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 its observations of its target world. The Juno spacecraft took five years to reach
Jupiter and 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 that 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.
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| Uranus. Credit: Lawrence Sromovsky, University of Wisconsin-Madison/W.W. Keck Observatory |
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
2020’s and early 2030’s. 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 previous 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
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.
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| Summary of several of the mission options examined by the ice giants study team. Click on the image for a larger view. Credit: Ice Giants Pre-Decadal Study Final Report. |
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 two plus 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.
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| Example orbital tour at Uranus that would provide multiple flybys of the major moons and close examination of the rings and atmsophere. Credit: Ice Giants Pre-Decadal Study Final Report. |
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 gasses. 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.
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| Current understanding of the interior structure of the gas and ice giant planets. Credit: Ice Giants Pre-Decadal Study Final Report. |
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.
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| Voyager 2 images of the five larger moons of Uranus. Credit: Ice Giants Pre-Decadal Study Final Report. |
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, and 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.
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| Voyager 2 image of Neptune's moon Tritan. Credit: JPL/NASA |
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.)
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| Example trajectory for a mission to orbit Uranus (Option 5 from the report). Credit: Ice Giants Pre-Decadal Study Final 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.
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| Conceptual design for a Uranus orbiter and atmospheric probe. Credit: Ice Giants Pre-Decadal Study Final Report. |
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 may, 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.
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| 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. Credit: Ice Giants Pre-Decadal Study Final Report. |
While the report focuses on a US-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.
