Uranus or Bust (and on a budget)

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.

Voyager 2’s cameras saw a near featureless cloud deck at Uranus.  More recent images from observatories looking in different portions of the spectrum have shown that Uranus has an atmospheric circulation system as complex as Jupiter’s or Saturn’s.  From a presentation on a Uranus atmospheric probe mission by Mark Marley and colleagues at NASA’s Ames Research Center.

A presentation by Mark Hofstadter of JPL and member of a Uranus science working group, has laid out the scientific case for a mission to this world and its prospects.  (A European vision of a similar scale Uranus mission can be read here.)

For Uranus, science objectives break into several broad classes of studies.  Scientists want to send a probe into the atmosphere for detailed composition measurements of the atmosphere to better understand how Uranus formed and how its structure has evolved.  Remote observations of Uranus to study its weather, understand its heat balance, and probe its interior are another priority.  Researchers would like to study the planet’s ring system many and many moons up close.  And finally, they would like to re-measure Uranus’ highly unusual magnetosphere which is unlike any other planet’s except for Neptune’s.

The ideal mission architecture would combine an atmospheric probe with an orbiter, as proposed by the Decadal Survey.  (You can find a copy of the mission study at this website.) The atmospheric probe would gather compositional information that is not available any other way.  An orbiter would stay within the Uranus system for prolonged observations of the planet, rings, and moons.  From orbit, the spacecraft would also traverse many locations within the magnetosphere allowing a study of its structure not possible from a flyby spacecraft that travels a single path.

The scientific case for exploring Uranus is solid and gaining attention.  NASA’s shrinking planetary science budget, however, can no longer support the approximately $1.8B orbiter and probe recommended in the Decadal Survey.  Mission architects, however, have begun to look at alternative, cheaper missions that could fit in the New Frontiers (~$1B) and Discovery (~$500M) mission programs.  These cheaper missions would reduce costs by conducting only a portion of the Uranus science goals.

The team examining Uranus science goals for the Decadal Survey divided the science goals into Tier 1 (highest priority), Tier 2 (high priority), and Tier 3 (highly desirable).  While the Decadal Survey endorsed a mission that would address goals from all three tiers, a New Frontiers mission would likely focus on only the highest priorities.

 Estimated costs of Uranus mission elements from the DecadalSurvey’s Ice Giants Decadal Study Mission Concept Study.  The Tier 1 (highest priority) science orbiter costs are near the cost of a New Frontiers mission.  Many observers have commented that the Decadal Survey mission costs were conservative (i.e., erred on the high side) so a New Frontiers Uranus orbiter might fulfill all the Tier 1 goals.

The most straightforward way to enable a cheaper Uranus mission would be to do either an orbiter or an atmospheric probe mission.  The Decadal Survey’s minimum orbiter mission cost (~$1.2B) was close to the close to the expected $1B cost cap of future New Frontiers missions.  The Decadal Survey report on the conceptual design of a Uranus orbiter-probe mission listed two Tier 1 goals:

“Determine the atmospheric zonal winds, composition, and structure at high spatial resolution, as well as the temporal evolution of atmospheric dynamics.”  Instruments: wide angle camera with multiple filters and a visible/near-IR (Vis/NIR) mapping spectrometer

“Understand the basic structure of the planet’s magnetosphere as well as the high-order structure and temporal evolution of the planet's interior dynamo.” 

An orbiter would allow many regions of the Uranus system to be explored.  This figure is from the mission concept study and shows the path of the orbiter in a Uranus-fixed coordinate system.

If the spacecraft’s orbit were designed to enable close flybys of Uranus’ larger moons, these three instruments would also address two Tier Two science priority objectives: “Remote sensing observations of large satellites,” and, “detect induced magnetic fields that would be indicative of interior oceans [in Uranus’ moons].”  However, the best orbits for meeting the Uranus observations are not the same as those for close flybys of the moons.  Accomplishing both would require a longer and more expensive orbital tour (~$26M more) that might not fit within a New Frontiers budget.

The three Tier 1 science instruments are based on mature technologies, and similar instruments have flown on numerous missions.  They could have a combined mass less than 15 kg.  By comparison, just the Cassini Saturn orbiter’s camera system is almost 58 kg and is just one of twelve instruments.  A Uranus mission that focused just on Tier 1 science would indeed be a tightly focused science mission.  (If money for a the full $1.8B Flagship mission were to become available, the mission would add fields and particle sensors, a mid-infra-red thermal detector to measure the distribution of thermal emissions, a narrow-angle camera, and a UV imaging spectrograph, and a nephelometer to detect clouds to the atmospheric probe.)

To my knowledge, no formal analysis of a New Frontiers class Uranus atmospheric probe mission has been performed.  However, the requirements and complexity of such a mission would seem to be similar to that of a Saturn atmospheric probe mission.  That mission has been studied and been judged to fit within a New Frontiers budget.  (A Uranus mission would have a longer flight time that would raise costs somewhat compared to a Saturn mission.) Requirements for the atmospheric probe portion of a Uranus orbiter also were examined by as part of the Decadal Survey and continue to be studied at NASA’s Ames Research Center. 

The Decadal Survey analysis ranked the goals for an atmospheric probe mission lower than those for the Tier 1 orbiter science.  A single Tier 2 goal was identified for the atmospheric probe:

“Determine the noble gas abundances (He, Ne, Ar, Kr, and Xe) and isotopic ratios of H, C, N, and O in the planet’s atmosphere and the atmospheric structure at the probe descent location.”  Instruments: Mass spectrometer and pressure, temperature, and acceleration/deceleration sensors.

Further analysis by the NASA Ames team suggests that the atmospheric probe should be able to survive to below the bottom of the methane clouds where the atmospheric pressure would be five times that at sea level on Earth.  (Measurements from below the lower water clouds – a key goal for the Galileo atmospheric probe at Jupiter – also would be beneficial, but these are too deep and the pressure too great at Uranus to achieve within any mission currently conceived.)  Uranus is believed to contain 30 to 50 times more carbon (a key element of methane) than would have been the mean in the solar nebula from which the planets formed.  A key question for planetary science has been how the enrichment of carbon and other elements occurred.  Measurements of the ratios of key elements and their isotopes would help determine whether Uranus-class planets are cores of planets that failed to grow into Jupiters or super-Jupiters or represent an entirely different path of planetary creation.

The proposed Saturn atmospheric probe would be carried to its destination by a simple carrier craft that would also act as a data relay that would collect and return the atmospheric probe’s data to Earth.  As envisioned by the Decadal Survey, the carrier craft would not carry any instruments of its own.  A Uranus atmospheric probe carrier craft also probably would not carry any instruments.

If the scientific community continues to support the same priorities as the team that examined Uranus mission goals for the Decadal Survey, the orbiter would address Tier 1 goals while the atmospheric probe mission would address Tier 2 goals.  That would give the nod to the orbiter mission.

There is one programmatic hitch to using the New Frontiers program for either a minimalistic orbiter or an atmospheric probe mission.  New Frontiers missions are selected from a list of candidate missions pre-selected by the Decadal Survey.  The current list includes missions to Venus, the moon, a comet, the Trojan asteroids, Io, and Saturn, but not Uranus.  However, NASA can change the list and has suggested (see this presentation) that it would be open to a request by the planetary science community to change the list in light of the smaller budgets that have become reality since the Decadal Survey.   There will also be a science community review of the Decadal Survey in the second half of this decade that will review the candidate list and could revise it.  Either approach would add a Uranus mission to the list of candidates, but Uranus would be in competition with five to six other exciting destinations.

The new interest in Uranus missions has also led mission architects to look at even lower cost Uranus missions than those described above.  In addition to the New Frontiers program, NASA has Discovery missions that cost approximately half that of New Frontiers missions at <$500M.  To date, none of the twelve selected missions will have flown beyond the asteroid belt.  Flying to any outer solar system destination poses considerable challenges within the Discovery program cost cap.  The long flight times require greater spacecraft reliability and require funding many years of flight operation costs.  Missions beyond Jupiter (and some within the Jovian system) also require plutonium power supplies, although NASA in the past has offered to help defray the costs of using an ASRG system. 

To fit within the Discovery cost cap, a Uranus mission would be limited to a simple flyby and likely would carry only one or two instruments.  The spacecraft also would need to be able to conduct science beyond what Voyager did in 1986, a tough task with just one or two instruments.  Recently, though, a proposed Uranus Discovery mission has been studied that would do that with just a single instrument.

To understand the proposed mission, let me give a bit of background.  Gaseous worlds have internal turbulence that generates acoustic wave oscillations that propagate to their surface.  (The closest equivalents in a solid rocky world are earthquakes that periodically produce movement at the surface.)  Solar astronomers have exploited this phenomenon through helioseismology to study the interior “seismology” of the sun.  The giant planets, including Uranus, can also be studied using the same technique.  (Uranus is referred to as an ice giant because the bulk of its composition (such as water) would freeze if exposed to space that far from the sun.  However, the pressure and heat within Uranus keeps the deep interior fluid except for a rocky core.) A Doppler spectrographic imaging instrument could measure the oscillations and reveal the interior structures of outer planets.  To date, however, no spacecraft has carried one to an outer planet.

Acoustic wave seismology for giant planets including Uranus.  From a presentation by Steve Matousek and colleagues at JPL on the Uranus Explorer concept.

A JPL team has proposed a Discovery-class mission that would carry a small, 35 cm telescope that would house both a Doppler imager and a visible camera.  The spacecraft would first flyby Jupiter to study its interior.  Approximately seven years later, the spacecraft would repeat those measurements at Uranus. 

The Uranus Explorers Doppler imager would fulfill two goals for Uranus: 

• ·     Determine the internal structure: bulk composition and density profile
• ·     Determine atmospheric zonal winds and dynamics.

Acoustic waves have been detected on Jupiter, but not so far for the other giant planets.  It may be that their greater distance puts the detection of these waves below the measuring threshold for Earth-based telescopes.  For Uranus, these waves may be especially weak.   Acoustic waves are created by internal motions produced by heat flow within the planet.  Key trace gases such as carbon monoxide in the upper atmosphere demonstrates that this convection occurs on planets such as Jupiter.  (Carbon monoxide forms in the deep, hot interiors but chemical reactions in the cooler upper atmosphere eventually converts it to methane.  For carbon monoxide to be detectable, it must be regularly replenished by convection.)  Uranus, however, is unique among the giant planets in having a low heat flow and a lack of these key trace gases.  If the Doppler measurements fail to measure acoustic waves at Uranus, this could put a threshold on the amount of internal convection within the planet.

The single, quick transit of the Uranus system would not provide time to meet all the science goals for studying the moons and rings.  However, during the hours around closest approach, the visible camera would address two other goals: 

• Determine the geology, geophysics, surface composition, and interior structure of large satellites.
• Determine the composition and dynamical stability of the rings and small satellites.

In researching this post, I have come to realize how diverse Uranus’ moons are.  Like Saturn, Uranus has many small moons in and near the ring system and a diverse group of medium-sized moons further out (see this presentation).  (Uranus, of course, lacks a truly large moon equivalent to Saturn, though.)  Uranus’ Ariel, like Saturn’s Dione, shows signs of internal activity that have partially resurfaced its surface.  Two of Uranus’ moons are large enough that astronomers suspect that they may harbor interior oceans. 

The JPL presentation focuses on demonstrating that a scientifically compelling Discovery-class Uranus mission is possible.  With further study, I suspect that a proposal team building on this start would find additional, fairly low cost ways to enhance the science.  Careful design of the flyby trajectory could have the spacecraft travel behind the rings and allow the transit of the radio signal from the spacecraft to measure the structure of the ring system. 

If an ultra-stable oscillator was including in the communications system, the radio transmissions could be used to measure gravity fields.  Improving on Voyager’s gravity measurements of Uranus itself would be difficult because the spacecraft would have to fly dangerously close to the top of the atmosphere where inner ring particles could present a hazard.  However, the craft could be sent past one of the moons to use gravity to measure its interior. 

International partnerships might allow adding a simple instrument or two like this to the craft with little cost to NASA.  One high priority instrument might be a visible/near-infrared mapping spectrometer that could measure trace gases in Uranus’ atmosphere.  Measurements of trace gases (or their absence) would provide a second way to estimate the internal turbulence of Uranus.  Alternatively, if the flyby spacecraft carried a magnetometer and its trajectory carried it close to one of the larger moons, it could look for changes in the magnetosphere that could confirm an ocean.  A magnetometer also would provide a second look at Uranus’ magnetosphere. 

The JPL presentation describing the proposal calls the mission simply the “Uranus Explorer.”  Because the spacecraft studies two very different giant planets, I think a better name might be something like, “Echoes: Giant Planet Seismology.”  JPL’s Uranus Explorer Discovery mission concept is in the early stages of study.  What I find exciting is that the results so far suggest that a compelling mission to Jupiter and Uranus could be possible within the tight cost constraints of the Discovery program. 

Editorial Thoughts: Before I did the research for this post, I had almost given up hope for a return to Uranus within my lifetime.  (I’m in my mid-fifties.)  Now it appears that a mission could fit within either the New Frontiers or the Discovery program, and I’m hopeful.  However, I also recognize that NASA’s shrinking planetary science budget means fewer missions will be flown.  Competition from other exciting solar system destinations will be fierce.  But I am hopeful.

As I wrote this post, I found myself wondering which of the mission options I personally find most appealing.  I concluded that I believe the combination of a flyby spacecraft with the Doppler imager-camera with an atmospheric probe would provide the combination of science that I believe is most compelling.  I don’t know if this could be done within the cost cap of a New Frontiers mission or not, and recognize that the scientific community may well decide that another mission configuration provides better science.  For me, though, the combination of examining the interior of two planets, measuring the composition of a new class of worlds with a probe, and imaging the other half of Uranus’ moons is compelling.  But I would be excited by any mission to Uranus.