Return to the Ice Giants

by Paul Gilster on June 18, 2014

Once New Horizons has performed its flyby of Pluto/Charon and, let’s hope, its reconnaissance of a Kuiper Belt object (KBO), what comes next in our exploration of the outer Solar System? Pushing further out, Innovative Interstellar Explorer grew out of a NASA ‘Vision Mission’ study and has been developed at Johns Hopkins University Applied Physics Laboratory by Ralph McNutt and team. Boosted by a Jupiter gravity assist, IIE would explore the interstellar medium some 200 AU and further from the Sun, using a plutonium-fueled 1 kW electric radioisotope power supply.

And then there’s Claudio Maccone’s FOCAL mission, which would target the Sun’s gravitational focus beginning at 550 AU, continuing well past 1000 AU for observations exploiting gravitational lensing effects. FOCAL has been the subject of intense study — Maccone’s 2009 book Deep Space Flight and Communications grew out of this decades-long work — and with both IIE and FOCAL we have the prospect of making observations of the medium through which any future interstellar mission would pass, having exited the heliosphere inflated by the solar wind from the Sun.

But there is much to do in the region between the gas giants, with their astrobiologically interesting targets Europa and Enceladus and the seductive Titan, and the inner edge of the Kuiper Belt. Here we are in the domain of the ice giants last visited by the Voyagers in the 1980s. A new paper from Diego Turrini (Institute for Space Astrophysics and Planetology INAF-IAPS, Italy) and colleagues makes the scientific case for a mission to Neptune and Uranus, to be flown by two identical spacecraft. The ESA-funded mission would have a launch date of 2034.

Neptune

Image: Neptune as captured by Voyager 2 in 1989. Credit: NASA/JPL.

I want to focus on Turrini and team’s discussion of planet formation this morning, because this is where the scientific payoff for a return to Uranus and Neptune is the most profound. The rising tide of exoplanet research is uncovering more and more planetary systems showing us that the old view of planet formation as an orderly process producing stable, well-spaced systems is incorrect. Systems around other stars are not necessarily patterned on what we see in the Solar System. In fact, it is we who seem to be the outliers, confronting a cosmos that produces a bewildering array of planetary configurations in which migration surely plays a major role.

Thus the ‘Jumping Jupiters’ mechanism that involves close gravitational encounters that occur after the original circumstellar disk has dispersed. In our Solar System, our changing views have led to the ‘Nice Model,’ which involves our own jumping Jupiter scenario, one tied to the era known as the Late Heavy Bombardment. Here we have a series of encounters between the giant planets, with interactions with what the paper calls a ‘massive primordial trans-Neptunian region.’ The end result is to take giant planets once closer to each other and to move Jupiter inward while migrating Saturn, Uranus and Neptune outward. The paper describes this scenario:

The importance of the Nice Model lies in the fact that it strongly supports the idea that the giant planets did not form where we see them today or, in other words, that what we observe today is not necessarily a reflection of the Solar System as it was immediately after the end of its formation process. Particularly interesting in the context of the study of Uranus and Neptune is that, in about half the cases considered in the Nice Model scenario, the ice giants swapped their orbits (Tsiganis et al. 2005). The success of the Nice Model in explaining several features of the Solar System opened the road to more extreme scenarios, also based on the Jumping Jupiters mechanism, either postulating the existence of a now lost fifth giant planet (Nesvorny et al. 2011) or postulating an earlier phase of migration and chaotic evolution more violent and extreme than the one described in the Nice Model (Walsh et al. 2011).

Mixing of the solid materials that make up the primordial Solar System would have occurred, with both inward and outward fluxes of ejected material affecting the composition of primordial planetesimals. When we look at the satellites of the gas giants today, we may be seeing material that was originally extracted by these processes from the inner Solar System and incorporated in their systems.

Uranus and Neptune would have been strongly affected by these events, with a giant impact involving Uranus that explains its sideways rotation. And we can see other evidence in the capture of Triton by Neptune, Triton being a moon that orbits in the opposite direction to its host planet. The paper continues:

…our view of the processes of planetary formation and of the evolution of the Solar System has greatly changed across the last twenty years but most of the new ideas are in the process of growing to full maturity or need new observational data to test them against. The comparative study of Uranus and Neptune and their satellite systems will allow to address the problems still open, as the ice giants were the most affected from the violent processes that sculpted the early Solar System and yet they are the least explored and more mysterious of the giant planets.

Turrini and team also make the case that based on data from the Kepler mission, about one star in every five should have a Neptune-class planet, but the only up-close data we have on this class of planet comes from the Voyager 2 flybys of Uranus and Neptune performed in the 1980s. Here the authors have to pause, for Kepler’s candidates have short orbital periods because of the nature of Kepler’s operations. Kepler can only detect ‘warm’ or ‘hot Neptunes,’ whose composition and dynamics will differ from the ice giants in our own Solar System. Even so, characterizing our ice giants is something we can do with existing space technologies, and it can offer up templates for interpreting the data returned from future exoplanet observations.

From observation of the satellite systems around the ice giants to study of planetary interiors, there is much to investigate in the outer Solar System. The ODINUS mission described in the paper would put a spacecraft into orbit around both Neptune and Uranus, an ambitious goal that would allow measurements with the same set of instruments in both systems, as well as studies of the interplanetary medium from different angular positions during cruise. The ESA’s Senior Survey Committee has already stated that exploration of the ice giants “appears to be a timely milestone, fully appropriate for an L class mission,” assuming financial support emerges.

There is no question we are going to get payloads back to Uranus and Neptune at some point, and the Turrini paper makes a strong case for the scientific validity of the effort in helping us understand our own system’s violent past and the results of our planet-hunting observations in other systems. The comparatively well studied Jupiter and Saturn are composed mainly of hydrogen and helium, while Neptune and Uranus are dominated by water, ammonia and methane along with metals and silicates, with hydrogen and helium making up a scant 25 percent. We obviously have much to learn about such planetary formation outcomes.

The paper is Turrini et al., “The Scientific Case for a Mission to the Ice Giant Planets with Twin Spacecraft to Unveil the History of our Solar System,” submitted to Planetary and Space Science (preprint).

tzf_img_post

{ 15 comments… read them below or add one }

ljk June 18, 2014 at 11:01

Galileo-style Uranus Tour (2003)

BY DAVID S. F. PORTREE 05.12.12 | 1:23 AM |

In a paper published in the Journal of Spacecraft and Rockets shortly before Galileo concluded its tour, Andrew Heaton of NASA’s Marshall Space Flight Center and James Longuski of Purdue University demonstrated that the Uranus system could support a complex Galileo-style tour. This was, they acknowledged, “contrary to intuition. . . because the Uranian satellites are much less massive than those of Jupiter.” A Galileo-style tour would be possible, they explained, because “the key to a significant gravity assist is not the absolute size of the satellite, but the ratio of its mass to the primary, and the mass ratios of the Uranian satellites to Uranus are similar to those of the Jovian satellites to Jupiter.” Titania and Oberon form a large outer pair equivalent to Ganymede and Callisto, they noted, while Ariel and Umbriel form a small inner pair equivalent to Io and Europa. The “Uranian satellite system is nearly a smaller replica of the Jovian system,” Heaton and Longuski wrote.

Full article here:

http://www.wired.com/2012/05/galileo-style-uranus-tour-2003/

ljk June 18, 2014 at 11:38

Uranus or Bust (and on a budget)

Posted by Van Kane for The Planetary Society

2013/07/09 17:38 UTC

Topics: Future Mission Concepts, Uranus, Uranus’ moons, Uranus’ rings

Given my interest in future planetary missions, I regularly look through lists of missions submitted to space agency mission selection competitions. I also read through the abstracts of mission concepts presented at the many planetary science and engineering conferences each year. Uranus is trending.

Why the interest now? First, the 2011 Decadal Survey ranked a $2B Uranus orbiter and probe mission as a priority to launch in the coming decade. (Alas, new budget realities make any such mission look 20 years away or more now.) Second, the Uranus-sized worlds are proving to be common in other solar systems and may be the most common type of planet in the galaxy. Our only up close examinations of planets in this class were the flybys of Uranus and Neptune in the 1980s by the Voyager 2 spacecraft that carried 1970s vintage instruments.

Third, NASA’s development of the light and relatively cheap ASRG plutonium-based power systems enables cheaper missions than were possible with the older, heavier power systems. And fourth, the changing outer planet alignments have made gravity assists from Jupiter and Saturn to shorten flight times to Neptune impossible the current mission planning window. Jupiter is still available for Uranus missions in the coming decade.

Full article here:

http://www.planetary.org/blogs/guest-blogs/van-kane/20130708-uranus-or-bust.html

A useful presentation on the proposed Uranus mission:

http://www.lpi.usra.edu/opag/iceGiant/01_Hofstadter_UranusMissionProspects_v6.pdf

ljk June 18, 2014 at 12:12

A Centauri Dreams article from January, 2014 on using an electric sail to get a probe rather quickly to Uranus:

http://www.centauri-dreams.org/?p=29805

Alex Tolley June 18, 2014 at 12:27

To test these idea of planetary formation, won’t we need a follow up on Kepler to determine the longer period exoplanets? Since we now know which stars have transiting planets, is there a cheaper way to do long period observations of these stars to detect and characterize these planets, or do we need another Kepler class telescope but with much longer observing periods (if that is possible from an engineering standpoint and even a Jupiter orbit would need 30+ years of observations – 3 orbits – to verify, a Neptune orbit would be out of the question)? I’m guessing that direct observation is going to be needed for these ice and gas giant exoplanets.

Brian Swiderski June 18, 2014 at 18:33

Cassini has been such a massive success at Saturn that I would suggest just sending two identical copies to Uranus and Neptune. It would be very expensive, of course, but the data (and breathtaking imagery!) generated by Cassini has proven priceless, so the added confidence of success and the depth of information and imagery to be gathered at Uranus and Neptune would more than justify the expense. Personally, if I were a billionaire, I would privately fund such missions.

And it’s not just the ice giants themselves that would reveal wonders: Very little is known of their moons, and very little has been seen of them from the brief Voyager flybys. Given how mind-blowing the imagery of Saturn’s moons has been from Cassini, just imagine what could await detailed imaging of Triton, Ariel, Umbriel, Oberon, etc. They look pretty exotic even from the relatively low-res images the Voyagers captured. Just imagine seeing them up close in serious detail, with Uranus or Neptune rising over the horizon. Inspiration is just as important a product of these missions as scientific data, and going to Neptune and Uranus would generate both in spades.

But perhaps since NASA is being squeezed, and ESA tends to be pretty conservative in its probe ambitions, and other countries are not yet advanced enough to handle outer solar system probes, the missions to Uranus and Neptune could be the first truly international probe missions? Perhaps they could even be the basis for founding an international space exploration institution in which governments, businesses, nonprofit organizations, and individuals can all participate and contribute?

RobFlores June 18, 2014 at 18:56

My personal view is that we will find some big surprises on Neptune.
This Planet has many peculiarities for it to be a simple Jovian, or even compared to Uranus.
The most Glaring is the tremendous wind speeds of it’s atmosphere in stormy
regions unmatched by anything in the solar system, at 21oo km/hr
This is sharp contrast to Uranus, (but this maybe partially due to Uranus’ axial tilt and lower density) whose atmosphere is so mild is creates little atmospheric banding, or turbulence. Related to this is the large amount of heat emitted by Neptune compared to Uranus. How planetary astronomers can still say both these planets are similar is odd, more proof that internal arrangement makes a difference I would think.

As to what a surprise would look like:
There might be something underneath the clouds, that is causing these contrasts. Maybe one candidate might be a thin Ice Layer, 3-4 Km constantly in a state of breakup and reformation. If it were a thicker solid layer of ice it would be apparent in the upper atmosphere and show up as a less chaotic atmosphere. Most citations show that Hydrogen and Helium
are predominant. But that is a volume proportion not a mass proportion.
Ice In this instance could mean either any of Water or Methane
The big question would be exactly what distance from the center is this Ice layer (if it exists at all) If this layer really exists it cannot be too far from the
apparent surface, or It’s effects would be far less dramatic. So If anything
resembling Slushy Ices, it should like in the upper 1/3 of the Atmosphere.

Brian Swiderski June 18, 2014 at 19:51

RobFlores,

The radically high wind speeds of Neptune are due to the very low viscosity of fluids at cryogenic temperatures. There’s very little friction in an atmosphere when it’s that cold. That’s also why both Uranus and Neptune are relatively bland-looking compared to Jupiter and Saturn – there just isn’t much going on that would interrupt the smooth air flow or introduce drastic color gradients. The air moves rapidly and smoothly around the planet, for the most part without interruption.

Glaas June 19, 2014 at 1:06

@RobFlores, Neptune is believed to have migrated outwards, passing Uranus, to define the inner limit of the Kuiper belt by absorbing or ejecting everything in its path. That history should’ve made it peculiar. In spite of its much larger distance, I think it would be more valuable to study Neptune than Uranus.

WILLIAM June 19, 2014 at 3:48

From an article on The Atlantic site:

http://www.theatlantic.com/technology/archive/2014/06/nasa-is-building-a-tiny-mothership-to-pioneer-distant-lunar-oceans/373020/

NASA Is Building a Tiny Mothership to Pioneer Distant Lunar Oceans

Suppose you’re a planetary scientist. You operate an unmanned spacecraft, surveying a distant moon in our solar system. Years of funding, engineering work, and long-distance space travel have all come together, and at last this machine—to which you have devoted so much of your life—is in place. And it’s just made an incredible discovery.

Maybe it’s a new kind of crater. Or an odd, unexpected mineral. Or the holy grail: liquid water.

It’s thrilling news—years of your career, vindicated! Now you have to wait. And lobby. And hope for the funding to come through. And wait for the next craft to get there.

As Brent Streetman, a researcher at the aerospace technology firm Draper Laboratories, told me earlier this week: “Once we find interesting things, there’s no way to access them. We have to wait for the next cycle of space exploration to that planet.”

Indeed, the NASA scientists tasked with extending humanity’s reach into space have two very different jobs. The first is posed by space and solved by engineering: It’s the actual work of sending tools, instruments, and (sometimes) humans millions of miles, to another place in space, intact. But the second one can be both much more mundane and much more infuriating: It’s the ongoing work of securing funding for space exploration from a capricious and dysfunctional Congress.

A new experimental spacecraft design anticipates the second problem with the techniques of the first. Draper Laboratories received funding this week from NIAC, NASA’s innovative concepts fund, for a two-phase space probe—technology that could both survey a planet and send instruments to its surface.

Where might such a probe go first? Its designers, led by Streetman, think it might be a good way to explore the only orb in the solar system believed to have liquid water: Jupiter’s moon, Europa.
Draper Labs
In its first stage, a small satellite about as large as a half-gallon of milk would orbit the moon. Using two highly accurate accelerometers, it could sense small changes in Europa’s gravitational field, eventually mapping the gravity of the entire surface. These detailed gravity maps could then suggest the location of watery oceans below the planet’s surface—or the openings to these oceans.

Once an ocean (or the entryway to one) was found, the probe would begin its second stage. The small satellite would release even smaller instruments over the interesting region. These “chipsats,” each no larger than a fingernail, could enter Europa’s thin atmosphere unharmed and float down to the surface.
“When there is an atmosphere, they flutter down like little pieces of paper, not like a rock,” said John West, leader of the advanced concepts team at Draper. He added that while they expect to lose some of the smaller “chipsats,” enough would be released that useful science could be performed.

Once deployed, the tiny chipsats would then send their measurements back to their orbiting mothership, which would in turn beam them back to Earth.

Both of the mission’s vehicles were pioneered in near-Earth orbit. The gravity-mapping satellite draws on cubesat technology, a set of tools and common plans that let satellites be cheaply produced. Last November, a team of high schoolers put a cubesat in orbit. The even smaller “chipsats” were first deployed as part of the space shuttle Endeavour’s final mission in 2011, in partnership with researchers at Cornell University. Cornell is also consulting on the project.

Europa was last studied at close proximity by NASA’s Galileo spacecraft. Over a decade ago, Galileo orbited Jupiter before the probe’s human overlords sent it careening into the gas giant’s atmosphere, in part to keep from contaminating Europa’s surface.

steven rappolee June 19, 2014 at 11:29

I have a rather warped idea on how to get payloads out to the outer planets using a hybrid chemical/NEP Centaur in space stage, and with out plutonium

http://yellowdragonblog.com/category/small-fission-reactorchemical-stage-hybrids/

Con ops;
Centaur and outer planet probe fire chemical propellant

Xenon gas is used as ullage to pressurize fuel tanks

LO2 tank contains an inert decadel survey small fission reactor

after chemical burn fission reactor starts up and radiator heats xenon gas in fuel tank, powers a set of Stirling or ranking engines to augment the reactor.

Centaur fuel tank walls and Centaur hull warms up and transmits heat and power to space craft and science payloads.

power to xenon ion engines requires the RS-25 engine nozzle to retract to avoid impingement

RobFlores June 19, 2014 at 15:05

BRIAN:

I though the winds were due to the large amount of energy
in the atmosphere compared to Uranus. It has been established that
per same amount of surface area, Neptune emits as much heat as
Jupiter. Could this mean that it’s core it either much more massive
than Uranus. and/or is somewhat different in composition.

Ashley Baldwin June 20, 2014 at 18:29

ODINUS is a mission of great vision . Geoff Marcy’s excellent work looking at the characteristics of the exoplanets discovered in the Kepler data and especially those with both mass and radius (and hence “bulk density”) has obliged us to see Uranus and Neptune in a different light. To date , let’s admit it, we have all seen them as the poor relatives of our solar system , dull compared to mighty Jupiter and its moons , beautiful Saturn , glowing warlike Mars and Dantesque Venus . No longer is there a clear divide between gas/ice giant and rocky terrestrial planets. Maybe there are “dwarf ice giants” in between Earth and its icy cousins.
Worse still , early solar systems seem to be planetary bowling alleys with “Jumping Jupiters “and “Grand tacks” and “Nice” models which are anything but , causing mayhem and leading to planetary ejections and the late heavy bombardment of displaced asteroids and comets ( or Kuiper Belt Objects, KBO, to give them their sexy title) , especially if you are the Principal Investigator of the New Horizons mission wanting a grant extension from NASA after your ship whizzes past exciting ,unknown Pluto in a few days after years travelling ( like visiting Beijing as a tourist after going through immigration) and needs another target.
Which brings us to ODINUS. Combining two M ESA missions into one L mission still only produces circa €I billion , with a free launch. That isn’t even deemed nearly enough to get one satellite to Europa , so how is it get two to Neptune and Uranus ,”straw man”or not ? Furthermore it is dependent on RTGs. Radio active isotopic thermoelectric generators , which uses the heat of the radioactive decay of plutonium 238 ( gentle cousin of bomb making 239) to create electricity to power the satellite in the outer reaches of the solar system that are too dark for solar panels to work. Only the US has this material available , and although it appears there is more about than let on till recently , it will be required to power atleast one more Mars rover ( another?) and the aforementioned Europa mission. Certainly not two speculative ESA missions. The ESA like maximum science return per euro , so I understand the ODINUS concept , but surely the best bet is get as much science payload on one satellite as possible and send it to the nearer Uranus . The Marcy data will no doubt be enhanced by further trawling through Kepler data plus the soon to come recently approved Kepler 2 and 2017′s TESS supplanted by critical spectroscopic characterisation from JWST’s NIRSPEC . Indeed TESS’ whole raisin d’être is to characterise gas and ice giants ( and hopefully a few Habitable zone “super earths” too) . This in itself should make a powerful case to help characterise Uranus/Neptune planets, especially given mission shaping data from exoplanets to compare too , but with infinitely better resolution given their proximity. Perfect combination, made even better as the satellite would arrive just after or even during the ESA Kepler plus M2 PLATO mission and with some luck , direct imaging via WFIRST-AFTA or even an Occulter telescope.
Space X already have several Falcon Heavy rocket designs at advanced development stage and atleast one of these would be ready in time to give a Uranus probe a hefty ( cheap) push on its way ( assuming Elon can tear himself away from Mars missions for a bit and sort out my PayPal account) in a nice old Saturn V style rather than slung via Memphis and Milton Keynes for a couple of centuries . That still leaves some bargaining to be done with US over the deal breaking RTG , which they have been understandably wary of giving to volatile European nations. Partnership perhaps? It is only one tincture windy RTG after all. Nevertheless , Uranus and Neptune ,like 80s music , are back in fashion. ( although thankfully they are here to stay )
Never mind using obscure Viking gods no one has heard of and sound like Swedish au pairs ,this probe is fighting for knowledge . Call it “Thor”. Roar.

steven rappolee June 20, 2014 at 19:37

somewhere on my blog or is it my linkitIN site I have a theory for gas rice giants that emit more energy the contraction would call for,
it involves natural reactors in the core of the planet, I have not come up with a convective mechanisms for how a core would segregate fissile materials to create a natural reactor as of yet

Michael June 21, 2014 at 11:31

@steven rappolee June 20, 2014 at 19:37

‘I have a theory for gas rice giants that emit more energy the contraction would call for, it involves natural reactors in the core of the planet, I have not come up with a convective mechanisms for how a core would segregate fissile materials to create a natural reactor as of yet’

If the planets cores have efficient fission reactors most of the material, U235, would have fissioned long ago, initial heating the planets greatly but then they would have cooled off. The most likely solution is multiple heat source processes in the condensation of gases, contraction and the heat of fusion of water changing into ice. Suppressed convection could have kept the heat in longer and efficient convection would have led to accelerated cooling as heat was churned to the surface from the mantle/cores, Uranus may have experienced this efficient convection process leading to result we have today.

Just my thoughts on the subject, we know little about the ice giants and that explains why we need to go visit them more thoroughly.

ljk June 23, 2014 at 9:08

Steve, I do not know if this is any help but here is a nuclear reactor made by nature on Earth:

http://apod.nasa.gov/apod/ap100912.html

Leave a Comment