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Outer Planet Exploration Strategies

I’ll wrap up this week’s outer planet coverage with a look at recent Cassini flybys of Titan, but I also want to put these accomplishments in the context of what we might do with future missions to the ice giants Uranus and Neptune like the proposed ODINUS missions we looked at yesterday. One-off missions to explore a planet and its satellites collect highly detailed data, but comparative studies of the giant planets require accumulating datasets separated by decades. Are there alternatives?

Let’s hold that thought as we look at Cassini in this light. The flyby designated T-101 occurred on May 17 and was highlighted by Cassini beaming radio signals over Ligeia Mare and Kraken Mare, the two largest seas on Titan. The idea here is to bounce the signals off the surface of the lakes so that they are received by the ground stations of the Deep Space Network here on Earth.


Image: Signals bounced off Titan can reveal important details about the moon’s surface. Credit: NASA/JPL-Caltech.

Called a bistatic scattering experiment, the radio signals encode information about Titan’s surface, including its solidity, reflectivity and composition. The attempt using the two seas in May demonstrated that specular reflections of the radio frequencies could be detected by the DSN. Essam Marouf (San Jose State), a member of the Cassini radio science team, describes the result: “We held our breath as Cassini turned to beam its radio signals at the lakes. We knew we were getting good quality data when we saw clear echoes from Titan’s surface. It was thrilling.”

The June 18 flyby, T-102, performed the same bistatic scattering experiment, during an approach that took the spacecraft just 3659 kilometers above the surface of the moon. Both flybys also experimented with radio occultation, as this NASA news release explains. Here a signal from Earth is sent through Titan’s atmosphere toward the Cassini spacecraft, which responds with an identical signal. Temperature and density differences can be teased out of the transaction, a method that has been used for several earlier occultations of Saturn.

The May flyby demonstrated that signal lock could be achieved quickly, making the method a useful tool in our efforts to track atmospheric variations during Titan’s changing seasons.

“This was like trying to hit a hole-in-one in golf, except that the hole is close to a billion miles away, and moving,” said Earl Maize, Cassini project manager at NASA’s Jet Propulsion Laboratory in Pasadena, California. “This was our first attempt to precisely predict and compensate for the effect of Titan’s atmosphere on the uplinked radio signal from Earth, and it worked to perfection.”


Image: Cassini team members react with excitement to the successful receipt of radio signals bounced off of Titan during a flyby on May 17, 2014. Credit: NASA/JPL-Caltech

If only we had a spacecraft as capable as Cassini orbiting Uranus and Neptune, as at least one commentator here noted after yesterday’s post on ODINUS, twin spacecraft that would orbit the two worlds in a new proposal for the European Space Agency. We could then imagine repeated flybys of Triton, that mysterious geologically active moon that circles Neptune in a retrograde orbit, while performing a variety of studies on the satellite system at Uranus. I want to drop back into the ODINUS paper to cite its statements on this matter, which remind us how little data we have on these moons since our only close encounter has been through the Voyager flybys:

The satellites of Uranus and Neptune are poorly known, mostly due to the limited coverage and resolution of the Voyager 2 observations. The Uranian satellites Ariel and Miranda showed a complex surface geology, dominated by extensional tectonic structures plausibly linked to their thermal and internal evolution… Umbriel appeared featureless and dark, but the analysis of the images suggests an ancient tectonic system… Little is known about Titania and Oberon, as the resolution of the images taken by Voyager 2 was not enough to distinguish tectonic features, but their surfaces both appeared to be affected by the presence of dark material.

As for Triton, it is itself a prime inducement to put such a mission together:

The partial coverage of the surface of Triton revealed one of the youngest surfaces of the Solar System, suggesting the satellite is possibly more active than Europa… Notwithstanding this, the surface of Triton showed a variety of cryovolcanic, tectonic and atmospheric features and processes… The improved mapping of these satellites, both in terms of coverage and resolution, would allow to study their crater records and their surface morphologies, which in turn would provide a deeper insight on their past collisional and geophysical histories.

Both Galileo and Cassini have given us in-depth looks at a gas giant and its satellites, and we now ponder the kind of follow-ups that will investigate specific areas like the interior of Jupiter (Juno) and specific Jovian moons like Ganymede, Callisto and Europa (the JUICE mission). Cassini’s successes have been spectacular, but I like the approach that Diego Turrini and his colleagues take when they observe that comparative studies of the gas giants involving separate missions demand the completion of each before a full assessment of their data can begin.

Instead, Turrini argues for ODINUS as “…two M-class spacecraft to be launched toward two different targets in the framework of the same mission.” The tradeoff is likewise obvious: While we get comparative planetary data — in this case on the ice giants Uranus and Neptune — in a shorter timeframe, we also need to produce two spacecraft and manage them simultaneously, likely limiting the amount of instruments in the scientific payload of each. Can we find a way to do this while still achieving the high-quality science we expect from separate dedicated missions?

It’s an open question, and every Cassini success speaks to the value of the more traditional approach, but we should be looking at ways to speed up the comparative data return in future missions. It will be interesting to see how ODINUS fares among its ESA critics. The Turrini paper cited here and yesterday is “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).


Comments on this entry are closed.

  • Tulse June 19, 2014, 9:51

    It’s hard to argue with Cassini’s success — it is arguably the most effective science device ever lifted off the surface of our planet, and at the very least is the most informative space probe ever.

  • ljk June 19, 2014, 11:39

    NASA wants to send a quadcopter drone to Titan

    Jesus Diaz

    Today 12:02 am

    NASA wants to search Saturn’s moon Titan for life but they’re having trouble coming up with a good way to cover a large territory and obtain samples. Now they think they may have a good solution: A 22-pound quadcopter that will work from a mothership. After reading about it, it’s a really cool idea.

    Larry Matthies—a Senior Research Scientist and the supervisor of the Computer Vision Group, in the Mobility and Robotic Systems Section of the JPL in Pasadena, California—thinks that this may be the only solution that can achieve mission objectives—the search for life or prebiotic chemistry in one of the places in the solar system more likely to have it—safely and at low cost and low risk.

    Full article here:


  • torque_xtr June 19, 2014, 13:16

    My guess is that the next craft to visit ice giants would be nanosatellites of cubesat-like proportions (but larger than chipsats envisioned in the previous article), equipped with some kind of very thin film solar cells, ion drives, laser comms and the scientific equipment which is reasonable to fit into 1 kg-class probe with the power restricted to several watts even with the cost of decreased performance – “better something than completely nothing”. Some kind of imagers, possibly with spectral capabilities, magnetometers of course, maybe even radars based on extendable antennas to study gas giant atmospheres and precise doppler lidars to study “seismology” by measuring vertical surface (big atmospheric masses) movements to the precision of a centimeter per second and acoustic waves (in the case if the high-pressure ices mantle is solid enough to experience neptune-quakes and the waves could penetrate into the upper atmosphere) Maybe it’s possible to extract much info about the internal mass distribution and properties of the material in the interior even with compact equipment.
    (and in particular, find if there is a H2/He(gas)-H2O-(liquid) boundary somewhere not so deep under the Neptune clouds or the ocean is hyper-pressurised and overheated to the supercritical state…)

    And the mission would be at least partially private and very likely fly considerably before the start of proposed flagman and serve as a very low-cost reconnaissance precursor mission to the main M-(L-)class craft, which becomes more and more feasible as the age of private space and miniaturized spacecraft advances…

  • Michael June 19, 2014, 15:35

    If we are to go to Titan I would like to see a glider operating.




    Aerogel balloon/glider hybrid (fully deployed), perhaps we could have them in the atmosphere of Uranus and Neptune or the other gas giants, it would save on design costs.

    @torque_xtr June 19, 2014 at 13:16

    ‘My guess is that the next craft to visit ice giants would be nanosatellites of cubesat-like proportions (but larger than chipsats envisioned in the previous article), equipped with some kind of very thin film solar cells, ion drives, laser comms and the scientific equipment which is reasonable to fit into 1 kg-class probe with the power restricted to several watts even with the cost of decreased performance – “better something than completely nothing”.

    …miniaturized spacecraft advances…’

    The problem with smaller is two fold, the first is radiation and the other is heavily affected by the first, the smaller we go electronic wise the more influence radiation has on electronics. There will need to be a trade off between radiation impact probability, its effects and the size of an electronic device. Dumb devices such as the optical lenses and power supplies could be used to shield the more sensitive electronics though, so careful design would be needed.

  • Andrew Palfreyman June 19, 2014, 22:06

    One of the best places to replenish our He in quantity, too

  • Ashley Baldwin June 21, 2014, 14:26

    In relation to ODINUS, please see my comments under ” return to the ice giants “.
    Nothing epitomises more the essence of how future planetary science will be carried out , not just in the solar system , but for extra solar planets too than spectrography.
    Exciting as pictures from Voyager , Galileo and Cassini are they are somewhat misleading. They are presented in both false colour and visible spectrum so as to appeal to a wider audience when in fact the most useful work is done in infra red , a longer wavelength that reveals more detail than visible as it is able to penetrate the dust and ice that so often conceals planetary targets . More and more telescopes see in the infra red and ground based telescopes are high up on mountains as the atmosphere ( water vapour in particular ) absorbs most infrared light . Indeed this is the main reason for space scopes. Even in good viewing areas like Msuna Kea and Chile , absorption is high .( the lowest ironically is in Antarctica) . This is one of the main reason for space based telescopes along with the general disturbance to viewing at all wavelengths presented by the atmosphere. The JWST sees best in infrared, not visible and as such is not the natural successor to Hubble , quite right, it will see much much, more even if it won’t show some of the sexy solar planet and galaxy pictures of Hubble .
    If there is to be a mantra to future astronomical observation it will be spectroscopy, spectroscopy, spectroscopy. Spatial resolution of images is almost impossible for sell but solar system targets so any information has to come from “point sources” and the only way they will reveal characterics ( but fortunately a lot) is via spectroscopy of the light they emit , which as we’ve seen basically means infra red . The principal instrument on the JWST is the NIRSPEC , Near infra red spectrograph, which will amongst others , target exo planets . The key then is photons. How much light the telescope can feed to the spectrograph for it to analyse. The more the better, improving the quality of spectroscopic resolution.( ability to spectate different spectral lines) . As with visual resolution even this depends on the light gathering ability of the telescope , in other words its aperture. And the time the telescope has to look at a chosen target . JWST time is at a premium with only about 15 % available for exoplanet observation. Dim , distant exoplanets take days to EMI wet enough light into a telescope for even a high quality spectroscope like
    NIRSPEC to analyse. Infrared light used because the difference between star and planet light release is least at this wavelength, a million times less than for visible but still a difference of over a billion times. Large as it is at 6.5 m , JWST will only get adequate light to analyse the characteristics ( basically the atmosphere) of only the largest , nearest exo planets generally orbiting dim red dwarf stars . Put exoplanet atmosphere spectrography into google and you will find numerous articles publishing the viewing times required for soectrographic characterisation. Sara Seager from MIT in particular has published extensively on this and lobbied hard for the ESA M2 to go for EChO ( an Exoplsnet spectrographic CHaracterisation Obsevatory ) ahead of planet finding uber Kepler PLATO. With Kepler , Kepler 2 , TESS , WFIRST and ground based telescopes we will soon have thousands of exoplanets but without knowing many of their characteristics , how can we assess them? Visual spatial resolution needs huge telescopes , or combinations of telescopes in interferometers spread over hundred Kilometer baselines and are decades ( atleast ) away , so we are left only with admirable spectrography . Even TESS has been cleverly designed to look for its “best” planets ( in terms of being in a habitable zone as far as possible from the parent M dwarf with a forty day plus orbital period as opposed to the TESS average of ten days) in the areas around the poles were the viewing field will overlap with that of the JWST and its all important NIRSPEC . The time spent on this depending on whether the initial 2 year mission is extended and the satellite functions that long.
    So the future of exoplanet exploration lies in infra red spectrography , space telescope supplemented by the new generation of giant ground scopes and their huge spectrographs ( increasingly in .Antarctica ) , with the largest telescopes possible or an interferometer that combines the input of several telescopes. These need to be , as with EChO, dedicated to exoplanets given the time taken to point at a target and get sufficient light In a way the terrestrial planet finder was mis named. It should have been called “Terrestrial Planet Characteriser “.

  • ljk June 24, 2014, 11:14

    What If Voyager Had Explored Pluto?

    23 Jun 2014

    The PI’s Perspective – Alan Stern

    As I mentioned in my previous PI Perspective, New Horizons crosses the orbit of Neptune, the outermost planet explored by the Voyager mission, late this August. Voyager’s flyby of Neptune was in August 1989, 25 years ago!

    Across flights launched in 1977 and spanning the entirety of the 1980s, Voyagers 1 and 2 performed the historic, first detailed reconnaissance of our solar system’s four giant planets (Jupiter, Saturn, Neptune, Uranus). The essentially identical Voyagers were launched with a core mission to explore the Jupiter and Saturn systems, and each spacecraft carried a powerful and diverse scientific instrument suite. After Saturn, Voyager 2 was tasked with reconnoitering Uranus and Neptune during an extended mission.

    Although Pluto’s orbital position relative to Neptune made it impossible for Voyager 2 to travel to it from Neptune, Voyager 1 actually could have reached Pluto after its Saturn flyby, had it been targeted to do so. In fact, NASA and the Voyager project actually considered this option, but eliminated it in 1980 – going instead with the very exiting but lower-risk opportunity to investigate Saturn’s large, scientifically enticing, cloud-enshrouded and liquid-bearing moon Titan.

    But if Voyager 1 had been sent to Pluto, it would have arrived in the spring of 1986, just after Voyager 2’s exploration of Uranus that January. As New Horizons approaches Pluto in 2015, it’s fun to think what we might have found almost 30 years ago had Voyager 1 – rather than New Horizons – been first to Pluto.

    Full article here:


    To quote:

    One could debate this kind of detailed comparison at length, but my overall conclusion is this: Had Voyager 1 been sent to Pluto in 1986, rather than New Horizons arriving in 2015, it would have made many spectacular discoveries, but with less data depth and diversity than New Horizons is likely to achieve. And, not knowing of the Kuiper Belt in 1986, Voyager would then have unknowingly gone on (as it actually did), sailing across the belt without even attempting flybys of primordial Kuiper Belt Objects, as New Horizons will.

    That said, consider how amazing it would have been in 1986 to rapidly discover, in the few weeks prior to a Voyager closest approach, that Pluto has an atmosphere; that Pluto has its own retinue of small moons, much like a giant planet; and that the much more volatile snows of nitrogen and carbon monoxide dominate its surface composition-rather than the methane snow as was then known.

    But make no mistake: Both spacecraft are extraordinary. And both have their own places in the history of humankind’s first exploration of the planets. Both projects are staffed by outstanding teams of engineers and scientists; and both projects are testaments to the leadership of the United States in exploring our solar system for all humankind.

  • ljk June 24, 2014, 11:37

    Not sending Voyager 1 to Pluto was a mistake, irregardless of how much more advanced New Horizons is.

    For one thing, as said in the article above, NH will not be able to see the winter hemisphere of Pluto due to where it is in its solar orbit in 2015, whereas Voyager 1 would have been able to map Pluto’s entire surface. How long will we have to wait for a Pluto orbiter and/or lander mission?

    We may have known less about the Pluto system in 1986 than we do now, but nothing beats having a space probe actually there making those discoveries. NH would be even better prepared for its flyby next year. Plus we would have an irreplaceable record of what Pluto and its moons (and rings?) were like in 1986, something no present or future mission can ever duplicate. We already know Pluto is not a stagnant place; it may even have active geysers like Triton and underground liquid water systems.

    Imagine where our knowledge of Uranus and Neptune would be if Voyager 2 had not visited those planets and their moons and rings in the 1980s, even with their “primitive” instruments. Even HST has its limitations when it comes to seeing distant bodies in the Sol system.

    In summation, we could have had a record of what Pluto was like 28 years ago. Not only would we have learned of all those moons that world has in addition to the ones already discovered since that time, we would have learned new things still unknown to us because we would have had a probe there.

    The whole “Is Pluto a Planet?” debate and the energy and time spent on this issue might have been avoided based on what Voyager 1 returned to us. We might also have redirected and/or create new missions based on the information from Voyager 1 at Pluto to that world and others.

    Yes, New Horizons may be fancier and technologically better, but it cannot travel back in time. The items NH also carries onboard for future discoverers, human or otherwise, are lacking in comparison to the amazing Voyager Interstellar Records, though the One Earth Message to be beamed to the probe after its main mission in 2015 can make up for them.


  • ljk June 24, 2014, 11:42

    Titan’s Building Blocks Might Pre-date Saturn

    23 Jun 2014

    (Source: Jet Propulsion Laboratory)

    A combined NASA and European Space Agency (ESA)-funded study has found firm evidence that nitrogen in the atmosphere of Saturn’s moon Titan originated in conditions similar to the cold birthplace of the most ancient comets from the Oort cloud. The finding rules out the possibility that Titan’s building blocks formed within the warm disk of material thought to have surrounded the infant planet Saturn during its formation.

    The main implication of this new research is that Titan’s building blocks formed early in the solar system’s history, in the cold disk of gas and dust that formed the sun. This was also the birthplace of many comets, which retain a primitive, or largely unchanged, composition today.

    The research, led by Kathleen Mandt of Southwest Research Institute in San Antonio, and including an international team of researchers, was published this week in the Astrophysical Journal Letters.

    Nitrogen is the main ingredient in the atmosphere of Earth, as well as on Titan. The planet-sized moon of Saturn is frequently compared to an early version of Earth, locked in a deep freeze.

    The paper suggests that information about Titan’s original building blocks is still present in the icy moon’s atmosphere, allowing researchers to test different ideas about how the moon might have formed. Mandt and colleagues demonstrate that a particular chemical hint as to the origin of Titan’s nitrogen should be essentially the same today as when this moon formed, up to 4.6 billion years ago. That hint is the ratio of one isotope, or form, of nitrogen, called nitrogen-14, to another isotope, called nitrogen-15.

    The team finds that our solar system is not old enough for this nitrogen isotope ratio to have changed significantly. This is contrary to what scientists commonly have assumed.

    Full article here:


  • ljk August 6, 2014, 10:02

    Juno Launched 3 Years Ago on Aug. 5, 2011 on 5 Year Journey to Discover Jupiter’s Genesis

    By Ken Kremer

    It’s 3 down and 2 to go for NASA’s Jupiter-bound Juno probe which marks a major milestone today, Aug. 5, celebrating its launch exactly three years ago today on Aug. 5, 2011 from Cape Canaveral Air Force Station, FL on a journey to discover the genesis of Jupiter hidden deep inside the planet’s interior.

    The solar powered probe has now traveled 80% of the distance on its five-year and 2.8 Billion kilometer (1.7 Billion mile) outbound trek to the Jovian system and the largest planet in our solar system.

    When it finally arrives at Jupiter on America’s Independence Day, July 4, 2016, Juno will become the first polar orbiting spacecraft at the gas giant.

    The probe and its trio of huge solar panels are cart wheeling through interplanetary space on the long voyage to Jupiter.

    Upon arrival at Jupiter, in July 2016, JUNO will fire its braking rockets and go into polar orbit and circle the planet 33 times over about one year. The goal is to find out more about the planets origins, interior structure and atmosphere, observe the aurora, map the intense magnetic field and investigate the existence of a solid planetary core.

    “Jupiter is the Rosetta Stone of our solar system,” said Scott Bolton, Juno’s principal investigator from the Southwest Research Institute in San Antonio. “It is by far the oldest planet, contains more material than all the other planets, asteroids and comets combined and carries deep inside it the story of not only the solar system but of us. Juno is going there as our emissary — to interpret what Jupiter has to say.”

    Full article here: