Solar-powered probe to view unseen parts of Jupiter

by Rachel Courtland

The Jupiter orbiter Juno will study the distribution of water in the gas giant and look for evidence of a solid core (Illustration: NASA/JPL)

After a two-year delay, NASA has given the green light to Juno, a $1 billion, solar-powered mission to Jupiter.

Juno was originally set to launch as early as 2009, but budget constraints delayed the next step in the probe's design.

Now, funding had been approved to build the spacecraft, the agency announced on Monday. Juno will launch from Cape Canaveral, Florida, in August 2011. After reaching Jupiter in 2016, it will orbit 32 times over the course of its year-long mission.

Juno will be the second orbiter to study Jupiter. The first such probe, Galileo, circled Jupiter's equator for almost eight years before plunging into the Jovian atmosphere in 2003.

Juno, on the other hand, will take up a polar orbit around the planet, skimming the poles at an altitude of 5000 kilometres. This trajectory will afford the spacecraft a view of unseen parts of the planet.

The probe will also fly between Jupiter's atmosphere and the intense radiation belts that girdle the planet. In the radiation belts, streams of charged particles could darken the glass covering the probe's solar panels, limiting their capacity and limiting the probe's lifetime.

Search for water

Juno will carry a suite of 11 science instruments, some of which will be used to measure the planet's gravity, magnetic field, and chemical makeup.

"We're really looking at composition," says principal investigator Scott Bolton of the Southwest Research Institute in San Antonio, Texas. "We're going after the ingredients of Jupiter so we can reconstruct the recipe."

Since oxygen is the third-most abundant element in the universe and in the Sun, many planetary scientists expect to see a fair amount of water in Jupiter. But Galileo's atmospheric probe, which entered Jupiter's atmosphere in 1995, saw little evidence of water.

"The Galileo probe was expected to measure water, but it failed," says Dave Stevenson of Caltech. Some suspect little water was found because the module descended through a dry patch in Jupiter's atmosphere.

Gravity field

Juno will attempt a more global search. The probe will use radio antennae to measure light absorbed by water and ammonia at six different depths in the atmosphere.

If water is present, it could fill out the picture of how the planet formed. Since water ice is thought to dominate the dust content in the early solar system, "the water is a tracer of the solid material that was added to the planet", says Stevenson.

Coupled with measurements of the planet's magnetic and gravitational fields, scientists could begin to build up a working model of the planet and put constraints on how it formed, Stevenson says.

Jupiter's gravitational field will be mapped by detecting how the planet's tug influences Juno's velocity. Those changes will be measured by looking for shifts in the frequency of signals passed between the spacecraft and the Deep Space Network, Earth's international array of radio antennae.

Solar power

Such measurements could eventually reveal whether Jupiter has a core of heavier elements that were once rock and ice. If Jupiter has a heavy core, it would suggest Jupiter formed by 'core accretion' - by slowly accumulating solid materials before later gathering gas to build its atmosphere.

Magnetometers on the craft will be used to map the planet's magnetic field. This map could be used to infer how intense pressures in Jupiter's interior squeeze hydrogen and change how it conducts electricity.

Other far-flung spacecraft, such as the New Horizons mission to Pluto, run on electricity generated by the heat of decaying radioactive material.

But Juno will be powered by three long, hinged solar panels that will unfold after launch. With the panels unfolded, Juno will span some 20 metres across.

The craft may be the most distant spacecraft to operate off solar power. The only other contender for the title is the European Space Agency's Rosetta spacecraft, which will briefly intersect Jupiter's orbit to rendezvous with the comet 67P/Churyumov-Gerasimenko in 2014.

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