The science of The Europa Report


I just watched The Europa Report, a 2013 movie about a crewed space mission to look for life on Europa, the smallest of Jupiter’s Galilean satellites (the four moons of Jupiter found by Galileo in 1610).  The movie was recommended by a friend, who was wondering if the science in the movie was accurate.  This is my attempt to answer her question.  To cut to the chase, I enjoyed the movie a lot, although there were a few problems with the science.  They weren’t big ones, however.  Spoilers follow, so be warned.

The movie is set five minutes into the future, after remote probing of Europa has revealed the possibility of under-surface life on Europa.  It’s been known since the Voyager probes that the moon’s surface is principally water ice at a temperature ranging from 50 Kelvin (-223 C) to about 125 Kelvin (-150 C), at least according to Wikipedia.  This is wayyyy too low for life as we know it, but cracks running through the surface indicate that there might be liquid water underneath the crust, raising the possibility that life might exist there.  The presence of water became more probable when water vapor (probably caused by plumes of water breaking through the crust) were detected in December, 2013.

First, a bit of science:  how can such an icy place have warmth enough for water underneath?  In one word: tides.  Really, tidal friction.  As Europa orbits Jupiter, the massive gravitational attraction which Jupiter exerts on the planet stretches it in the direction of Jupiter, and squeezes it along the other two directions.  This stretching and squeezing leads to heating of the moon as it orbits the planet.  Europa is an icy ball – it’s covered by an “ocean’’ of ice which is probably several hundreds of kilometers thick, and a water ocean underneath that, with a relatively small rocky core.  Therefore, it seems that one of the conditions for life (liquid water) exists on the planet.

This has been a staple of science fiction stories since its discovery in the late 1970’s.  The first novel I know of which featured Europan life was 2010: Odyssey 2 by Arthur C. Clarke, the sequel to the 1966 novel and movie 2001: A Space Odyssey.  In the novel, the mysterious aliens responsible for the Monolith turn Jupiter into a star to allow the Europans to develop beyond their primitive life forms and become a technologically advanced species.  The Europa Report owes a great deal to 2001 and its sequel: it has a very similar look and feel to 2001, and of course it is based on a mission to find life on Europa.  In a conscious homage to the older movie, NASA plays Johann Strauss, Jr.’s “Blue Danube” waltz as the mission is starting, in reference to the space station docking scene from 2001.

The movie is very different from 2001, however, although the sound track is nearly as good (very high praise from me…)  It is a “found footage” movie (although in this case, “sent footage” is more accurate.)  It concerned a crewed mission to Europa, sent there because the  plumes of water and other evidence indicate the possibility of life.  The movie includes Neal deGrasse Tyson discussing how he wants to go ice fishing on Europa (I think it’s real footage), and some pretty accurate discussion of the motivation and science of the expedition.

The first point to make is that the movie science is very good, by Hollywood standards.  It’s one of the most accutate I’ve seen in a while.  I’m going to nitpick about the science, but my readers should keep this overall point in mind.  Let’s start with the space travel:

First off, it takes the spaceship 22 months to reach Jupiter and its moon system.  Why 22 months?  Probably because that’s how long it took the Voyager spacecraft to reach Jupiter from Earth.  I don’t know this for a fact, but my suspicion is that the writers and director just used values from a handy mission which already went there instead of trying to do the rather difficult job of estimating the mission time.

It’s not a crazy estimate.  Jupiter has an average distance of 5.2 AU from the sun (5.2 times the average distance of Earth from the sun).  The spacecraft won’t travel on a straight line from Earth, so it’s hard to say exactly how far the spacecraft will have to travel.  However, if we assume something like 5 AU total distance, the average speed is about 14 kilometers per second (very roughly 30,000 mph).  This isn’t crazy, but there will be difficulties with doing this.

The ship flies by the moon and Mars while en route, so it’s reasonable to assume that the ship was using a gravitational assist speed boost from both bodies.  Again, not a crazy idea, but only very detailed calculations would show if that’s feasible or not.  Unfortunately, the movie never mentioned if that was part of the plan.  One issue which I had with the movie was tha the cast seemed remarkably blasé when passing Mars.  Also, if you’re going to pass it, why not drop off a probe while you’re doing so, or at least spend some mission time getting data?  The crew doesn’t seem to have too much else to do.

One concern of a crewed mission is that of coming home.  For that, you will need fuel to return home at the end of the mission.  To estimate the amount of fuel you need, you have to calculate what are called “Delta v” costs for the rocket – more or less the changes in rocket speed needed to go to Jupiter, maneuver in the system, and return home.  The rocket equation is unforgiving: for chemical fuels, every “delta V” change of about 4 to 5 kilometers per second requires a multiplicative factor of about three in rocket fuel mass to payload mass. (The exact value depends a bit on the type of rocket fuel). Getting the ship away from the gravitational attraction of the Earth and into an orbit headed for the outer solar system is far and away the most difficult problem – it requires a “delta v” change of some 13 kilometers per second, including the effects of air drag.  We can probably assume another six or seven for getting to Jupiter, maneuvering and return, even with any hypothetical gravitational assists from the moon and Mars.  An overall “delta v” budget of about 20 kilometers per second seems right.  20 is 5×4, meanind that the ratio of fuel to payload will be between about 80 and 240 times the payload mass. To put this into perspective, the Voyager spacecraft was launched from a Saturn III rocket; the total mass of the Saturn was about 60,000 kg, while the mass of the Voyager payload is 770 kg (information taken from Wikipedia).  Not all of the Saturn III rocket was fuel, of course; the fuel to payload ratio was probably no better than about 50.  (I can’t find the specific information on this, however, and the fact that the Saturn is a multistage rocket complicates things.)  A Saturn III rocket wouldn’t be big enough– at least, so long as the rocket for the Europa mission is using chemical fuels, which it seemed to be.  The footage used to show the liftoff was clearly of a smaller rocket carrying an uncrewed satellite payload.

It gets worse because a crewed mission is by neccessity much higher mass than an uncrewed one.  There were six crew.  The Apollo 11 Command and Service Module had a total mass of 28,000 kilograms to house half the number of crew for a few days rather than several years (including the return voyage.)  You’d probably need a much larger craft for this mission.  This in a nutshell is why crewed spaceflight is tough.

On to some other issues:  as the ship moves farther from Earth, it will take longer and longer for messages to go from Earth to the ship and back again because of the finite speed of light.  It takes 8.3 minutes for light to go a distance of 1 astronomical unit.  Even as close as the Moon, there should have been a noticeable 1.7 second delay between a crewmember speaking to NASA and the response.  As they pass Mars (closest approach about 0.5 AU), the delay in communication should have been at least 8 minutes – 4 to get to Earth, and 4 to return.  I never saw this mentioned, and it never seemed to take the crew any time to talk to mission control.

(Spoiler alert) Another issue is that the crew never returns – all are lost on the voyage out or due to a series of disasters on Europa.  Some of these disasters seem avoidable, especially (part of) the first tragedy, in which a crewmember is lost en route.  I will only say this about it:  under current NASA regulations, astronauts never attempt a space walk unless there is some means to get back to the spacecraft: either a line or an all-axis maneuverable harness.  This was the kind of problem which could have been avoided with 20 cents of rope from the hardware store…

More nitpicks: on Europa, someone says that the temperature outside is absolute zero.  It’s not – the average Europan temperature is about 50 above absolute zero, warm enough for hydogren to be liquid.  Scientists are pretty picky about this; I can’t imagine any trained scientist making this goof.  Also, you can clearly hear the drill cutting into the ice from outside the ship, despite the surface being in vacuum.  This is something that really bugs me  – sound waves are vibrations in matter.No air, no sound.  If the sound was traveling through the ice and into the ship through the hull, it should have been very faint and distorted.

And now to my final two points:  first off, all of the tragedies were avoidable with the simple expedient of sending a robotic mission to Europa to explore the moon.  Yes, it’s not as glamorous or dramatic (Jerry Pournelle has rather famously said that nobody ever gives a robot a ticker-tape parade), but it could be done at a tiny fraction of the cost of a crewed mission, and no danger to anyone.  The narrator of this  “documentary” defined their end as successful in the face of tragedy, but I’m not inclined to agree no matter how great the discovery they made before dying.  It could have just as easily been made by robot.  Of course, you probably couldn’t make a movie about the dramatic robot mission to Europa to discover life there.

Secondly, it’s a sad commentary on science fiction in the media that movies like this, where there is at least some attempt to get the science right, come around only about once every decade.  I think it is possible to have drama and accurate science at the same time, as this movie and movies like Avatar and 2001 show.  I don’t dislike things like Star Wars or superhero movies with their flagrant disregard for the laws of physics – if nothing else, it’s grist for the critics mill.  But it’s too bad that the right stuff comes along so little.  Oh, well – Sturgeon’s law, I guess.

For the budding science fiction writer: More local color


The third in our sequence of posts for those who are starting to write science fiction.

So now you’ve picked your world and the star it circles.  You’ve figured out how far away it should be.  What should the world look like?  How can you make it distinct?

There are limitless ways in which you can pursue this.  Exotic fauna is one way: make the animals different from those on Earth.  We’ll explore this in an upcoming post.  I’m going to concentrate on the flora in this one: what should vegetation look like on your planet, assuming it has vegetation at all?

Flowers come in all shapes and sizes and colors.  The local quality of light may make some differences, but you are pretty much free to choose what you want in this regard.  However, leaf color is probably going to be determined by the star you’ve picked.

Stellar spectra

Light curves from three types of stars

The figure shows a schematic diagram, representative only,  of the spectrum of different classes of stars.  This shows the relative amounts of power which the star emits as a function of the wavelength of light.  Human eyes are insensitive to most light: our eyes are most sensitive in the wavelength region in which our sun emits the most light.

On Earth, leaves are green because Earthly chlorophyl absorbs blue (shorter wavelength) and red (longer wavelength) light predominantly.  Why this is so is a little bit tricky.  The chemical pathways by which plants absorb light and convert it into energy are complicated, and involve no less than about 8 photons per reaction.  Red light has low energy photons but can be used directly for photosynthesis.  Blue light has high-energy photons, but can be “down-converted” to lower energy to make them useful for photosynthesis.  Because of this, photons of wavelengths which appear blue and red to our eyes are used for photosynthesis on Earth and are absorbed, while middle wavelength photons, i.e., green light, is reflected,  leading to leaves being green.

Vegetation under different stars is probably going to look different.  Dr. Nancy Kiang and her colleagues at NASA have investigated the most likely chemical pathways for energy uptake for vegetation on planets circling other stars.  Note that the M-class star emits less light in toto than the sun does.  Dr. Kiang and her group theorized that plants on worlds circling M-class stars would need every available photon; if they absorb all light, they would appear black.  On the other hand, planets circling F-class or brighter stars would have too much light, especially in the short wavelength spectral region.  They might have to reflect the blue light, leading to blue-looking leaves.

These are just a few of the details which the science fiction writer needs to keep in mind.  In the next post, we’ll discuss how the distance of the planet determines the length of the year.

How long can you tread water?


The new blockbuster disaster movie Noah looks like it might be harmless, if simple-minded, fun.  I want to see it, if for no other reasons than watching Russell Crowe and Anthony Hopkins ham it up, and to see if Emma Watson, a one-time student at my alma mater, can make it big past Hogwarts.  I had to explain to my kids why I kept on yelling “Voopa-voopa-voopa… DING!” and “You want a hint? How long… can you tread… water? Hah hah hah hah hah,” every time I saw the trailer.  (My wife has ordered them to poke me if I try to do this when we see it.)

Why bring this up here? There’s a teaching point which can be applied to any disaster movie:  How reasonable is it from the standpoint of basic science?  The science fiction writer Isaac Asimov already observed that the rainfall rate would been enough to swamp an aircraft carrier, let alone a smallish ship (by today’s standards).  However, it’s fun to think about.

I want to talk about an aspect of such a disaster which Asimov didn’t cover:  where’d the water come from?  And how much energy is involved in putting it where it needs to go?  We can make some simple estimates using energy methods.

Maybe the water came from the oceans.  According to NOAA (no relation) the average depth of the ocean is about 14,000 feet (roughly 4,000 meters.)  Since there is three times as much ocean area as land, we can imagine (somehow) emptying the oceans to cover the land to a depth of three times as much, or some 50,000 feet – more than enough to do the job.  It would take a heck of a lot of energy to do this, however.  This is a very large volume of water: under some reasonable assumptions it’s something like a million trillion cubic meters.  The mass of that much water is a billion trillion kilograms, or a million trillion tons.

Let’s say we evaporate the oceans so that it rises into the sky and falls as rain onto the land.  This doesn’t happen under ordinary circumstances – the Earth  isn’t hot enough to make this happen.    The energy needed to evaporate all of the oceans is about the same as the amount of energy which would be used by our world civilization in 9 million years.  Another way to put this: it rained for forty days and forty nights.  Therefore, the total rate at which this energy would need to be put into Earth’s climate system is some one billion trillion watts – about seven thousand times the total power which the Earth gets from the sun!  A power input that large would destroy all life on Earth by boiling everything alive.

If the water came from elsewhere, say a rain of comets, which are largely water, the problem gets worse.  The comet impact speed would be about the same as the speed of Earth in its orbit around the sun (30 kilometers per second, or 67,000 mph.)  It’s a hefty speed.  The speed is so high because the comets would be traveling around the sun in about the same way in which the Earth does. It could be somewhat higher or lower depending on exactly how the collision happens.  The total impact energy is given by (1/2) x (the mass) x (impact speed ) x (impact speed), or about seven hundred thousand trillion trillion joules. (Yes, I wrote that right – trillion trillion.) The comet or asteroid which struck Earth 65 million years ago killing the dinosaurs had an impact energy about one million times less than this.  With a series of impacts like this, the flooding would be the least of Noah’s problems.  He’d have to worry about mile-high waves each time one of the comets struck, the fact that Earth’s atmosphere would be more water than air after the impacts, the complete and utter darkness shrouding the land due to kicked up dust…  And how do we get rid of the water afterwards?  You need about the same amount of energy to push it back up into space.

Amazingly enough, real disasters like this have befallen the Earth, luckily before any life existed on it.  The best theory scientists have on how the Moon formed was due to an impact over four billion years ago by an object about the size of Mars.  The impactor had a mass a few hundred times that of all of the oceans on Earth, but the impact would have “only” been ten times more energetic that our hypothetical cometary scenario because the collision was a “slow” one (about 4,000 meters per second, or 8,900 mph).  Luckily, there are no more objects that size in the solar system which could collide with Earth.

Two points: 1) Always start with the energy involved if you’re trying to decide if movie “science” is reasonable;  2)  I’m still looking forward to the movie.  Expect a review if it violates still more basic science.

Thoughts on Project Orion


I did an interview a few days ago with Michio Kaku on his radio show “Science Fantastic”. It’ll be broadcast sometime this weekend.   It was a strange conversation, at least from my point of view.  I’d never given a long radio interview before and the whole thing seemed rushed and stressful.  Kaku wanted to emphasize the weirder and more outré aspectes of science fiction; fitting, perhaps, from the program title, but most good science fiction rests on a firm basis of known science.

One good thing did come from the program.  Kaku and I were discussing starships.  I, perhaps unwisely, told him that my personal favorite idea for a starship was Project Orion, the truly audacious suggestion that we could propel spacecraft using nuclear bombs as propellants.   Dr. Kaku raised a very good objection to this, which is what I want to write about here. (As an aside, there is a new NASA program to develop a crewed rocket propelled by conventional engines called “Orion” – don’t get the two confused!)

Before I go on with this topic, let’s discuss nuclear rockets in general:  spaceflight enthusiasts have always wanted rocket fuels which have higher energy density than chemical propellants.  This is because it takes a lot of energy to get a rocket moving at the high speeds it needs (25,000 mph or more) to reach destinations like the moon or farther.  Chemical propellants like combinations of liquid hydrogen and oxygen work, but just barely. From the 1950′s through the 1970′s nuclear energy was seen as a viable fuel alternative – the Nobel laureate physicist Richard Feynman may have been the first person to realize this, but a number of science fiction writers including Robert Heinlein also came to this conclusion.

The problem is that harnessing the large amounts of energy, millions of times higher per kilogram than chemical energy, is difficult.  The simplest way to do it is to take a gas (usually hydrogen) and heat it to very high temperatures and pressures using the reactor.  The gas escapes through a nozzle and the spaceship is pushed forward.  However, materials science limits this.  If the temperature gets too high, you start melting the nuclear fuel itself.  This limits the fuel ejection speed to about twice what chemical propellants can do.

Project Orion wanted to get around this by building a big spaceship with a “pusher plate” separating the crew from the engines.  Nuclear bombs would be ejected behind the ship and blown up.  The resulting explosion propelled the ship forward.  It seems daft, but a team led by two very good scientists, Ted Taylor and Freeman Dyson, worked out many of the practical details.  The effective fuel ejection speeds which you could get were five or six times, maybe even more, than you could get from conventional chemical fuels.  It also seems that you could keep the crew safe from radiation, and keep the explosions from destroying the ship.

You can’t build a spacecraft based on this principle today.  Trying to make it work would probably violate the 1963 treaty banning above-ground nuclear testing.  Dr. Kaku made the very good point that Ted Taylor himself stopped working on bomb design because the small hydrogen bombs which the ship would use would make very good terrorist weapons.  This caused me to stop and think things over.

I’m of two minds about Orion.  On the one hand, I am very worried about the prospects of nuclear terrorism.  I grew up in the DC suburbs, and was terrified as a teenager by the prospect of nuclear war with the Soviet Union.  Today isn’t much less scary, as smaller nations have gotten the bomb, and mounting pressures due to the world economy and the environment seem to make war and terrorist acts more and more likely.  Developing small nuclear weapons seems foolish in this light.

On the other hand, the Orion idea is the only feasible method using current technology to move a payload at very high speeds – perhaps at speeds of up to a few percent of the speed of light.  People have discussed other methods of powering starships, for example fusion reactors and matter-antimatter annihilation. (Yes, people have studied this seriously – it’s not just from Star Trek!)  However, neither could be done now: antimatter has an equivalent cost of  several trillion dollars a pound, and can only be produced in microscopic quantities today.  No one knows how to make a fusion reactor on Earth, let alone put one in a spacecraft, although recent developments may be promising.

If humanity wanted to send a space probe to the Alpha Centauri system, four light years away, the only conceivable means to do it using today’s technology would be something like Orion.  It wouldn’t be easy or cheap, and it wouldn’t happen any time soon.  It might not be possible, but this method would be the one with the best chance of success.  It would also take decades for the spacecraft to reach the system; any government funding a project like this would need to be farsighted in a way which most aren’t today.  Whether it would be worth doing is something which our society, or perhaps our society fifty years from now, would have to decide for itself.

The issue here is that any propulsion system for any spacecraft is a weapon in disguise.  Conventional rockets are made from highly explosive materials, as we saw in the Challenger disaster.  The higher the energy density, the worse it gets: in the extreme case, if you somehow could make a rocket propelled by antimatter, simply turning the engine on too close to Earth could sterilize it!   You can’t deweaponize a starship.

The best science fiction investigates the problems created or issues raised by new discoveries or new technologies.  I’ve never read a story or seen a movie or TV show which really looks at the societal problems which would be caused by the development of the Orion concept into a truly workable spacecraft.  Perhaps it’s time to write one.

PS  If you like what I write, or even don’t, please leave a comment!