William Hubbard is on a NASA mission for the first time in a career that spans the space era, and he has another NASA “gig” lined up later this year.
Hubbard’s first target is Jupiter, a planet he has studied since being hired by Gerard Kuiper in 1972 as one of the first professors in the graduate school of the University of Arizona’s Department of Planetary Sciences.
He is a co-investigator for NASA’s Juno mission, which will make 32 passes of the giant planet from pole to pole, ducking beneath the electromagnetic rings that encircle it and would destroy any spacecraft that cruised through the intensely radioactive environment created by them.
Hubbard, an expert on planetary interiors, will be analyzing data from a super-sensitive device that will measure the planet’s gravitational pull on the spacecraft, revealing its shape and its interior composition in ways never before seen or known.
The measurements of acceleration are extremely sensitive, he said. “If a mosquito were to land on your head, it would increase your attraction to Earth by about a milligal. We can measure to within one-one-hundredth of a milligal.”
Juno’s science mission begins July 4.
Later this year, NASA’s Cassini spacecraft will perform the “Grand Finale” of its 12-year observations of Saturn’s moons and rings. Before Cassini crashes into the planet, it will orbit it at close quarters, taking gravitational measurements comparable to those being collected on Jupiter by Juno.
It’s something of a “grand finale” for Hubbard as well. He will retire from his teaching post at the UA within a year but intends to continue research with the data gathered by Juno and Cassini for years to come.
He built his career on theoretical modeling of the interior structures of Jupiter and Saturn. Now he gets to test those theories with some actual measurements.
“It’s ironic that he’s never been on a mission before, because he’s been a part of LPL’s history for quite a while,” said UA Lunar and Planetary Laboratory Director Tim Swindle.
“In large part, he was one of the people driving Juno — his ideas and how you can test what’s going on in the interior of these planets,” Swindle said.
“He is, and has been for years, the expert in trying to figure out what’s going on in the interiors of Jupiter and Saturn.”
At age 75, Hubbard is having as much fun as he’s ever had and his family shares the enthusiasm. When NASA engineers fire Juno’s rockets to slow it down for insertion into orbit around Jupiter on July 4, Hubbard will be at the Jet Propulsion Laboratory, along with his wife, Jean, his daughters and their spouses, and his six grandchildren.
Hubbard said the mission has taken on a new scientific urgency with the recent discoveries of exoplanets around nearby stars. “When I started with the mission, exoplanets hadn’t been discovered. Now there are thousands and most are ‘Jupiters,’ ” Hubbard said.
Exoplanets are tough to find, and the biggest of them are the first to be discovered, he said. Giants like Jupiter.
Jupiter isn’t a round sphere. It is stretched at the poles, and Hubbard expects Juno’s instruments will find it misshapen elsewhere. Its moons exert tidal forces on its swirling atmosphere of hydrogen and helium.
Hydrogen, subjected to the gravitational force of a planet 300 times the mass of Earth and heated to temperatures hotter than the surface of the sun, is a viscous liquid, capable of experiencing tides like water on Earth.
“The shape will provide clues about what lies beneath the surface and the gravitational measurements will constrain the size and mass of the planet’s core,” Hubbard said.
Hubbard and colleague Burkhard Militzer, of the University of California at Berkeley, have revamped earlier models of Jupiter’s structure for comparison to the data gathered. They expect Juno’s instruments will back them up.
“We have a new equation of state which describes how hydrogen and helium should behave under those pressures,” said Hubbard.
“There should be a core,” he said, made of elements other than hydrogen and helium. “We think the Pioneer description isn’t quite accurate, so we’re sort of sticking our necks out a little bit.” NASA’s Pioneer 10 and 11 spacecraft took close-up images of Jupiter more than four decades ago.
The existence of a core is a critical question for the mission, said Scott Bolton, Juno’s principal investigator. “If there is, it tells us something: Rocks formed before Jupiter did.”
Bolton, of the Southwest Research Institute in San Antonio, Texas, said in a televised NASA news conference Thursday that Jupiter’s mysteries hold keys to understanding planet formation in our solar system and in others.
“We’re puzzled. It looks a lot like the sun, almost all hydrogen and helium.”
But it also has “an enrichment of heavy elements,” such as carbon, nitrogen, sulfur and the noble gases.
Looking into the interior of Jupiter will allow scientists to “learn the recipe for a solar system,” he said.
Juno will also send back the best close-ups ever taken of Jupiter, he said, and take the first photos of its polar regions and the intense auroras created by its astoundingly energetic electromagnetic field.
Juno, the size of a basketball court with its solar panels deployed, will spin through its orbit about 3,100 miles above Jupiter’s surface. Its electronics are encased in half-inch thick titanium and its sensors are additionally armored for protection from the “scariest radiation environment” in the solar system, according to the mission’s radiation expert, Heidi Becker of the Jet Propulsion Laboratory.
“It’s a planet on steroids,” said Bolton. “Everything about it is extreme.”
