Decades of data collected by the Hubble Space Telescope have allowed an international team of astronomers to take a fresh look at what is happening to Uranus. The new analysis, published in the journal Nature Astronomy, calculates the spin of one of the least studied planets in the Solar System with unprecedented accuracy.
A human-made object has visited the seventh planet from the Sun only once. On January 24, 1986, NASA‘s Voyager 2 spacecraft flew past it, collecting some of the most comprehensive data about Earth’s distant neighbor that scientists have ever had access to. Among them were some surprising facts, such as the fact that Uranus’ magnetic field was strongly tilted and displaced. By comparing measurements of this field, astronomers were able to estimate the planet’s rotation period at 17 hours, 14 minutes, and 24 seconds.
However, this calculation included an error of 36 seconds. This may seem insignificant, but as the astronomers note in their new paper, it was enough to prevent observers from finding the planet’s magnetic axis less than two years after Voyager 2’s visit.
This uncertainty is because giant planets like Uranus pose unique challenges for scientific observers. While Uranus is not technically a gas giant because it has a solid core, it is difficult to see what is actually going on in all that gas as powerful winds rip through the thicker atmosphere.
Credit: NASA, CSA, and the European Space Agency: NASA, ESA, CSA, STScI
An attempt to reanalyze the ultraviolet data collected by Voyager 2 in 2009 failed to improve our understanding of how fast Uranus rotates. To solve this mystery, astronomers turned to the Hubble Space Telescope, which began taking images of the planet’s ultraviolet aurorae in 2011. Similar to auroras seen on Earth, such as the Northern Lights, auroras on Uranus are caused by particles striking the atmosphere and interacting with the magnetic field. In the following years, more images were taken, allowing us to observe spectacular light shows under different solar wind and magnetospheric conditions.
The analysis of the Hubble images, combined with data collected by Voyager 2, allowed for a more accurate calculation of Uranus’ rotation speed. What they found really fits the initial error: according to their calculations, Uranus rotates at a speed of exactly 17 hours, 14 minutes and 52 seconds. The error still remains, but it has decreased to 0.036 seconds.
“Our measurement not only provides an important benchmark for the planetary science community, but also solves a long-standing problem: previous coordinate systems based on outdated rotation periods quickly became inaccurate, making it impossible to track Uranus’ magnetic poles over time,” said Laurent Lamy, an astronomer at the Paris PSL Observatory who led the research team, in a statement. “Thanks to this new longitude system, we can now compare astronomical observations spanning almost 40 years and even plan a future mission to Uranus.”
The mission he is referring to is an as-yet unplanned probe that the National Academies of Sciences, Engineering, and Medicine have included in their list of top NASA priorities for 2022. The mission of this spacecraft will be to map Uranus’ gravitational and magnetic fields, a mission that will be made easier with this new discovery.
But (and there’s always a “but” with Uranus), this mission currently exists only on paper. NASA, like much of the federal government, faces an uncertain future, so it’s unclear when humans will dive back into this distant gas wonder, if ever.
