Uranus up close: What proposed NASA 'ice giant' mission could teach us

An image of Uranus taken by Voyager 2 on Jan. 14, 1986.
An image of Uranus taken by Voyager 2 on Jan. 14, 1986. (Image credit: NASA/JPL)

A mission to Uranus is now the top future-mission priority of NASA planetary scientists, and exploration of this mysterious ice giant may shed light on a kind of planet now known to be one of the most common in the universe, researchers say.

Since astronomers discovered the first exoplanets orbiting distant stars more than 30 years ago, one of the most common kinds of alien worlds that scientists have detected are ice giants. Whereas gas giants such as Jupiter and Saturn are, as their name suggests, mostly gas, ice giants such as Uranus and Neptune are rich in heavier elements.

Much remains unknown about Uranus and Neptune. Whereas the solar system's six innermost planets — Mercury, Venus, Earth, Mars, Jupiter and Saturn — have all had spacecraft in orbit around them collecting insights for scientists, to date Uranus and Neptune have only experienced flybys more than 30 years ago, both from NASA's Voyager 2 probe in 1986 and 1989. These brief encounters yielded tantalizing views of the planets that left behind more questions than previously imagined, Kathleen Mandt, a planetary scientist at The Johns Hopkins University Applied Physics Laboratory in Maryland, told Space.com.

Related: Photos of Uranus, the tilted giant planet

This dearth of knowledge on the ice giants has led the planetary science community to select a mission to Uranus as the highest priority for NASA's next large-scale "flagship" mission in the National Academies of Sciences, Engineering and Medicine's 2023-2032 Planetary Science Decadal Survey.

"The potential findings will be groundbreaking in the same way that the Cassini mission has revolutionized our understanding of Saturn, its moons — especially Titan — and its rings," Mandt said.

The proposal involves both an orbiter to gather data about Uranus over time and a probe dropped into Uranus' atmosphere to scan the planet from the inside. The aim of the mission — currently given the placeholder name of the Uranus Orbiter and Probe (UOP) — is to explore how Uranus and the rest of the solar system formed and to help solve mysteries regarding the planet, its moons and its rings. The mission is recommended to last for at least five years.

One of Uranus' strangest features is the fact that, unlike the solar system's other planets, Uranus is tilted so far that it essentially orbits the sun on its side, with the axis of its spin nearly pointing at the star. 

"The tilt is crazy — it's the only planet in the solar system that is completely on its side," Mandt said.

This unusual orientation might be due to a collision with a planet-size body, or several small bodies, soon after Uranus formed. "We can figure out if this is true by studying what the planet is made of and its interior structure," Mandt said. 

The sideways nature of Uranus causes the planet to experience extreme seasonal variations over its 84-Earth-year orbit unlike those of any of the solar system's other planets, and what little astronomers can see of the world from Earth cannot explain what they know of its weather patterns. The UOP can help shed light on Uranus's atmosphere, with the probe gathering detailed wind and temperature data at one location and the orbiter collecting information across the entire planet, Mandt said.

Related: Why scientists want NASA to send a flagship mission to Uranus

Uranus is tipped on its side, and scientists aren't sure why.

Uranus is tipped on its side, and scientists aren't sure why. (Image credit: X-ray: NASA/CXO/University College London/W. Dunn et al; Optical: W.M. Keck Observatory)

Uranus's tilt also limits what astronomers can see of its moons. For example, Voyager 2 could only image the southern hemispheres of Uranus' satellites. What the probe did see was unexpected. Uranus's five largest moons, which scientists predicted were cold, dead worlds, being too small to hold much of the heat from their creation, all showed evidence of recent surface activity. This raises the possibility that one or more of these moons, such as Ariel, Titania and Oberon, could have potentially habitable liquid water oceans underneath ice shells.

The UOP will image the surfaces of the moons in their entirety to search for ongoing geological activity. It will also measure whether their magnetic fields vary in their interiors due to the presence of liquid water, Mandt said.

In addition, Uranus has nine very dense, narrow rings around it that suggest the existence of "shepherd moons" whose gravitational influence kept these rings from rapidly spreading out and losing their sharp edges. The UOP can help search for these extra shepherding moons, and also analyze the unexpectedly dark ring particles, whose composition is clearly different from that of the surfaces of Uranus' moons.

The mission to Uranus may also shed light not only on the origins and evolution of the solar system, but those of distant planetary systems as well.

"There are so many ways that the ice giants will help us to learn about exoplanets," Mandt said. "One of the largest groups of exoplanets that have been discovered are similar in size and mass to Uranus and Neptune. We want to know what these planets are made of and how the interior is structured. We also want to know more about the weather on the planet and how that compares to similar exoplanets."

As giant planets form and migrate over time, they play major roles in the birth and development of other worlds. Although scientists have gotten close looks at the solar system's two gas giants, they also need more data on Uranus and Neptune to reconstruct the solar system's history. The UOP's probe can analyze nitrogen isotopes and levels of noble gases in Uranus' atmosphere to help verify which model of giant planet formation and migration may be the most accurate, Mandt said.

"We can see evidence in exoplanet systems that giant planets migrate in many different ways — the most obvious one is hot Jupiters that must have formed far away and moved in really close to their stars," Mandt said. "Knowing how our planets formed and migrated helps us to know what did and didn't happen in exoplanet systems."

In addition, Uranus' magnetic field is quite unusual, in that it is not only tilted 60 degrees from the planet's axis of spin but also offset from the center of the planet. It remains a mystery as to how a planet can produce such a field, Mandt said.

The UOP is recommended to launch by 2032 to help the spacecraft use Jupiter's massive gravity to slingshot it toward Uranus. This would mean the mission would arrive well before Uranus' northern autumn equinox in 2050, ensuring full visibility of the moons. Trajectories after 2032 that do not use Jupiter's gravity but still arrive before the equinox are possible, but would deliver a smaller probe into orbit carrying fewer instruments or take longer to arrive.

"We will learn about how and where Uranus formed, what it's made of, and how the interior is structured," Mandt said. And the mission to Uranus may pave the way to its more-distant cousin Neptune as well, she added.

Mandt detailed the proposed mission in a paper in the Feb. 17 issue of the journal Science.

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Charles Q. Choi
Contributing Writer

Charles Q. Choi is a contributing writer for Space.com and Live Science. He covers all things human origins and astronomy as well as physics, animals and general science topics. Charles has a Master of Arts degree from the University of Missouri-Columbia, School of Journalism and a Bachelor of Arts degree from the University of South Florida. Charles has visited every continent on Earth, drinking rancid yak butter tea in Lhasa, snorkeling with sea lions in the Galapagos and even climbing an iceberg in Antarctica. Visit him at http://www.sciwriter.us

  • progan01
    I should be very interested in any theory that can explain how a single planetary collision could tilt not just Uranus but its rings and major satellites to the same precise degree. That's a billiard shot I'd pay a DOLLAR to see.
  • billslugg
    The collision that turned Uranus on its side did not also tilt the existing rings and moons, it destroyed them and created a new set. Any off center hit would put a huge accretion disc around the planet which would catch any existing rings and moons that happened to be on a different orbital plane. During their orbit they would be forced to cross the new plane and would collide with it. Any gravitationally bound system of random items in orbit eventually forms a single plane.
  • progan01
    The energy required to shift the rotational axis of a planet is considerably in excess of the amount of energy required to disintegrate the planet entirely, leaving a ring about the Sun that would, due to the effect of orbital harmonics, disperse to other stable orbits, likely colliding with other bodies already present, such as moons, and the planets themselves, leaving no Uranus whatsoever. No. The Solar System is not a billiards table with planets knocking into each other and bouncing away, nor do they change their orbital momentum post collision. It is now evident that Mars and Earth both suffered such collisions in their early history, transforming both planets, but not noticeably altering their rotational axes. We should be living on an entirely different world with radically different seasons if rotational axes could be altered so easily.

    The 'Uranus collision' theory is intellectually bankrupt and needs to be discarded before we make any headway in understanding of the planet. For my money, it's much more likely that Uranus is a captured outer planet formed in the same stellar nursery around a different star, and ejected from its original orbit due to the effect of orbital harmonics among the planets of that other system. We have postulated that our Solar System lost a gas giant world due to the perturbations of the orbits of the forming planets. If we could lose a planet, could we not gain one in more-stable times after formation? We need to probe Uranus and determine its physical makeup, which I suspect is similar to that of the other planets but differing in some elemental isotopes not found here in the same proportions. I find this postulate far more convincing than the notion that a whack can change a planet's rotation axis AND destroy all its moons at the same time. Preposterous.
  • billslugg
    Here is a simulation from NASA that shows an impact was sufficient to turn Uranus on its side and change its rotation without being powerful enough to even strip away its atmosphere.
    Planet-Shifting Collision Shaped Uranus’ “Rolling” Rotation | NASA
    Here is another article from NASA, one that posits the most likely origin for Uranus was formation around the Sun and then migration outwards
    In Depth | Uranus – NASA Solar System Exploration
    Do you have references for any of your claims?
  • progan01
    Didn't you notice that the 'simulation' you posted showed only a two-dimensional animation of the potential impact, with no explanation for how or to what degree this would change the planet's rotational axis? Not to mention that nothing is said about the destruction of rings and moons and their miraculous 'resurrection' in the new 'tilted' system, as you insist happened. What do you think this example proves, other than your attempt to invoke the name of NASA to justify a physically impossible astrometric change?

    I already cited the migration of the planets after formation, which this NASA blurb on Uranus mentions in passing, and in less depth than I have already provided. Not that there aren't better examples, but given your attempt to rely on inapt authority to carry your case, there's no point in bringing more and better evidence in.

    You might refer to this LiveScience article on changing the Earth's rotational axis for the amount of force required to do so, which ought to give you an idea of the scale of energies involved:
    We haven't yet seen the destruction of a planet, but we can calculate how much energy it would take to reduce, say, Earth to powder -- the energy equals three-fifths of the gravitational constant, times the mass of the planet squared, and divided by the radius of the planet. Or 2.25 x 10^32 joules. You may refer to this article and the accompanying Scott Manley video for how this determination was derived:
    But this would completely eliminate the gravitational binding energy of the Earth, erasing it utterly; reducing it to chunks would presumably take a good deal less. Much less than the energy required to change its axis. Your cited simulation provides no figures as to the energy released or recombined in the 'reconstructed' Uranian system, so I can't say that you've 'proved' anything other than a preference to use a big name to assert a conclusion you cannot yourself prove or justify.

    I also note that you did not bother to counter the possibility that Uranus is a 'captured' world. I must therefore conclude that you agree it is as likely an event as the migration of the planets through orbital resonance. If you have actual figures and probabilities of any sort of planetary axial change, feel free to bring them. I can't find a one.

    Mind, now, human beings have already altered the Earth's rotational axis through climate change and the loss of hundreds of billions of tonnes of polar ice. But not by 97.77 degrees. See this Guardian article on the issue: https://www.theguardian.com/environment/2021/apr/23/climate-crisis-has-shifted-the-earths-axis-study-shows
    I look forward to receiving your data. If any.
  • Mememe
    progan01 said:
    I should be very interested in any theory that can explain how a single planetary collision could tilt not just Uranus but its rings and major satellites to the same precise degree. That's a billiard shot I'd pay a DOLLAR to see.
    Computer modeling shows that when a planet turns its rotational angle its moons eventually follow and turn to what you call the same precise degree. It has to with the centre of gravity which is strongest at a planet’s equator. The theory is completely plausible.
  • progan01
    I take it you have never worked with a gyroscope, or owned so much as a toy one. Or you would have some personal experience of the amount of force required to change the direction in which a gyroscope, or any turning cylinder or ring, is rotating. This is what is meant by rotational energy, and it is by no means trivial. And it is certainly most difficult to change. I invite you to purchase a toy gyroscope, set it spinning, and then use a hammer to hit it to change the direction in which it spins. I daresay you will run out of both gyroscopes and hammers before you succeed.

    I don't know what sort of computer modeling you have referenced, if any, but we have no examples in nature of any sizable astronomical body changing its spin axis, for the reason stated above. Or any orbiting bodies such as moons following suit. There simply is no physical law for an orbiting body to match the direction of spin of the primary object it orbits. Were this true, then long ago all the satellites launched into Earth orbits - even polar and westward-launched - would have settled into a west-to-east equatorial orbit and no other.

    For these reasons, as well as the others I have stated, the postulate that some collision resulted in 'tilting' the spin of Uranus is entirely erroneous. I look forward to your results hammering your gyroscope. You may attempt the same experiment with a rotating car wheel, if you prefer. I expect the same results regardless.