Monday, August 25, 2014

Europa: How Less Can Be More

Three factors make exploring Europa hard.  First, we want to explore an entire complex world, and mapping its features requires acquiring vast amounts of data.  Second, Europa lies far from the Earth, which necessitates capable communications and power systems (read, “expensive”) to return the data to Earth.  Third, Europa lies well within the harsh radiation fields surrounding Europa, which both requires significant radiation hardening (again, read, “expensive”) and limits the life of any spacecraft that explores this world.  These factors can make a mission concept that seems like less actually be more.

The limiting factor on science for most planetary orbiters is not the time the instruments can make observations.  Rather it is the time available to return data to Earth because many instruments can gather data far faster than the communications system can transmit it to antennas on Earth.  (There also are a limited number of antennas to listen to planetary spacecraft, so few missions receive continuous coverage, and spacecraft often cannot continuously transmit either because they must turn to observe the planet or the planet itself blocks communication.) 

To get a sense of the challenges, compare the problems of exploring Mercury and Europa.  A mission to Mercury must deal with the intense heat coming from both the sun and the planet surface.  However, a spacecraft designed to overcome that challenge can continue to function until its fuel is exhausted.  As a result of the luxury of spending years in orbit around Mercury and the fact that Earth is never more than 222 million miles away, NASA’s MESSENGER mission has been able to return terabytes of data to Earth.  Between its orbital insertion in March 2011 and March 2012, the spacecraft generated 2.3 TB of data to be archived by NASA.  The mission continues to operate today, so the total returned to date should be substantially more.  The maximum data rate for this $446M (2008 dollars) mission is 104 kilobits per second.

By comparison, it’s cold at Jupiter, but it is the intense radiation around Europa that limits spacecraft life.  Different mission studies have assumed lifetimes in orbit between one ($1.6B estimated cost, 2015 dollars) and nine months ($4.7B estimated cost), many times shorter than the approximately four Earth years MESSENGER will have at Mercury.  While Jupiter is never closer than approximately 2.7 times as far from Earth as Mercury, more capable spacecraft systems would allow data rates of around 135 kilobits per second.  With a lifetime of one month in orbit, the data return would be around 334 gigabits, and with a lifetime of nine months, around 4.5 TB.  (Different mission designs made different assumptions about data return, so nine month mission data return isn’t a simple multiple of the one month mission data return.)

Comparison of data return for different Europa mission concepts with the data archived from NASA's Mercury MESSENGER orbiter from its first terrestrial year in orbit.  Credit: Kane

These challenges for exploring Europa have been well known since the Galileo Jupiter orbiter in the 1990s all but proved that Europa likely has a vast ocean that could harbor life that lies under a relatively thin icy shell.  As mission planners and budget directors have wrestled with this problem, we’ve been through at least five distinct eras of Europa mission planning.  (There have also been various proposals by independent teams for simpler and cheaper missions, which may or may not have been feasible for their proposed costs.)

Immediately following the Galileo spacecraft’s discoveries, JPL conducted preliminary mission studies that envisioned a capable spacecraft using conventional technology to orbit Europa.

In the late 1990’s, NASA’s then Administrator redirected efforts to a mission concept that would use yet-to-be-developed technologies (the X-2000 project) to dramatically lower mission costs to the neighborhood of MESSENGER’s cost.  By the time the program was cancelled in 2002, mission estimates had shot from around $190M to $1.4B (early 2000’s dollars). 

Not to be outdone, the next NASA administrator proposed the Battlestar Galactica of missions, the Jupiter Icy Moons Orbiter (JIMO) that would orbit the moons Callisto and Ganymede in addition to Europa.  This mission depended on the development of radically new capabilities such as space-rated nuclear fission reactors to power the spacecraft.  This $16B concept died quietly when the administrator left NASA.

NASA's concept for the Jupiter Icy Moons Orbiter.  Credit: JPL/NASA.

If the previous two efforts were perhaps fanciful, the next effort, the 2008 Jupiter Europa Orbiter (JEO) concept was based solidly on feasible technology.  This highly capable spacecraft would have conducted extensive studies of the Jovian system before beginning nine months in orbit around Europa with a highly capable instrument suite.  This was the mission any fan of Europa really wanted.  Unfortunately, an estimated $4.7B price tag doomed the concept.

The bulk of the data that would have been returned by the 2008 Jupiter Europa Orbiter concept would have been produced by the ice penetrating radar, infrared spectrometer, and a trio of cameras.  Credit: Kane

Following the JEO studies, NASA conducted studies of three missions that each would have a firm cap of $2B: an orbiter, a multi-flyby spacecraft, and a lander.  It was quickly realized that the latter would not be feasible until a previous mission had better studied Europa’s surface to find the best combinations of most scientifically while still safe landing sites.

That left the choice of an orbiter that would spend 30 days circling the moon and a multi-flyby spacecraft that would spend less than a cumulative 6 days close to Europa during 34 flybys.  The scientists who reviewed the two missions solidly backed the multi-flyby concept (that has evolved into the current Europa Clipper concept).

So how can 6 days of science be better than 30?  For the comparison that follows, I’ll use the assumptions of the 2012 studies.  Since that time, the capabilities of the multi-flyby concept have been substantially enhanced into the Europa Clipper concept.  Because the orbiter concept didn’t have the additional two years of fine-tuning of the multi-flyby craft, comparing their 2012 conceptions allows comparison of equally developed concepts.

Comparison of data that would be returned by instrument for the 2012 multi-flyby and orbiter mission concepts.  Credit: Kane

Between each of the flybys, the multi-flyby spacecraft would have seven to ten days to transmit data stored during each brief encounter back to Earth.  That would let the multi-flyby craft have up to a year of time to transmit its data compared to just 30 days for the orbiter.  The result would be almost three times as much data returned to Earth.  (Differing assumptions about how much of the time antennas would listen to the spacecraft mean that the amount of data returned is not a multiple of time.)

The larger data return of the multi-flyby spacecraft would enable the spacecraft to carry two high priority instruments that generate large amounts of data.  The more data hungry of these, the ice penetrating radar, would study the structure of the icy shell beneath the surface.  This would allow scientists to study whether bodies of water are trapped within the ice between the surface and the ocean below and fracturing of the shell.  The radar might penetrate through the shell to the top of the ocean to measure the total depth of the icy shell.  These measurements will help scientists understand how material is transported between the ocean and the surface.

The second instrument, a short-wave infrared spectrometer, can identify materials exposed on Europa’s surface and map their distribution.  Scientists believe that Europa’s surface exposes materials transported from the ocean below, where we can easily see it and eventually study it with a lander.  The interaction of materials on the surface with Jupiter’s radiation field creates chemicals that may be transported to the ocean below to be available for use by any life.  This spectrometer would map the presence and distribution of these materials across Europa’s globe.

Both the 2012 orbiter and the multi-flyby spacecraft would carry a third data-hungry instrument, a topographic imager that would map the surface.

 Not all potential instruments require high data rates.  The orbiter would have carried a trio of instruments that required measurements from around the globe: a laser altimeter to measure surface tides to enable estimates of the thickness of the icy shell and a magnetometer and plasma instruments that would have enabled estimates of the volume and salinity of the underlying ocean.  Unfortunately, the measurements of these instruments are lower priority than those for a radar and shortwave infrared spectrometer.  (In the 2012 study, the multi-flyby spacecraft also would have carried a heavy, power-intensive, but low data rate mass spectrometer that would directly sample material sputtered from the surface.)

The importance of the ice penetrating radar and mid-IR spectrometer tipped the weight of opinion in favor of the multi-flyby concept.  Given a limited number of encounters that would fly over just a tiny fraction of Europa’s surface, they key was to distribute those flybys to fly over key locations. 

With two years of further study, the multi-flyby concept has evolved into the Europa Clipper concept has added an additional eleven flybys (for a total of 45) and several instruments compared to the 2012 concept.  By balancing the placement and number of encounters with many months to return data, the Europa Clipper concept would enable a $2B mission that conducts the most crucial measurements of the $4.3B JEO concept.  The $1.6B orbiter concept couldn’t match this feat.

However, the Europa Clipper is not NASA’s plan for a Europa mission.  White House budget analysts and NASA’s senior management are looking for a $1B concept that wouldn’t do the job of the Europa Clipper but would still do significant science.  Earlier this summer, they reportedly received six proposals that target this cost cap.  NASA ’s managers are examining the proposals to ensure that they are both fiscally and technically feasible within the budget.  In the meantime, they are not releasing any information about the types of missions proposed.

From what I understand, much of the scientific community and many NASA managers are skeptical that a meaningful mission can be done within a $1B budget.  Sometime in the coming months we will learn whether NASA thinks any of the proposals have merit.  If they do, then the broader scientific community will weigh in with its assessment. 

I’ve argued in a previous post that a $1B mission is likely technically possible, but I have doubts about whether it could address enough high priority science to be worth the expenditure.  The coming months will see if I’m proved wrong or not.


In the meantime, NASA continues to refine the Europa Clipper concept, which so far has shown the best balance between doing more with less to perform the critical science for the next step in exploring this world.

Current concept for the Europa Clipper mission, which is an evolved version of the 2012 multi-flyby concept.  Credit: JPL


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8 comments:

  1. Saturn doesn't create high radiation fields. This means in addition to the ice radar, you can land acoustic (water, pressurized sublimated vacuum ice caves) sensors, and keep stationary gravity sensors, all also looking for water on Enceladus.

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  2. ...the Ice Moons are a potential source of ice for the cores of Sentinel asteroid diverters, if a silicon core is unworkable.

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  3. The Keystone GarterOctober 11, 2014 at 1:26 PM

    Thread hijack: I can't find a proper thread.
    ...many tachyon transponder seeds (600? 600 in transit?).
    Every time you send tachyons (backwards) they pollute the timeline. The hope is them and us are still here after time and space co-ordinates change with every msg making it hard to find your target.
    It took 100+ years to breakeven after their Nuclear Holocaust (back to IR). Their players are better than ours even at MLB rules. Their reflexes are superior becaure tech is integrated into their bodies. I'd guess after breaking even, they went toward mood sensors and possibly dark-mood altering devices. To prevents suicides for example.
    Only when in a sharp state of mind, so is an out for Religions if msg is true. I guess Al-Qaida-like entities can moderate to avoid attacks that harm humanity enough to threaten the whole Universe. If there is a war in the universe against AI, it is pretty cool to be on the right side and a lot our allies are addicted to advanced drugs rather than fighting AI. They don't know what happens at the beginning of the universe where the tachyons (all?) arrive. Thus, they focus msgs on only a few people (and whatever else I suppose).

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  4. The Keystone GarterOctober 20, 2014 at 9:19 AM

    So...the immediate conclusion appears to be Starshade and MWG observatories like it are important. Which is mostly materials science R + D and robotic deployment expertise...
    There was a woman who came up with a 4 part mantra to explore surrounding star systems. The last word was "Remember". Which is to avoid sending any risky cargo. If you can't enforce it, don't build it is a mantra for a lot of things...I suppose we are to prevent AI from occurring around our surroundings, minus the tachyon part.
    After their war, their survival was not assured. All you ever needed to know... is maybe a Presidential Endorsement but it also is how they survived the aftermath. I assume water pipes would've been intact. Which is why they are okay interfering: their history probably gave them an advantage for harnessing/avoiding dangerous technologies.
    They can't reveal art without the underlying motive (technology). Admit 100+ years after their war, they launched a spacecraft, so yes, they had something like Googie.

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  5. The Keystone GarterOctober 20, 2014 at 9:20 AM

    They like kindergarten as a time to test for mental illnesses. I've divied up travel infrastructure into tachyon, non-massive/non-detectble, and massive. Apparently, sending mass alarms the recipient.

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  6. I've figured out their most probable tachyon system and there are more future technologies to figure out. They gave me clues: sending a probe doesn't necessarily alarm and their planet is invisible. This means we can look for aliens at present space programme missions without necessarily declaring war. It also suggests they have sent a c/sub-c tachyon transmitter. They mentioned I don't reply in real-time but is tachyons so doesn't matter. Is delayed about 8 mins...I think there is a tachyon relay transmitter in the Sun and maybe a less powerful one in Earth. They said bombardment might be a bluff. Feynmann would be proud of my drawings.
    I'd assume they were ahead of us in time: that they'd developed tachyons 20B years after Big Bang. It could be there are ways to cut sideways but we are probably in the same light-cone. Thesis:
    They invent tachyon tech before us, maybe 12B yrs after BB. The first world line (I've always thought every physicist is wrong about there being an infinity of them; instead 2^2^8^8...numbers of worldlines. Their first tachyon world line were good guy aliens. They or any alien AIs in parrallel worldlines can't modify their own past light cone much or msg never sent. So they are modifying our "past"; I'd bet each human worldline or at least the critical points of inflection: Chaos Theory I'll book up. I think the voices are sent to us at c or below c speeds, and aren't an "All Good Things" existential threat to the universe. When I reply, it is backwards in time to them: The Bomber kicker might be right (he was never focused on FGs): There is an explosion of new world lines as a result. Maybe some are deleted?? If true, they sent a tachyon transmitter to us at or below c, and they are likely capable of wiping us (using a coarse method) out if we make AI or anything else.
    I'm happy hockey players and Heads of State don't think I'm crazy but the conservative politics doesn't help me deal with things. I went for depression medication and Dr. Hegdekar thought I was crazy for saying I give policy advice without pay. If I'm to live in a city that is the descendant of Scottish Enlightment ideals needed to manage these risks/rewards, I want anyone educated under inferior curricula to have a miserable life. I'm a utilitarian equal is a good chunk of why they chose me.

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  7. ...they wanted me to figure out it is likely not a bluff. They are helping to make me a good person (so far). In other world-lines I might not have been sucha good person. My select WMD surveillence system or some other future technology might have been used once I die, by someone else, to create AI.
    ...If we see a Milky Way world with life, and we launch a ship to stop them from developing AI, and they build telescopes and see us 1/2 way, it looks like we have just declared war. This is an issue for any propulsion system able to reach the stars (not VASIMR).

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  8. The reference 21 months ago to sea hawks not being sure whether to cheer for the Falcons was a triple entendre: Bird-of-Prey. Sun is a double entendre for teaching your son.
    Their world didn't have as much biodiversity as ours does. I'd like computer modellers to try to estimate what worlds would have the most biodiversity; what geologies, surfaces, atmospheres. The highest such scores would likely give rise to animal life.
    ...they destroyed an entire line of agri-animals in the Aftermath...I've classified the graphic details but is a warning to world leaders and military officials.
    I'd also have thought the first AI would be secret research, but apparently it a commercial app that when everywhere, my pre-Oct sensor network wouldn't be able to do anything about. Maybe the market forces threat is some iteration of my sensor network.
    ...waited until I learned some details of a sensor network before negotiating.
    If anyone has a better idea for Contact they'd like to know it (2nd request after don't build AI).
    Galaxies away answer to location fills me with wonder but makes me realize there may be malevolent actors nearer.

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