Planets

Juno Begins Jupiter Free-Fall

The NASA spececraft has reached "apojove," 5 million miles from Jupiter. Now it is falling and will come within 2,600 miles of the Jovian clouds on Aug. 27.

Image: This is an artist's impression of NASA's Juno spacecraft firing up its main engines to slow it for orbital capture around Jupiter on July 4. Credit: NASA/Lockheed Martin NASA's Juno mission reached the furthest point in its first orbit around Jupiter on Sunday -- a location known as "apojove." Now, and for the next 27 days, the solar-powered robotic mission will be at the mercy of Jupiter's hefty gravity, pulling it to within 2,600 miles of its thick, cloudy atmosphere. This will be Juno's point of closest approach, called "perijove."

RELATED: Juno Sends Back First Orbital Photo of Jupiter

Juno reached the gas giant on July 4, completing a flawless 35-minute rocket engine burn to slow it down enough to be captured by Jupiter's gravity. This was a culmination of five years of journeying through the inner solar system since launch on Aug. 5, 2011.

WATCH VIDEO: Juno Is At Jupiter. Now What?

"For five years we've been focused on getting to Jupiter. Now we're there, and we're concentrating on beginning dozens of flybys of Jupiter to get the science we're after," said Juno principal investigator Scott Bolton, at Southwest Research Institute in San Antonio, in a statement.

Whereas Juno's science instruments were powered down during its dramatic orbital insertion, the mission will be wide awake when the spacecraft reaches perijove on Aug. 27.

RELATED: Juno Arrives at Jupiter

"We're in an excellent state of health, with the spacecraft and all the instruments fully checked out and ready for our first up-close look at Jupiter," added Juno project manager Rick Nybakken, at NASA's Jet Propulsion Laboratory in Pasadena, Calif.

Juno is currently carrying out its first of two 53.4-day orbits. Known as "capture orbits," these orbits are by their nature very elongated (or eccentric). When the two capture orbits are complete, Juno will once again fire up its engines to slow itself once more to complete its orbital dance with the gas giant, shortening its orbit to just 14 days. These shorter orbits will allow Juno's science mission to begin.

But that doesn't mean we have to wait over 100 days before we start seeing photos from Jupiter's orbit. The mission team is planning on opening Juno's "eyes" during close approach later this month as a trial run to check out the spacecraft's instrumentation before it begins its grueling science campaign deep inside Jupiter's radiation belts.

RELATED: Watch Juno's Final Approach to Jupiter

Juno's mission is to reveal the science behind Jupiter's interior, including precision measurements of the huge planet's gravitational field and global magnetism. It's also going to have an incredibly close view of the gas giant's thick atmosphere, revealing its composition and how it likely evolved. Ultimately, Juno hopes to answer how Jupiter was formed and, by extension, provide an insight to the evolution of the solar system.

GALLERY: These Missions Have Journeyed to Jupiter

NASA's Juno spacecraft is due arrive at Jupiter on Monday, becoming only the second satellite in history to orbit the solar system's largest planet. Here's a look at the previous journeys to Jupiter.

Image: Juno spacecraft gets a tally-ho on Jupiter and its four largest moons -- Io, Europa, Ganymede and Callisto – on June 21, with 6.8 million miles to go. Credit: NASA/JPL-Caltech/SwRI/MSSS

Exploration of the outer solar system began in March 1972 when NASA's Pioneer 10 spacecraft blasted off from Florida to begin a 21-month long trip to Jupiter. The probe became the first to travel through the asteroid belt beyond Mars and the first to make direct observations of an outer planet in the solar system. Its primary mission was to study Jupiter's winds, magnetosphere, radiation belts and moons, especially Io. After flying past Jupiter in December 1973, Pioneer 10 headed off in the direction of the star Aldebaran, though it will take the intrepid robot 2 million years to get there. NASA lost Pioneer 10's signal in 2003.

Image: Progressively larger images of Jupiter were relayed back to Earth by the Pioneer 10 spacecraft in 1973. The most dramatic moment was after closest approach and after the spacecraft was hidden behind Jupiter. Credit: NASA

NASA's debut mission at Jupiter was followed in April 1973 by launch of Pioneer 10's sister satellite, Pioneer 11, which not only studied Jupiter, but also took advantage of the planet's gravitational pull to slingshot itself out to Saturn. During Pioneer 11's flyby of Jupiter in December 1974, the spacecraft made the first observations of the planet's polar regions and took the first close-ups of Jupiter's Great Red Spot, a massive storm that was first observed by telescopes on Earth 350 years ago. At closest approach, Pioneer 11 passed 26,570 miles over Jupiter's cloud tops, three times closer than Pioneer 10. Pioneer 11 was last heard from in 1995. It is headed toward the constellation Aquila (The Eagle) and will pass near one of its stars in about 4 million years.

Image: First image of Jupiter's polar region, taken by Pioneer 11, in 1974. Credit: NASA

Both Pioneer 10 and Pioneer 11 were outfitted with gold-plated aluminum plaques with diagrams of a man and a woman, the location of the sun and Earth in the Milky Way galaxy and other information as a message to any potential extraterrestrials who find the probes.

Credit: Pioneer Project/ARC/NASA

Armed with knowledge about Jupiter's intense radiation environment, NASA launched a pair of follow-on missions, Voyager 1 and Voyager 2, in 1977. (Voyager 2 actually blasted off first – on Aug. 20, 1977 -- followed 16 days later by Voyager 1.) Voyager 1 flew by Jupiter in March 1979, then reached Saturn in 1980. It became the first spacecraft to reach interstellar space in 2012.

Voyager 2 sailed past Jupiter in July 1979 and Saturn in August 1981. Voyager 2's mission was extended so that it could visit Uranus in 1986 and Neptune in 1989. It is presently in the outermost layer of the heliosphere, where the solar wind is slowed by the pressure of interstellar gas.

Astronomers used the twin Voyager spacecraft to discover that Jupiter has faint rings, lightning and a Great Red Spot that changes color. They also learned that Jupiter's moon Io has active volcanoes.

Image: Plume from erupting volcano on Jupiter's moon Io imaged by Voyager 1 in 1979. Credit: NASA

In October 1989, NASA's space shuttle Atlantis blasted off with the Jupiter-bound Galileo space probe aboard. It was only the second (and last) shuttle flight to deploy a planetary spacecraft. After an upper-stage rocket burn, a pass by Venus and two Earth flybys to pick up speed, Galileo finally reached Jupiter on Dec. 7, 1995. Unlike all previous missions to Jupiter, Galileo put itself into orbit for what turned out to be seven years of close-ups studies of the giant planet and its entourage of moons.

Data collected by Galileo provided evidence of an ocean buried beneath the icy surface of Europa, and possibly one on Callisto as well. It also showed that Jupiter has thunderstorms many times larger than Earth's and that Ganymede, the largest moon, has its own magnetic field.

Image: The puzzling geology of Jupiter's ocean-bearing moon Europa comes into sharp focus in images taken by the orbiting Galileo spacecraft in the late 1990s. Credit: NASA/JPL-Caltech/SETI Institute

Galileo carried a companion probe that was released about six months before the orbiter's arrival for an independent free flight into Jupiter. Slicing into the planet's atmosphere at 106,000 mph, the probe descended about 100 miles into planet's atmosphere, relaying data for 58 minutes before it was crushed, melted and/or vaporized in the intense heat and pressure. During its descent, the probe measured winds of 450 mph – stronger than anything on Earth. Initially, the probe's data showed that Jupiter was drier than expected, but scientists later realized it descended in what was later referred to as a "hot spot."

Image: Artist's impression of the Galileo probe descending into Jupiter's atmosphere on Dec. 7, 1995. Credit: NASA

Several missions, including NASA's Ulysses solar probe, the Saturn-orbiting Cassini spacecraft and the Pluto-bound New Horizons, took advantage of Jupiter's immense gravitational pull to pick up speed or adjust their flight paths to reach their intended destinations. In exchange, NASA got some bonus science at Jupiter -- and some glorious pictures.

Image: Departing view of Jupiter taken by the Saturn-bound Cassini spacecraft on Jan. 15, 2001. Credit: NASA/JPL/University of Arizona