (4,667 km) of the cloud tops.

The six-year US$1.1 billion mission will be the ninth to visit Jupiter and the second to orbit the giant. Its mission is to study the gas giant's atmosphere, structure, and magnetosphere over the course of 37 orbits, which will see it come closer to the cloud tops even than NASA's Pioneer 11 spacecraft, which came within 27,000 mi (43,000 km) in 1974.

Jupiter and its mini-Solar System of 67 moons (and counting) have been surprising scientists for over 400 years. Before the invention of the telescope, it was just the third brightest object in the night sky and the second slowest planet as it moved majestically across the heavens. Today, we know that it's the largest planet in system and could contain a thousand Earths with ease. It holds many clues to the early history of the Solar System, acts a giant laboratory for understanding meteorology, provides us with clues about how stars form, and even how life may have started on Earth.

Galileo and co.

Jupiter has been throwing curves to scientists for four centuries. When Galileo turned his first primitive telescope on the planet in 1610, he was astonished to find four moons circling it – the ones we now know as the Galilean moons: Ganymede, Io, Europa, and Callisto. In a time when the scientific orthodoxy stated that the science was settled and that everything in the heavens revolved around the Earth, this observation was gunpowder. It provided the first evidence that some bodies didn't orbit our planet and gave credence to the new Copernican system that put the Sun at the center of the Universe.

Beyond this, the new moons were seized upon as a clock in the sky that any mariner could focus on and tell the time for purposes of navigation by checking their positions in the sky against earthbound calculations. It was a nice idea, but by the 1670s it was obvious that when Jupiter was on the opposite side of the Sun to Earth this "clock" was off by 17 minutes because (surprise!) the speed of light is finite. So Jupiter introduced a new physical constant and started science on the road to Relativity.

Beyond its moons, Jupiter doled out new revelations to those who peered at it through telescopes. It turned out to have a diameter of 88,846 mi (142,984 km) at its equator and its ten-hour day spun the giant so fast that it flattened visibly at the poles. Instead of a surface there were colored bands of clouds that swirled about, and there was the great Red Spot that seemed to be some sort of weather formation, yet remained there for century after century.

Even by the mid 20th century Jupiter was still surprising observers. When the first radio telescopes were turned on it in 1955, the planet blasted out incredible bursts of radio energy as if nuclear explosions were going off. In addition, Jupiter was giving out more heat than it absorbed from the Sun, indicating that there were some very interesting dynamics going on at its core.

The first probes

One would think that when NASA sent the first eight missions to Jupiter things would settle down, but they just got weirder. Even getting to Jupiter was an oddity. It's a truism that in spaceflight the first step is the hardest. Most missions expend most of their fuel just trying to get out of the Earth's atmosphere and the deepest part of the planet's gravity well. Getting to Jupiter turned out to take just as much energy again as it does to reach low Earth orbit to begin with. Worse, once at Jupiter, getting about its system of moons was as hard as traveling between the planets.

This was the proverbial hard nut to crack.


It also turned out to be a useful one because all that gravity produced by that massive planet could be very useful. By approaching Jupiter on a parabolic trajectory, a spacecraft could use the planet's gravity as an invisible slingshot to hurl it off at much higher velocities. This is what happened during the Pioneer 10 and 11 missions, which flew past Jupiter in 1973 and 1974 respectively. The additional oomph they picked up increased their speed to the point where they became the first manmade objects to set on an interstellar trajectory and break free of the Solar System forever.

During their very brief visit, the Pioneer probes provided scientists with the most detailed view yet of Jupiter. This was anticipated, but the intense belts of radiation that surrounded the giant were not. Bombarding the spacecraft with ten times the radiation they'd been designed to withstand, the belts caused multiple malfunctions, especially in the imagers. NASA quickly had to go back to the drawing board to harden future probe designs and recalculate trajectories to keep missions away from the heaviest radiation belts.


In 1979, two more flyby missions reached Jupiter – Voyager 1 and 2. Carrying a more sophisticated suite of instruments and a record containing greetings from Earth to some distant civilization among the stars, the Voyagers each had about 48 hours each to study the Jovian system and the result was, again, a bag of the unexpected.

Data sent back by the spacecraft showed that the Red Spot was an incredible storm – a hurricane 17,000 mi (28,000 km) across that swirled counterclockwise and was filled with and surrounded by complex eddies. Jupiter itself had a ring of debris surrounding it like a cheap knockoff of Saturn's more spectacular version. Meanwhile, Io exhibited the first active volcanoes found off Earth, Ganymede's icy surface was split by plate tectonics, and Europa yielded the first evidence that it might have a subterranean ocean under its crust that could harbor life.

Passing shots

Not all of the probes that visited Jupiter were intended to study it. The slingshot maneuver turned out to be so useful that it became a standard procedure for NASA when it wanted to send spacecraft to various places in the Solar System while using the minimum of propellant. Since 1992, three of these used Jupiter as a boost point.

The first of these was the Ulysses solar probe, which flew by Jupiter on February 8, 1992. Unlike Pioneer and Voyager, Ulysses was intended to study the polar regions of the Sun. To put it into a very high-inclination orbit, it swung by Jupiter and revisited the planet again in 2004, though at a more respectable distance of 149 million mi (240 million km).

Artist's impression of New Horizons(Credit: NASA)

Another passerby was the Cassini probe bound for Saturn. It came within 6 million mi (10 million km) of Jupiter on December 30, 2000. Not wanting to miss an opportunity, NASA ordered it to take about 26,000 images to create the most detailed map of Jupiter yet with a resolution that can identify features as small as 37 mi (60 km) across.

Finally, there was the New Horizons mission to visit Pluto. It passed by at a distance of 1.4 million mi (2.3 million km) on February 28, 2007.

Galileo orbiter mission

However, the most productive probe hands down was the Galileo orbiter mission that arrived at Jupiter on December 7, 1995 to begin an eight-year detailed survey of the Jovian system. The first spacecraft to orbit Jupiter, it was specially hardened against radiation for its prolonged stay, but Jupiter was still handing out surprises as the radiation levels again exceeded the craft's design, causing malfunctions, including electrical arcs between rotating and nonrotating parts, loss of data, and kicking the spacecraft into safe mode on several occasions.

Despite this, Galileo carried out multiple flybys of all the Galilean moons and the first of the moon Amalthea. In December 1995, it launched a probe at Jupiter itself, which descended into the atmosphere on a parachute and returned telemetry before it was finally crushed and burned. In addition, Galileo confirmed the existence of an ocean under Europa's surface, discovered a magnetic field surrounding Ganymede, and the complex plasma currents between Io and Jupiter.

Galileo was destroyed in a controlled impact with Jupiter on September 21, 2003 to avoid biological contamination of Europa after the spacecraft shutdown for good.

Juno approaches

Named after the Roman goddess and wife of Jupiter, the Juno mission lifted off from Space Launch Complex 41 at Cape Canaveral Air Force Base on August 5, 2011 atop an Atlas V booster. Even with this much power behind it, the spacecraft still needed to execute a flyby maneuver of Earth on October 9, 2013, passing within 347 mi (559 km) to build up enough speed to reach Jupiter.

On July 4, Juno's LEROS 1b main engine will open its debris shield and fire for 35 minutes as it builds up 145 lb of thrust as it burns a mixture of hydrazine and nitrogen tetroxide. Meanwhile, attitude control will be maintained by the 12 thrusters of the monopropellant reaction control system (RCS).

Solar panels

One way in which Juno has already made history is that it's the first outer-planet probe to use solar panels – the farthest that any solar panel has ever traveled from the Sun. NASA says that this is due in part to the plutonium shortage that is stalking the American space program, as well a result of advances in solar cell technology.

Though Jupiter only gets four percent as much light as Earth, the three symmetrically placed panels weighing 340 kg (750 lb) generate 486 W of electricity. Each panel measures 2.7 x 8.9 m (9 x 29 ft) and the entire array has the surface area of a basketball court – the biggest ever sent by NASA into deep space. It's an arrangement that has worked so far, but the space agency has had to calculate orbits designed to keep the panels in the sunlight as much as possible to power the two 55-amp-hour lithium-ion batteries.

Radiation vault

The biggest hazards are the belts of radiation around Jupiter. Similar to the Van Allen belts of Earth, the Jovian belts are so radioactive that Juno is equipped with the "Juno Radiation Vault," which is a box with one-centimeter thick walls of titanium weighing about 400 lb (172 kg) that protects the computers and avionics along with the spacecraft's hardened wiring.

NASA says that the vault will provide enough protection to allow the spacecraft, with care, to survive for 20 months, by which time secondary emissions caused by high-energy electrons striking the vault will degrade the electronics. However, some instruments, such as the JunoCam and Jovian Infrared Auroral Mapper (JIRAM) will only survive about eight orbits before they are expected to fail.


So why is Juno going to Jupiter? NASA says its objectives are to learn more about how Jupiter formed and its dynamics. Does it have a rocky core that shows that it formed like the Earth, or is its core made of hydrogen, so it was formed by a collapsing gas cloud like the Sun did? What is Jupiter's magnetic field like? How is the planet's mass distributed? What is the nature of the giant aurorae at the poles? How does the atmosphere vary from place to place and at various depths? What's the structure of its magnetic field?


To answer these questions, Juno is equipped with a suite of instruments. The most familiar is its camera/telescope, which will send back images of the planet, though this is only expected to survive for about seven orbits before radiation destroys it. Other instruments include:

  • Microwave radiometer (MWR). This measures electromagnetic radiations in the microwave spectrum. Its purpose is to look for water and ammonia in the deep layers of the Jovian atmosphere down to 500 to 600 km (310 to 373 mi).

  • Jovian Infrared Auroral Mapper (JIRAM). This is a near-infrared instrument for looking at the upper layers of the atmosphere to detect water, methane, ammonia, and phosphine while tracking cloud movements.

  • Magnetometer (MAG). MAG is designed to map the Jovian magnetic field in three dimensions as a way of learning more about Jupiter's internal dynamics.

  • Gravity Science (GS). This maps Jupiter's gravitational field to sort out the distribution of mass inside the planet.

  • Jovian Auroral Distribution Experiment (JADE). This measures the angular distribution, energy, and velocity of low-energy ions and electrons in Jupiter's aurorae.

  • Jovian Energetic Particle Detector Instrument (JEDI). This measures the angular distribution, energy, and velocity of high-energy ions and electrons in Jupiter's aurorae.

  • Radio and Plasma Wave Sensor (Waves). This instrument measures radio and plasma spectra in the auroral region as a way of mapping the currents in the aurorae.

  • Ultraviolet Imaging Spectrograph (UVS). As the spacecraft rotates, the UVS uses a slit spectrograph to record ultraviolet light.

In addition to the instruments, Juno also carries a plaque provided by the Italian Space Agency commemorating the achievements of Galileo Galilei and three of the farthest-traveled lego figures in history. Made of aluminum instead of plastic to withstand the conditions of space, the trio depict the gods Jupiter and Juno, and Galileo with his telescope.

NASA also hopes to boost public interest and involvement with an invitation for people to vote on the NASA website as to where to point the JunoCam onboard color camera.

What's next?

So far, NASA says that Juno is in excellent health and behaving normally as engineers continue to evaluate the spacecraft's trajectory and send instructions for any needed course corrections. Because of the great distance and the harsh, unpredictable environment, Juno will not only carry out the July 4 insertion burn autonomously, but is also programmed to evaluate and compensate for any malfunctions during the maneuver.

If the insertion is successful, Juno will carry out two 53-day orbits before a second course correction burn sends it into a 14-day polar orbit. What will happen afterwards isn't certain, but Jupiter will no doubt have a few surprises in store.

View gallery - 16 images