Pasadena-Managed Juno Spacecraft Rewrites Jupiter’s Vital Statistics for the First Time in 50 Years

For nearly half a century, the accepted measurements of Jupiter — the solar system’s largest planet — rested on six data points collected by spacecraft that flew past the gas giant in the 1970s and never looked back. Those numbers treated Jupiter’s roiling atmosphere as though it were standing still.

It isn’t. And the difference, it turns out, matters.

An international research team led by the Weizmann Institute of Science in Israel has used 26 radio soundings from NASA’s Juno spacecraft — managed by the Jet Propulsion Laboratory in Pasadena — to produce what the researchers describe as the most precise measurement of Jupiter’s size and shape ever recorded. Their findings, published February 2 in the journal Nature Astronomy, show the planet is about 8 kilometers narrower at the equator and 24 kilometers flatter at the poles than previously estimated. The team’s methodology, for the first time, accounted for the powerful winds that distort Jupiter’s figure.

“Textbooks will need to be updated,” said Prof. Yohai Kaspi of Weizmann’s Earth and Planetary Sciences Department. “The size of Jupiter hasn’t changed, of course, but the way we measure it has.”

The correction sounds modest on a planet roughly 143,000 kilometers wide. But Jupiter’s radius serves as the calibration standard astronomers use to characterize giant planets orbiting distant stars. When a telescope detects an exoplanet passing in front of its host star, the dimming pattern is compared against known gas giant dimensions to estimate the distant planet’s size. A more accurate Jupiter means more reliable readings of worlds far beyond our own solar system.

The breakthrough came from a technique called radio occultation. As Juno passes behind Jupiter from Earth’s vantage point, its radio signal travels through the planet’s thick atmosphere before reaching the Deep Space Network, which includes ground antennas in Goldstone, California. JPL manages both the network and the Juno mission.

By measuring how Jupiter’s ionosphere bends and delays those signals, scientists can map the planet’s temperature, pressure, and density at varying depths and, from that data, calculate its dimensions with far greater precision.

“Those missions provided a foundation, but now we got the rare opportunity to spearhead the analysis of as many as 26 new measurements made by NASA’s Juno spacecraft,” said Dr. Eli Galanti, the senior staff scientist who led the Weizmann research team.

The opportunity itself was unplanned. When NASA extended Juno’s mission in 2021, the spacecraft’s new trajectory brought it behind Jupiter relative to Earth for the first time — a geometry its original orbit never achieved. Beginning in February 2023, on its 53rd orbit, Juno began radio occultation experiments that were never part of the original mission design.

Maria Smirnova, a Ph.D. student in Kaspi’s group, developed a technique to process the raw data from those experiments, converting the bent radio signals into detailed atmospheric maps.

“We tracked how the radio signals bend as they pass through Jupiter’s atmosphere, which allowed us to translate this information into detailed maps of Jupiter’s temperature and density, producing the clearest picture yet of the giant planet’s size and shape,” Smirnova said.

The previous measurements, taken by NASA’s Pioneer and Voyager missions, established Jupiter’s equatorial radius at 71,492 kilometers, with an uncertainty of about 4 kilometers. The Juno data places it at 71,488 kilometers — with an uncertainty of just 0.4 kilometers, an order-of-magnitude improvement. The polar radius dropped from 66,854 kilometers to 66,842 kilometers, a 12-kilometer reduction, with the same tenfold precision gain.

“These few kilometers matter,” Galanti said. “Shifting the radius by just a little lets our models of Jupiter’s interior fit both the gravity data and atmospheric measurements much better.”

A key reason the old numbers were off: they did not account for Jupiter’s fierce atmospheric winds, which reshape the planet’s figure. Jupiter completes a full rotation in less than 10 hours — the fastest of any planet in the solar system — producing wind patterns that influence its overall shape. The Weizmann team explicitly modeled those wind effects, correcting a methodological gap that had persisted since the Pioneer era.

The Juno mission launched on August 5, 2011, and entered orbit around Jupiter on July 4, 2016. JPL, a division of Caltech, manages the mission for NASA’s Science Mission Directorate. The principal investigator is Dr. Scott J. Bolton of the Southwest Research Institute in San Antonio. Lockheed Martin Space in Denver built and operates the spacecraft.

“This research helps us understand how planets form and evolve,” Kaspi said. “Jupiter was likely the first planet to form in the solar system, and by studying what’s happening inside it, we get closer to understanding how the solar system, and planets like ours, came to be.”

The techniques developed for the study will also serve the team’s analysis of data from the European Space Agency’s JUICE mission, which launched in 2023 and carries a Weizmann-designed instrument for observing Jupiter’s atmosphere.

A year from now, some of those textbooks Kaspi mentioned may carry the new numbers. The planet, of course, hasn’t noticed — it just keeps spinning, one full turn every 10 hours, indifferent to the decimal places closing in on it from Pasadena.