Van Kane • July 6, 2017

How we would explore Uranus or Neptune

This article originally appeared on Van Kane's blog and is reposted here with permission.

One fact dominates the planning for any mission to Uranus or Neptune: They lie far from the sun. Reaching one of these worlds takes so long that by the standards of almost any other mission, the spacecraft is already old before it would begin observations of its target world. The Juno spacecraft took five years to reach Jupiter. The Cassini spacecraft took almost seven to reach Saturn. A mission to orbit Uranus, by contrast, could take twelve years just to reach its destination, with a launch in May 2031 and arriving at Uranus in May 2043.

As a result, each of these worlds has been visited only once by the 1970s-vintage Voyager 2 spacecraft, which performed hectic observations during brief flybys in the mid and late 1980s. The paucity of information on these two planets has left blank chapters in our understanding of the solar system. The planetary science community wants to return with a spacecraft that stays for in-depth observations and carries modern instruments. Toward that end, a NASA-chartered team of scientists and engineers published a report this month outline options for returning to these worlds.

Lawrence Sromovsky / W. M. Keck Observatory

Clouds on Uranus

False color images of both hemispheres of Uranus reveal the differing altitudes of clouds. The color balance was chosen to make the highest-altitude clouds white, middle-altitude clouds greenish, and lowest-altitude clouds deep blue. This color balance choice is responsible for the bright red color of the rings, which are actually gray, not red.

Uranus and Neptune are a distinct class of planets in the solar system. The gas giants Jupiter and Saturn are primarily composed of hydrogen and helium. The ice giants Uranus and Neptune, by contrast, are believed to have formed further from the sun where ices would have been more common. As a result, these are water worlds (although the water is under tremendous pressure and believed to be in an ionic state and mixed with ammonia and methane). Above the water are atmospheres of hydrogen and helium and below rocky cores. However, the report notes that with our current knowledge, Uranus and Neptune, “challenge our understanding of planetary formation, evolution and physics.” We also have learned that planets of their size are a common class of worlds orbiting other stars.

We need to return to these worlds to both understand our own and other planetary systems.

In one way, exploring the solar system is like building medieval cathedrals—it is a process that takes generations. If the plans laid out in the report come to fruition, almost six decades will pass between the Voyager 2 flybys of these worlds and their next visit. Approximately a quarter century will pass from today to arrival. This is not unusual. Nearly a quarter century will also pass between the first serious discussions I’m aware of for a dedicated mission to Europa and the expected arrival of the Europa Clipper spacecraft. Now is the time to begin serious planning to take advantage of the good flight opportunities around 2030.

The goal of the report is to provide the planetary community a range of options it can use when they consider goals for planetary exploration in the 2020s and early 2030s. That process, called the Decadal Survey, looks across the solar system and recommends a balance of missions to address top scientific questions within the expected budget. The last Survey, covering 2013 to 2022, ranked a mission to Uranus as the third priority after a rover to cache Martian samples and a mission to explore the habitability of Europa. Available funding allowed development of those latter two missions to begin this decade. The possible start of work on a mission to Uranus was deferred to the 2020s. With that delay, the changing alignment of the planets opens up the alternative to explore Neptune instead of Uranus.

The first use of the report will be by a committee conducting a mid-term assessment of the current Decadal Survey. According to Dwayne Day, the study director of the National Academies' mid-term assessment, the review committee has already been briefed about the ice giants study, and has also heard from the Outer Planets Assessment Group about future outer planets missions. Dr. Day was involved in running the planetary decadal. He notes that one of the challenges that group faced was that few planetary mission studies had been done prior to the last Decadal Survey, which limited the options the survey members had when they started. That created a crush during the study to develop new mission concepts and conduct mission evaluations while the survey was underway. Having the ice giants study in hand prior to the development of the next Decadal Survey is a real asset.

The report is a menu of options: perform a flyby only (not recommended), orbit Uranus or Neptune or both (each have unique characteristics), possibly deliver a probe that would enter the atmosphere (recommended), and carry 3, 7, or 13 (recommended) orbiter instruments. There’s also a choice of launch vehicles.

With all these options, it can be difficult to answer what was my basic question in reading the report: When we return, how will we explore whichever world is prioritized in the next Decadal Survey? In this post, I take one set of options and look at that question. For anyone reading the report, this generally follows Option 5 for a Uranus orbiter but with seven instead of three orbiter instruments listed for this option. I also look at how the goals would change if Neptune were selected instead.

I recommend Jason Davis' post on the Planetary Society's blog for additional background on the scientific reasons for returning to these worlds.

Ice Giants Pre-Decadal Study Final Report

Ice giants mission options

Summary of several of the mission options examined by the ice giants study team. Click on the image for a larger view.

Transit, Arrival, and Orbit

For the most part, the cruise to Uranus would have the spacecraft in quiet mode, with periodic status checks with home. Venus and Earth encounters en route could provide opportunities to check out the instruments and observation modes. Many of the trajectories include a flyby of Jupiter that could present opportunities for new science. The spacecraft would carry a new class of instrument, a Doppler imager (more on this later), that could extend our knowledge of the planet’s interior following the Juno mission. (The report doesn’t discuss opportunities for Jupiter science, but given the extensive observations by the Cassini and New Horizons spacecraft as they flew by Jupiter, an ice giant mission is likely to do so, too.)

Long range observations of Uranus would begin 85 days before arrival, and an atmospheric probe would be released 25 days after that. The hours around arrival would be crowded with relay of the data from the atmospheric probe, the orbital insertion burn, and a pass so close to the planet that the spacecraft skims the tenuous upper fringe of the atmosphere.

Once in orbit, the spacecraft would spend 2+ years studying the Uranus system. Each standard science orbit would take approximately 50 days. During distant portions of the orbits (greater than 20 Uranus radii), the narrow angle camera would observe the entire planet and the ring system. Closer in, the spacecraft divides its time between high resolution observations of the clouds, rings, moons and the magnetosphere. The example tour presented in the report would include two close flybys of the moon Titania and three each for Oberon, Umbrial, Miranda, and Ariel. The report notes that at the end of the mission, the orbiter might perform a series of orbits that has it, as the Cassini spacecraft is doing at the end of its Saturn mission, fly between the inner ring and the top of the atmosphere. If done, these orbits would allow close up gravity and magnetic field measurements.

Ice Giants Pre-Decadal Study Final Report

Example orbital tour at Uranus

This tour would provide multiple flybys of the major moons and close examination of the rings and atmosphere.

Science

The report lists twelve science goals. Depending on the instrument compliment, the full range of the Uranus system could be explored: the interior of the planet, the dynamics of the atmosphere, the many minor and five large moons, the ring system, and the magnetosphere. To keep this post to a readable length, I’ll focus on just the top two—determine Uranus’ composition and interior structure—and the examination of the major moons to determine which if any might be ocean worlds.

We now understand that Jupiter, Saturn, Uranus, and Neptune migrated following their formation to eventually reach their present orbits. Understanding the location and manner of the formation of an ice giant would provide missing puzzle pieces to understanding the history of the earliest outer solar system. The goal is to measure the precise ratios of key elements and isotopes in the atmosphere because they act as fingerprints identifying the actual circumstances of planetary birth and evolution.

Addressing these questions would be the job of the atmospheric probe. During its descent through the upper atmosphere to a depth where atmospheric pressure equals at least ten times the sea level pressure on Earth, its mass spectrometer would measure the composition of the gases. Additional instruments would measure temperature, pressure, density to provide context (and contribute to understanding the planet’s weather). If space within the probe and budgets permit, this probe could carry additional instruments such as an instrument to detect cloud layers.

Ice Giants Pre-Decadal Study Final Report

Our current understanding of the interior structure of the gas and ice giant planets

As with the ice giant’s formation, our understanding of their interior structure is poor. The current model has an outer gaseous atmosphere composed primarily of hydrogen and helium, a large inner ocean composed primarily of ionic water, and a rocky core. The existing data from the Voyager 2 flybys, however, is ambiguous. The primary instrument to address this question on a new mission would be one that has never flown on a planetary spacecraft. A Doppler imager would look for oscillations at the top of the atmosphere caused by motions at a range of depths within the atmosphere and ocean. Just as seismic waves in rocky worlds reveal their interior structure, these atmospheric motions would reveal the interior structures of gas and ice giants. The same method is used to study the interior of the sun. The report notes that these measurements hold the promise to revolutionize the study of outer planet interiors over the current methods that use planets’ gravity fields to study interiors. (Measurements during a Jupiter flyby would be as novel as those at Uranus or Neptune.) The measurements, however, are data hungry, requiring images be taken as frequently as every two seconds during the approach to the planet.

NASA / JPL-Caltech

Voyager 2 images of the five larger moons of Uranus

Satellites are shown with their correct relative sizes. The inner three (left) are black and white images, the outer two are false color.

The five major moons of Uranus fall within the same size range as Saturn’s medium sized moons such as Enceladus, Mimas, and Dione. Voyager 2’s low resolution (except for the inner most of these moons, Miranda) observations revealed that all five show signs of varying degrees of past resurfacing from cryovolcanism and tectonic activity. The two innermost, Miranda and Ariel, appear to have extensive resurfacing while the two outermost, Titania and Oberon, are large enough that they may have liquid oceans between their outer icy shells and inner rocky core. Titania has comparatively few large craters, suggesting a younger surface, than the more cratered Oberon. Titania also has a series of surface ridges similar to those on Europa’s surface. The middle of these moons, Umbriel, appears to have the least altered and most battered surface.

During the satellite tour, the orbiter’s camera would image their surfaces to allow scientists to reconstruct their past geologic history. If the craft carries more than just the minimal core instrument complement, imaging spectrometers would be used to measure the surface composition, which likely includes material erupted from the interior. The magnetometer would be used to search for induced magnetic fields at the moons that would strongly suggest a present interior liquid ocean. (This is how the Galileo spacecraft all but confirmed the existence of Europa’s ocean.) Radio tracking of the spacecraft’s signal would be used to measure the sizes of the icy shells, any ocean, and the rocky cores. The wide range of sizes (from Miranda’s 472 kilometer diameter to Titania’s 1577 kilometer diameter) and geologic variety makes the Uranus system a laboratory for understanding how systems of moons formed and evolved and their potential for providing habitats for life.

If Neptune Instead of Uranus

The report describes two options for missions that target Neptune instead of or in combination with Uranus. Because Neptune lies further from the sun, an orbiter and probe mission would require a larger and more expensive launch vehicle than the Uranus mission, plus a solar electric propulsion stage to provide an additional velocity boost beyond what the launch vehicle and the gravity assists from Earth and Jupiter can provide. Total flight time to Neptune with this combination would take thirteen years—one more than to Uranus without the solar electric propulsion stage.

NASA / JPL-Caltech

Voyager 2 image of Neptune's moon Triton

For this mission, the science goals for studying the planet, its rings, and minor moons are essentially the same as for Uranus. The key difference is that Neptune possesses one extremely large moon, Triton, that is likely a sister world to Pluto that was captured from the Kuiper belt. Triton has a thin atmosphere, erupting (at least at the time of the Voyager flyby) geysers, and possibly an ocean beneath the ice shell. This moon would be a focus of the orbital mission with 36 encounters. The science goals would be similar to those for the larger Uranus moons with the addition of studying the composition of the atmosphere with an ultraviolet spectrometer as was done by the New Horizon’s craft at Pluto.

The report also briefly discusses the possibility of sending an orbiter to Uranus and a flyby spacecraft to Neptune. For this option one orbiter would conduct the in-depth studies the committee felt were essential while the flyby craft would expand the studies to the second ice giant. One of the two craft would carry an atmospheric probe. In this case, the Neptune flyby would likely be similar to a Uranus flyby considered by not recommended in the report. The Neptune flyby craft would conduct approach science as described for the Uranus orbiter above. The craft would then likely conduct a close flyby of Triton to provide single close up examination of it before heading into the deep outer solar system and possibly into interstellar space.

Getting There

It’s possible to traverse the realm of the ice giants quickly—the New Horizons spacecraft reached even more distant Pluto in just under a decade. However, that spacecraft was intentionally kept as light as possible to allow a high velocity launch. While New Horizon’s was a bantam-weight scout, scientists want a highly capable orbiter for what likely would be a once-in-several-generations mission to orbit one of these worlds. The proposed ice giant orbiters would be approximately 2000 kg without fuel, or about five times the mass of the New Horizon’s spacecraft, but about the same as the Cassini Saturn orbiter.

Even if the launch vehicle existed to directly fling the heavier ice giant orbiter to its destination, a New Horizon style mad dash wouldn’t be possible. When it reached Pluto, that spacecraft was going so fast that it would have been impractical to carry enough fuel to insert itself into orbit. The mission design for an ice giant mission becomes a trade off between speed and the mass of the fuel needed to brake from that velocity into orbit. Using the a mid-range commercial launch vehicle such as the Atlas V, a reasonable balance results in the twelve-year flight listed above.

Using a more expensive Delta IV Heavy, a year to a year and half can be cut from the transit time. If an SLS booster is available with its greater launch ability, the flight time can be cut by four years. Adding a solar electric propulsion unit to provide a boost in flight could cut the flight time a year. Any combination of these latter options comes with the trade off of a higher, and possibly much higher, overall mission cost. (When it becomes available, the Falcon Heavy will provide an additional option.)

Ice Giants Pre-Decadal Study Final Report

Example trajectory for a mission to orbit Uranus

(Option 5 from the report)

Another challenge for exploring these worlds is that the sun is too faint for solar power so radioisotope power supplies would be required. Over time, the components of these supplies degrade, reducing power to the spacecraft. (The radioisotopes also decay, but that loss is slower.) An enabling technology for these missions is an enhancement already under development that boosts power delivery late in a mission’s life by incorporating longer-lived components (for those of you who follow these technologies, this is the enhanced Multi-Mission Radioisotope Thermal Generator, or eMMRTG). The proposed designs would carry either four or five eMMRTGs. The Curiosity rover, by comparison, carries just one of the current generation MMRTGs. At the projected end of these missions, the combined output from the multiple eMMRTGs will be less than either four or five 100 Watt light bulbs, depending on the number carried.

The report notes that the expected power will require turning instruments on and off because not all can operate at the same time. That act stresses the electrical components such as solder joints. Instruments for missions to Uranus or Neptune, the report notes, may require additional levels of redundancy be built in to ensure they can operate for the full length of the missions.

Another challenge imposed by distance is returning the data collected. At Uranus, the data rate would be approximately 0.37 to 0.52 gigabits per day, or approximately one-fifth to one-seventh the rate planned for the Europa Clipper mission which will operate at the much closer Jovian system. The report notes that arbitration for the available data bandwidth between the different scientific investigations will be “complex.”

The Next Steps

As discussed in the introduction to this post, planning missions for the outer solar system is a long game. A key decision will come in the rankings of priorities of missions in the next Decadal Survey. Then, assuming a high ranking, the size of NASA’s planetary science budget will determine its capabilities, and whether just one or two of these worlds will be explored. A mission to either of these worlds would have similar costs—ranging from around $2 billion to $2.3 billion depending on options—to other large planetary missions such as NASA’s Curiosity rover.

Ice Giants Pre-Decadal Study Final Report

Cost-benefit analysis

The estimated relative science benefit (vertical axis) versus estimated cost of different mission options using the minimum three instrument payload recommended for the flyby or orbiter spacecraft. The study team concluded that the limited science from a flyby mission would not be worth the cost savings from the much more capable orbiter mission. Sending missions to both planets was judged to approximately double the science return but at substantially higher costs.

While the report focuses on a U.S.-only mission, the study was done with the hope that exploring these worlds could be done as an international project. Scientists in Europe have proposed their own missions. The report recommends a further study to explore the options for collaborative missions to these worlds. Working together, a mission becomes more affordable to both agencies and the chances for a more capable mission, or even missions to both worlds, seems more likely. Other space agencies might also be interested. The case for exploring these worlds is compelling, and I am optimistic that we will.

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2017年7月6日星期四