Wednesday, July 1, 2015

Review: Declaration by James Patrick Kelly

Spoiler alert: I give away events unfolding in Kelly's story. It appeared in the March 2014 edition of Asimov's Science Fiction Magazine.

"Declaration" is a thought provoking extrapolation of existing trends. In the style of Spinrad, I will use this review as an excuse to jump up on a soapbox and deliver my own opinions.

In this tale more and more people are dwelling in softtime. Softime is what today's internet might evolve into, a shared online virtual reality. Depending on sophistication of interface, the virtual reality can be fully immerseive.

A significant part of the population are severely disabled and can't interact with the world using their meat bodies. These disabled people are known as stash. They are more or less stashed in coffin like life support cubicles.

The government mandates that everyone spend an allotted time in hardtime, a.k.a. reality. Stash revolutionaries want to spend all of their existence in softtime. The story title Declaration refers to the revolutionaries' Declaration of Independence. They want to sever their connections with the real world.

But would the revolutionaries become independent of hardtime? In Kelly's story, that's not clear. Are the stashed people dependent parasites or do they provide services and do meaningful work? Robots are ubiquitous in the story but I get the impression machines haven't fully replaced humans. There still seems to be need for meat to interact with the world to maintain infrastructure and take care of business. For example the main character turns her stash brother to prevent bed sores, even though the brother has a carebot.

Primitive versions of these interfaces already exist. For example motion capture sensors control virtual puppets in movies like Shrek or Avatar, as well as virtual avatars in computer games.  Neuroscientist Miguel Nicolelis has implanted a Brain Machine Interface into the cortex of monkeys that they've used to control virtual avatars.

From inner space back to outer space

Virtual avatars aren't the only puppets controllable by motion capture or brain machine interface. Nicolelis' monkeys have also used their cerebral implants to control remote robotic arms. A paralyzed person using a Nicolelis exosuit did the opening kick in the 2014 World Soccer Cup. Surgeons use motion capture sensors to operate surgical telerobots.

Kelly's story has robots as well as a sophisticated brain machine interface. Given telerobots, exosuits, and robotic prosthetics, the boundary between hardtime and softtime blurs. There are hard as well as soft avatars.

Telerobots are becoming major game changers. They are doing work in places too hard to reach or dangerous for humans. British Petroleum uses them to build oil drilling infrastructure on the seafloor. Planetary Resources hopes to use them to mine the asteroids. Paul Spudis and Bill Stone hope to use robots to prospect and establish mining infrastructure on the moon.

Kelly is correct a severely disabled person would want to use a Brain Machine Interface every waking hour whereas a healthy person only a fraction of the time. I believe the severely disabled will be the most practiced users of telerobots. They could be the most intrepid explorers, the most able builders, the heroes of a coming age.

Fashions in Science Fiction

A Brain Machine Interface story from yesteryear was Anne McCaffrey's hopeful and uplifting The Ship That Sang. More recently we have Kelly's bleak dystopia Declaration.

Kelly's story has a lot of currently fashionable themes: overpopulation, terrorism, limited resources, alienation, inevitable decay. More often than not modern SF is gloom and doom exploring catastrophic failure modes of technology. Such storites are worthwhile, we should certainly try to anticipate and avoid possible calamities.

But we also need stories exploring technology's potential for good. I yearn for a return to tales about new frontiers and the triumph of human spirit over adversity.

We need hopeful as well as cautionary tales. Without hope there is no reason to get up in the morning.

Tuesday, June 30, 2015

Making the tether catch

In Phobos - Panama Canal of the Inner Solar System, Doug Plata had asked about tether catches:

How much time would one have to attach to the tether's end? Since it is connecting two different orbits then I'm imagining that it would be fairly brief. If one misses the connection, then what?

I liken a catch at apoapsis to catching a ball at the top of it's bounce. For a brief time, the ball hangs motionless -- and then gravity pulls it back down. The less the acceleration, the longer the ball will hover at the top of a toss.

Regions of the tether that feel a substantial net acceleration will have a greater need for fast reflexes and good timing. The regions of the tether closer to the balance point can catch at a more relaxed space. Catching at the balance point would be like docking with the I.S.S.

For an example I will use the ellipse common to the Phobos and Deimos tether:

The larger red ellipse is the path a payload would follow dropped from the foot of the Deimos tether and/or if thrown from the top of the Phobos tether. At peri and apoapsis, this path matches the speed of the tether. So the moons could exchange payloads while using virtually zero reaction mass.

Note: When I use directional words like top, foot, above, below, up or down, I'm using Mars as the center. Down means Marsward.

Making the catch at the Deimos tether foot

Both the payload and Deimos tether foot are traveling about 1.18 km/s. But it is the relative velocity that counts. After all, I am traveling 30 km/s as earth circles the sun and so is my computer monitor. Do I worry about a catastrophic collision with my computer monitor? Not since I'm moving about zero km/s with regard to my computer.

Catching at the foot of the Deimos tether:

30 minutes before the catch the payload is trailing the foot by a few kilometers and is about 55 kilometers below. It's traveling about 136 miles per hour with regard to the tether, most of that velocity is vertical.

1 minute before the catch, the relative speed is only about 5 mph.

I'll compare this to driving a car. Driving down the road at a leisurely 30 mph, I step on the brakes. I don't stomp on them, mind you. Just decelerating at my usual careful pace, it takes 6 seconds to come to a full stop. Now look at the payload 5 minutes before the catch: 22.6 mph. Compared to my Sunday driving, this payload is moving like a turtle coming out of hibernation.

From Phobos throw to Deimos catch is an ~8 hour trip. During the vehicle can make measurements of it's distance and velocity with regard to the Deimos tether foot and compare it to optimal distance and velocity.

If catching below a tether's balance point, the payload would rendezvous with the tether at the trailing edge. If the tether is in a prograde orbit, the payload would land on the western end of a ramp:

In this cartoon I have a quadpod on wheels entering on the west end of the ramp. The wheels are only partially for comic relief. Wheels would actually be helpful landing on a platform.

The quadpod is a fanciful design not really relevant to tethers. I use it because it can quickly make slight adjustments to speed along any direction. The closing velocity is almost completely vertical. There will be a time when the space ship is quite close to the tether and falling up at a high speed. So it would be good to be able to do a slight tap on the brakes or gas pedal.

Also to give a little error room to the landing on the west ramp, I imagine a folding west end of the ramp that can extend itself after the ship has matched altitudes.

Acceleration, net weight

At this part of the Deimos tether, centrifugal acceleration is -.0681 m/s2 and Mars gravity is .1017 m/s2. Net acceleration is .034 m/22. A Sumo wrestler weighing 400 pounds on earth's surface would weigh 1.4 pounds. A coin falling out of your coat pocket would take ~8 seconds to reach your foot (assuming distance from coat pocket to foot is one meter).

Making the drop to Phobos

To send a payload on it's way to Phobos, simply roll of the east edge of the ramp.

Making the catch at the Phobos tether top

Catching at the Phobos tether:

1 minute out, the tether is moving 18 mph. A faster pace than the Deimos tether catch, but still much more leisurely than me rolling my car into the driveway from a Sunday drive.

Acceleration, net weight

At this part of the Phobos tether, centrifugal acceleration is -.536 m/s2 and Mars gravity is .4027 m/s2. Net acceleration is -.133 m/22. A Sumo wrestler weighing 400 pounds on earth's surface would weigh -5.44 pounds.  A coin falling out of your coat pocket would take 4 seconds to reach your foot (assuming distance from coat pocket to foot is one meter).

From the point of view of someone on Mars, the acceleration would seem upward. Looking through a telescope, they'd see the Sumo wrestler bumping against the ceiling like a helium balloon.

Making the drop to Phobos

To send a payload on it's way to Deimos, simply roll of the west edge of the ramp.

Some general rules for vertical tethers

This is an illustration for any vertical tether in a circular orbit. It also applies to Clarke style beanstalks.

The red orbits below the circular balance point's orbit move faster than the tether except where they cross the tether. At crossing points the orbits move the same speed as the tether. I explain here how tether matching orbits are found.

For points on the tether below the balance point's  circular orbit, entrance/catching ramps are on the trailing edge of the tether (the west end for tethers in prograde orbits). To drop to lower orbits, roll off the leading edge (east end of the ramp in prograde orbits).

For points on the tether above the balance point's circular orbit, entrance/catching ramp are on the leading edge of the tether. To throw to high orbits, roll off the trailing edge.

Making catches in steep acceleration gradients

Things are less relaxed as the tether extends further from the balance point. As soon as I have time, I will look at a Phobos tether whose foot extends into Mars upper atmosphere. The net acceleration at this foot would be about 3 km/s2 or about a third of an earth g.

Wednesday, June 17, 2015

Phobos--Panama Canal of the Inner Solar System

My post Orbital Momentum as a Commodity describes how a tether with a healthy anchor mass can catch and throw payloads. I tried to think of ways a tether might restore orbital momentum lost during a catch or throw. Two way traffic is one way to pay back borrowed momentum.

Well, Mars' moon Phobos masses 1.066e16 kg. With this huge momentum bank, catching and throwing payloads would have less effect than a gnat hitching a ride on a Mack truck. A Phobos anchored tether could catch and throw for millennia with little effect on Phobos' orbit.

The tether illustrated above doesn't suffer the enormous stress of a full blown earth elevator or even a Mars elevator. It could be made from Kevlar with a taper ratio of about 11.

Access to Mars

The tether foot pictured above moves about .6 km/s with regard to Mars surface. This is about 1/10 of the ~6 km/s the typical lander from earth needs to shed. Mars Entry Descent and Landing (EDL) would be vastly less difficult.

Some have suggested Phobos 1.88 g/cm3 density indicates volatile ices. If so, the moon could also be used as a source of propellent. A Phobos propellent source would make EDL even less of a problem. However Phobos' low density might also be due to voids within a rubble pile.

On page 2 of the Acceleration of the Human Exploration of The Solar System with Space Elevators Marshall Eubanks takes a look at how the foot of Phobos-Anchored Martian Space Elevator (PAMSE) might interact with Mars' atmosphere:
The orbital eccentricity of Phobos amounts to 283 km, which is by coincidence comparable to the effective depth of the Martian atmosphere for satellite drag (typically ~ 170 km, but subject to variations due to atmospheric events such as dust storms). The average relative velocity between the lower tip and the surface of Mars is only 534 m/sec, roughly Mach 2 in the cold Martian atmosphere, and slow enough that it should not cause significant heating of the tip. This raises the interesting possibility that the PASME tip could dip down deep into the atmosphere to leave or recover payloads or perform reconnaissance, acting as a supersonic airplane for the period near periapse when it is near the surface.
Eubanks' 534 m/sec is a little slower than the .6 km/s of my tether tip. This might be because I had placed my tether tip 300 km/s above Mars' surface thinking atmospheric friction would destroy a lower tether foot. Eubanks' analysis has changed my view.

In the Facebook Asteroid Mining Group, Eubanks noted:
The orbit of Phobos is equatorial, and there is a big mountain in the way, Pavonis Mons, the middle of the Tharsis volcanoes, straddling the equator and by far the highest obstacle in the path of the elevator tip. Maybe a railroad on top of the volcano could match speeds with the elevator tip, once every 3 days or so (when the orbit and volcano aligned). If so, you would have up to 3 minutes to shift cargo on and off. 
as well as
…the cool thing is that the tip can be something like a tethered airplane (with wings and flaps, etc.) and you should be able to use that to control oscillations. I was hoping to get money to begin actually "testing" this (i. e. in simulation), but, alas, not so far. 
Remember, too, with the PAMSE the counterweight has ~ infinite mass, and so any oscillations have to end there. (of course, anchoring a PAMSE in Phobos is left as an exercise for the reader.)
If Phobos is indeed a loose rubble pile, anchoring the elevator would be difficult. So while Eubanks eased my anxieties on oscillations and atmospheric friction, he calls my attention to a problem I hadn't thought of.

Access to Earth

6155 km above Phobos the tether is moving faster than escape velocity with a Vinf of 2.65 km/s. This is sufficient to toss a payload down to a 1 A.U. perihelion. This could provide most of the delta V for Trans Earth Insertion.

A ship coming from Earth would have a Vinf of 2.65 km/s and so rendezvous with this part of the tether might be accomplished with little propellent.

Access to the Main Belt

7980 km above Phobos the elevator is moving with a Vinf of 3.27 km/s, enough to hurl payloads to a 2.77 A.U. aphelion. This part of the tether might send/receive payloads to/from the Main Belt. There are a lot of asteroids with healthy inclination, though. So there would be substantial plane change expense at times.

Possible Mars exports to the main belt

One thing about the Main Belt, the pace is much more leisurely. Ceres moves about 1º every 5 days. In contrast earth moves about 1º a day and a satellite in low earth orbit moves about 4º a minute.

So a month-long, low-thrust ion burn over there looks a lot more like an impulsive burn than it does in our neck of the woods. I believe high ISP ion engines are well suited for travel about the Main Belt.

The inert gas argon can be used as reaction mass for ion thrusters. Mars' atmosphere is about 2% argon. It is also about 2% nitrogen and 96% carbon dioxide with traces of oxygen and water. Mars also has respectable slabs of water ice at the poles.

Mars would be a good source of propellent for the entire belt as well as CHON for the volatile poor asteroids in the inner main belt.

Ion engines don't have the thrust to weight ratio to soft land on the larger asteroids. But asteroids often have high angular velocity (in other words, they spin fast). High angular velocity combined with shallow gravity wells make asteroids amenable to elevators.

For example the balance point for a Ceres elevator would only be 706 km above Ceres surface, that is the altitude of a Ceres-synchronous orbit. To provide enough tension to remain erect, the elevator would need to extend to an altitude of 2000 km. At 2000 km, the tether tip is moving about .46 km/s, a good fraction of the 2.82 km/s needed fro Trans Mars insertion. If this Ceres elevator is Kevlar, taper ratio would be about 1.02.

If extended to an altitude of 14,500 km, the Ceres elevator top would be moving fast enough for Trans Mars insertion. This would require a taper ratio of around 5 for a Kevlar tether.

Incremental Development

The tether pictured at the top of this post is ~14,000 km long with a taper ratio of 11 for Kevlar. While much smaller than a full blown Mars elevator, this elevator would still be a massive undertaking. But the whole thing doesn't need to be built overnight. Early stages of the elevator would still be useful.

Pictured above a Deimos tether drops a payload to a Phobos tether.

At apoapsis of the large ellipse, payload velocity matches the Deimos tether foot. At periapsis, the velocity matches the speed of the Phobos tether top. Thus payloads can be exchanged between these Martian moons using practically zero reaction mass.

After descending the Phobos tether, the payload can be dropped to a Mars atmosphere grazing orbit.

These tethers are a lot shorter than 14,000 km tether we were talking about and taper ratio is close to 1.

No Moons to Dodge

A full blown Mars elevator capable of throwing payloads to the Main Belt or even earthward would have to dodge Deimos as well as Phobos.

A Phobos elevator for flinging payloads to Ceres ends well below Deimos' orbit. And of course a Phobos anchored tether doesn't need to dodge Phobos.


Tsiolkovsky's rocket equation and big delta V budgets are touted as show stoppers for routine travel to Mars' surface or the Main Belt.

With judicious use of tethers and orbital momentum, rhinoceros sized delta V budgets are shrunk to hamster sized delta V budgets. No bucky tubes needed, ordinary materials like Kevlar can do the job.

Wednesday, June 10, 2015

Mass parameter and ITN

It seems like every other post I'm singing the praises of EML2. I'm also enthusiastic about L1 and L2 necks for big moons orbiting gas giants.

So why do I diss the Sun Earth L2 or the Sun Mars L1? 

Robert Walker put it fairly well:

I've no idea why you think there's some essential difference between e.g. transfers between moons in the Jupiter system and transfer between planets around the sun. Mathematically it's the same situation, multiple masses around a central planet or sun. Obviously the moons of Jupiter are larger compared with Jupiter than planets are compared to the sun, and the orbits are far shorter. But they still have Hohmann transfer orbits, and hill spheres, and lagrange points, and these tubes, which lead out from the lagrange points. 

It's The μ

The big reason is mass parameter. This quantity is often denoted μ in discussion of 3 body mechanics.

It's common to choose units so that mass of central and orbiting body sum to one.

For example if central mass were 90% percent of the system's mass, central mass would be .9 and orbiting body would be .1. In this case μ would be 1/10.

The small the μ, the closer L1 and L2 get to the orbiting body.

What paths do payloads follow when nudged away from the orbiting body at L1 or L2? Well, we can notice a few things about L1 and L2:

The L1 and L2 have the same ω (angular velocity) as the orbiting body.

L1 and L2 are collinear with central and orbiting bodies.

It just so happens I have a diagram of collinear points all having the same ω. It's what I use to model vertical tethers. By scaling this diagram it could also be used to model space elevators. (Space elevators are a special case of vertical tether where the tether foot coincides with planet surface and circular orbit of the balancing point coincides with planet synchronous orbit):

Eccentricity Vertical Tether Conics = |1 - r3|

Release a payload from any point on the tether and the path will be conic section having eccentricity
|1 - r3| where distance from center to balancing point is 1 and a point's distance from center is r.

But this diagram was derived using 2 body mechanics. When nudged away from the orbiting body's Hill Sphere, the payload will quickly enter a regions where the central body gravity dominates and conic sections are fairly accurate.

But while in the neighborhood of the Hill Sphere, there's a short interval when the path should be modeled using both the accelerations of central and orbiting body:

While falling away from the moon near L1,  a payload surges ahead while the moon tugs it backwards and a little up. This has the effect of lowering the apo and periapsis as well as rotating line of apsides in a prograde direction. A payload nudged from L2 away from the moon will lag behind the moon. While in the lavender region, the moon pulls the L2 payload forward boosting peri and apo-apsis as well as rotating line of apsides forward. L2 necks throw higher and L2 drops lower than corresponding points from a vertical tether.

Here are orbital sims for various mass parameters where colored pellets are nudged with slightly different velocities from L1 and L2:

μ = .1

μ = .001

μ = .00001

μ = .000001

Notice as μ shrinks, the orbits get closer and closer to what the tether model would suggest. For μ = .000001, apogee is very close to point of release and eccentricity of ellipses is approaching |1 - r3|. As the Hill Sphere shrinks the lavender two body zone gets thinner and the 2 body model becomes increasingly accurate.

μ for sun-earth is .00000304 and μ for sun-Mars is .000000323. The paths from the planets' L1 and L2 necks don't go far and there's not much variation.

What About Gravity Assists? Just Look At Rosetta

"Well sure, nudging a payload from SEL2 doesn't get us much past a 1.07 A.U. aphelion" an ITN defender replies. "But earth gravity assists can boost that aphelion. Look at Rosetta's March 2005 gravity assist -- it boosted an earth like orbit to an aphelion past Mars."

So let's look at the Rosetta gravity assist.

We can see the March 2005 gravity assist gets Rosetta past Mars orbit. And the orbit from launch to gravity assist looks pretty earth like, right?


The orbit from launch to gravity assist is an ~.9 x 1.1 A.U. ellipse with an eccentricity of around .11. When r = 1 A.U., flight path is about 5 degrees. As it approaches earth, Vinfinity is about 2.6 km/s:

In contrast, Vinf of an orbit departing from SEL1 or 2 will be about .3 km/s. Rosetta's initial orbit couldn't be accomplished via a WSB from an L1 or L2 neck.

Further, a payload from SEL2 remains outside earth's orbit, it does not cross. The closest it comes it .01 A.U. during which time it's flight path is zero. The same is true of an orbit nudged from SEL1:

Synodic Period

Another thing to consider is synodic period. A way to think of synodic period is how often one runner laps another as they race about a circular track. If both runners are going nearly the same speed, it will take a long time.

Synodic period of orbiting bodies is |(T1 * T2)/(T1 - T2)| where T1 and T2 are bodies' orbital periods. Orbital period of a 1.01 x 1.06 ellipse is 1.053 years. Synodic period is 19.88 years. So the payload wouldn't even come close to the earth until almost two decades later!

When the payload finally does lap the earth 19.88 years later, it will be 43º from perihelion.

Instead of being .01 AU from earth, the payload will be more like .02 or .03 A.U. from earth. It won't get close to earth until nearly 8 synodic periods later. That's about 160 years.

The tinier the μ, the bigger the synodic periods of payloads released from L1 and L2 necks.

Synodic Period with a big μ

In closing I'll take a look at synodic period of something dropped with from Earth Moon L1. When it comes to μ's the earth moon's .012 is the 900 pound gorilla of the solar system. It's the biggest I know of except for Pluto Charon's .104.

Dropping from EML1, payloads fall into an approximately 100,000 x 300,000 km orbit:

Period is about 11 days. Synodic period is 20 days. So within a month's time these pellets will fly by the moon when they're near apogee:

EML1 and EML2 can do lots of stuff within a fairly short time. I have seen some crazy stuff running earth moon sims.

But zoom out and it gets boring. I've let sun earth sims run for centuries without seeing any drama. The L1 and L2 necks for the sun/rocky planets are a bunch of duds.

Does the ITN include Gravity Assists?

Given big enough  μ's, some WSBs can snake by another body which can lend a gravity assist.

Does that mean gravity assists are part of the ITN? No. We've been using gravity assists for many decades. They were in common use before Ross' or Belbruno's techniques came on the scene.

Some claim astrogators are now using Belbruno's or Ross' techniques to find opportunities for gravity assists. I haven't seen any evidence of this. So far as I can tell mission planners are still finding such opportunities the old fashioned way: looking for needles in a hay stack. In other words with persistence and hard work.

Monday, June 8, 2015

Orbital Mechanics Coloring Book

“The most sophisticated people I know - inside they are all children. ” 
― Jim Henson

I agree with Henson -- many of the folks I most admire are just big, playful kids. Henson (of the Muppets), Theodor Geisel (aka Dr. Seuss), Carl Barks (Uncle Scrooge, Donald Duck, Huey, Dewey and Louie), Stan Lee (Marvel Comics) are a few of favorite artists/kids.

My coloring book is aimed at kids. It's aimed at adults too. At least I hope there's a lot of adults like myself that like kid stuff.

I try to give info on the conic sections is a visual, accessible manner as well as have some fun. Here's some of the pages:

Click on image to see larger version.

The above are a few of the book's 40 pages. It's my hope it informs and entertains kids as well as adults. Getting folks interested in outer space is also a goal.

It is available at Conic Sections & Celestial Mechanics Coloring Book by Hop David.

My other coloring books can be found on my Amazon author's page.

Saturday, June 6, 2015

Colonization - where first?

This was one of the topics suggested by Doug Plata -- Colonization O Neillian vs lunar colony - Where first?

Neither Is Likely

For review I'll recap a discussion I've been through a number of times:

Skeptic: Humans are adapted to life on earth's surface. Outer space is too hostile for humans to call home. 
Hop: Man's extended phenotype has enabled humans to live in places they're not adapted for. Without animal skins, shelters and fire, our ancestors couldn't have settled northern Europe. 
Skeptic: Well, sure. But outer space is a lot more hostile than northern Europe. 
Hop: And our extended phenotype is a lot more sophisticated than our prehistoric ancestors. 
Skeptic: The advanced extended phenotype that could enable humans to live off planet requires a massive infrastructure and population. Until such a massive infrastructure and population is established off world, the outpost will remain dependent on earth.

And that's where Skeptic wins the argument. Space enthusiasts do indeed suffer from Home Depot Syndrome. Need grow-lights? Go to Home Depot. Solar array? Home Depot. Gaskets and seals? You get the idea. It's so convenient to get many diverse products that it's easy to forget they come from a vast mining, transportation and manufacturing infrastructure.

To establish such an infrastructure off world would take a long and costly effort. Who's going to make that sort of investment?

Not governments, at least not with the present zeitgeist. When the general populace finds New Horizons more interesting than Caitlyn Jenner, I might change this view.

How about Musk? He's going to launch a huge constellation of sats providing communication to the 3rd world. He's going to make fully re-usable spaceships and cut cost of spaceflight 100 fold.

A worst case scenario for Musk's constellation is Iridium redux. Maybe Musk's scheme will be more successful. Electronics for a com sat now takes less volume and mass. And Elon is enjoying some success in reducing launch costs.

Let's look at a best case scenario. Musk establishes a thriving communication monopoly for the 3rd world. He'll be like Mexico's Telmex monopoly owner Carlos Slim but 10 times as rich. Is this likely? I don't think so. Fiber optics and cell phone towers are already being erected through out the 3rd world. And if LEO com sats are as lucrative as Musk hopes, other competitors will move in and attempt to take market share. But for the sake of argument let's assume Musk becomes a Carlos Slim on steroids. Further let's assume fully reusable, cheap spacecraft (also questionable but for the sake of argument…)

Even then, there's not enough to colonize Mars. A base almost wholly dependent on earth. maybe. But not a colony. I don't believe Musk would have enough to make a self sufficient colony in the Gobi Desert. See my discussion of the Home Depot syndrome above.

Walden Ponds Sans Home Depots

Space enthusiasts like to imagine Closed Ecological Life Support Systems (CELSS) that provide our needs without importing lots of food and water from planet earth. They also like to imagine 3-D printers that could make a huge variety of parts and supplies from a few feed stocks. Throw in some basic tools like a lathe and mill. Maybe we could get by without a huge infrastructure.

Is a small, self sufficient, nay - thriving outpost possible? Maybe it is. If so, that could be a source of income for would be space colonizers.

Toss some of these colony seeds in the Gobi desert. Or Siberia. Atacama Desert. Arctic tundra. Earth's wastelands are a tropical paradise compared to Mars, the moon or asteroids. With time these colony seeds grow and you have self sufficient, thriving metropolises where there used to be barren dirt.

A reader who doesn't know me may think I'm being snide and sarcastic. But I'm serious. The notion of seeding barren places with self sufficient Walden Pond style Kibbutz settlements is perhaps doable. Maybe we'd need the massive infrastructure beneath the Home Depot tip of the iceberg. Maybe not.

Developing this ability is a prerequisite for colonizing Mars. If someone does develop that technology, it could be a source of revenue.

My Suggestion for the First Pre-Space Kibbutz

La Rinconada is described as the highest city in the world. The place is very unpleasant. At 5.1 km, it's hard to breathe. It's cold. It has no plumbing or sanitation system. Gold is the reason people endure these hellish conditions.

Would be space colonizers like to imagine starting out with buried Bigelow habs and then later burrowing or building walls with ISRU materials to contain atmosphere. Airlocks would be imported at first and later built with ISRU materials. The CELSS would convert nasty sewage and CO2 into food and fresh air.

What better place to try this than La Rinconada? Habs with warm, clean, pressurized air would be a Godsend to these people. Not to mention clean water to drink and sanitation facilities. La Rinconada miners would pay good money to enjoy the amenities space enthusiasts assume CELSS would provide.

The First Space Economic Incentive for developing CELSS

Some suggest necessity is the mother of invention. That if the choice is do or die, colonists will develop CELSS and self sufficiency. In my opinion that is putting the cart before the horse.

However it may well be that we won't develop self sufficient CELSS until humans are spending long stints beyond LEO. My notion of earthly seed colonies is unlikely. There are sovereign governments, private land owners, environmental impact statements and mountains of red tape standing in the way.

If space mining does happen, I'd expect the first mines to be on a Near Earth Asteroid parked in lunar orbit.

Space mining would probably be mostly robotic. But there may be some need for a human presence. See my post Who Needs Humans? If so, transporting humans to lunar orbit and back would be expensive and dangerous. There would be some incentive to make longer stays possible and thus reduce number of back and forth trips.

Some of the first steps I'd expect Planetary Resources or Deep Space Industries to make:

A modest spin hab. Enough spin gravity to enable flush toilets, showers and draining sinuses would be a huge boon for worker morale. It's possible a small amount of gravity could largely mitigate atrophy problems. We won't know until we try.

Plants. Green plants make a much more pleasant environment. Of course plants provide oxygen and cleanse CO2 from the air. Fresh vegetables are also a morale booster.

Sewage Treatment. I understand this is one of the more difficult problems for CELSS. But rich organics are valuable and mass imported from earth is extremely expensive.

Should Planetary Resources or Deep Space Industries develop good CELSS on rocks in lunar orbit, they'd be in a much better position to send humans to rocks in heliocentric orbits. And while smaller rocks amenable to retrieval are economically interesting, the bigger rocks have more resources.

Early asteroid habs would certainly not be self sufficient. But it is possible to reduce needed imports over time. And if the asteroid mines are making money and trading exports, self sufficiency isn't necessary.


If my goal were winning in Vegas, I'd place my bets on humans never getting past LEO with the exception of rare flags and footprints publicity stunts.

But I am an unrepentant optimist. So my money is on early asteroid mining outposts gradually becoming less dependent on earthly imports.

Even being a die-hard optimist, I would still bet against O'Neill cylinders or massive planet side colonies. At least in this century.

But given a gradually growing presence in the asteroids, I believe massive O'Neill Islands in heliocentric orbit would eventually come to pass. As well as cities on the moon, Mars, Ceres and beyond.

Thursday, May 28, 2015

Orbital Momentum as Commodity

To The Moon, Alice

A tether needs to be substantially more massive than it's payloads. Else a catch/throw would wreck the tether orbit.

In my last post I described how near earth asteroids parked in retrograde lunar orbits might be used as anchors and momentum banks for lunar orbital tethers.

What could we use to anchor a vertical tether in earth orbit?

When a satellite in geosynchronous orbit satellite dies, it's sent to a graveyard orbit about 300 km above geosynch. According to IO9 there are more than a hundred sats in this graveyard. Here is an estimate of 371 dead GEO sats totaling 668 tonnes. There will be more as time goes on.

Boost one of these dead sats 25 km higher than the graveyard orbit and dangle a 25 km tether. The tether foot will be moving about 10 kilometers/hour or about 5.5 mph with regard to the sat graveyard. As satellites are caught, momentum can be added via ion engines. With an ion engine's high ISP, the satellite collector's reaction mass can be small with regard to the mass of the satellites collected.

There's little plane change delta V since most the GEO sats are equatorial.

This would cleanse the geosynch neighborhood of orbital debris. When the dead sats are collected into a single mass, cross section is much smaller than a multitude of satellites. Thus likelihood of debris generating impacts is much smaller.

Some parts of these dead sats may be salvageable. Many may still have working solar arrays, for example. Although it is possible salvage issues could be a road block to this scheme. Those parts not salvageable would still have value just as a source of orbital momentum.

Boost this ball of dead sats to an orbit 10,000 km higher than GEO and extend a tether 10,000 km up and 7,000 km down. Now we have a well anchored 17,000 km tether. Tether foot is 3,000 km above geosynch so there's no chance the foot will hit an GEO sat. There is also less chance of tether damage from an impact with orbital debris.

A payload released from this tether top will reach the moon.

If we use Kevlar with a tensile strength of 3.6 giga pascals and density of 1.44 grams per cubic centimeter, taper ratio for this tether is 1.18. Tether to payload mass ratio is less than 1.

The tether center is moving about 2.78 km/s. Grave yard sats move about 3.06 km/s. So delta V to raise the ball of sats is only .28 km/s.

From LEO it takes around 3.2 km/s to rendezvous with this tether foot. About what it takes for a normal Trans Lunar Insertion from LEO. This was disappointing to me.

However this tether could receive mass from an asteroid parked in a lunar orbit. It could then send asteroidal mass to lower orbits.

If lower tethers were receiving mass from above as well as from earth, it might be less costly to give the lower tethers a substantial anchor mass.

Here are three possible tethers: the super GEO tether already described, a sub MEO tether and a super LEO tether:

Each of these tethers is positioned to avoid the high satellite/debris density areas: GEO (Geosynchronous Earth Orbit), MEO (Middle Earth Orbit, home of GPS and other global positioning sats), and LEO (Low Earth Orbit).

Tether to payload mass ratio is also less than one for both the Super LEO and sub MEO tethers.

The apogees and perigees of these red ellipses match the tether velocities at rendezvous. So almost no propellent would be used for catches and throws.

From LEO it takes .5 km/s to reach the Super LEO Tether. From there the relay of tethers can deliver spacecraft to the moon.

Once In The Moon's Neighborhood...

The Super GEO tether tosses payloads to a 384,400 km apogee. 384,400 km is the moon's distance from earth. Velocity of this ellipse's apogee is about .54 km/s while the moon's speed is about 1.01 km/s. Vinf with regard to the moon is about .48 km/s.

In my earlier post I described an asteroid anchored tether that could toss stuff from the moon with a Vinfinity of 1.6 km/s. Tether to payload mass ratio would be around 3. If we only need a Vinfinity of .48 km/s, a less massive tether would do:

Tether center's 24,200 km from the moon's center. Tether foot 13740 km, tether top 39000 km. If we use Kevlar with 3.6 giga pascal tensile strength and 1.44 g/cm^3 density, the tether to payload mass ratio is less than 1/4.

Dropping from this tether foot, a payload would impact the moon at 2.23 km/s.

It'd also be somewhat easier to park a near earth asteroid in this higher lunar orbit. See the Keck Report for a proposed method to park rocks in lunar orbit. For asteroids with orbital energy just above zero it is doable to park them in an high lunar orbit.

Up Momentum as a Valuable Commodity

Catching from a lower orbit and throwing upwards will sap a tether's orbital momentum. Earlier I had mentioned that ion engines using xenon as reaction mass could restore momentum.

But asteroidal mass from above is a source of up momentum.

With two way traffic, the need for xenon is reduced. Momentum boosting maneuvers could be balanced with momentum sapping catches and throws. The need for reaction mass would be largely eliminated.

An asteroid anchored tether in lunar orbit would be helpful in capturing more asteroids to lunar orbit. There's a lot of near earth asteroids in accessible orbits, so there's a massive source of up momentum.

Electrodynamic Tethers

How about using Lorentz force to change orbital momentum? As an electron moves up (away from earth's center) it passes through earth's magnetic field and the tether is pushed east, boosting momentum. Sending electrons down will give a westward push, reducing momentum.

But this relies on one way current. If the circuit is closed, electrons move up as well as down and there's no net Lorentz force.

The ionosphere can close a circuit in low earth orbit. The tether can pick up electrons from the ionosphere as well as discharge electrons into the ionosphere.

But the tethers described here are above the ionosphere. Using Lorentz force doesn't seem to be an option.

But I don't know that much about electrodynamic tethers, I could be wrong. If someone corrected me, it'd be a pleasant surprise.

Why I Don't Like Rotovators

Why my obsession with vertical tethers? Yes, rotovators could be shorter. But there's several reasons I don't like rotovators.

Taper Ratio and Tether to Payload Mass Ratio

Tension in rotovators comes from so called centrifugal force, ω2r. At the Space Stack Exchange 2012rcampion takes a look at a sling's tether taper ratios. Campion's results look a lot like Moravec's equations.

As tether tip speed grows, taper ratio soars. Short tethers capable of a good throw would be quite massive. According to this pdf, a LEO to GTO rotovator capable of tossing 5 tonne  payloads would need to mass 50 tonnes (3rd paragraph, page 4). A 10 to 1 tether to payload mass ratio.

In contrast, a vertical tether's tension comes from centrifugal force and gravity, ω2r - μ/r2. Gravity mitigates stress from centrifugal force and taper ratio grows a lot more slowly. All the vertical tethers described in this post have a tether to payload mass ratio less than 1.

Vertical tethers do need an anchor mass. But anchor mass can be very useful. I'd like to see lots of solar arrays at the tether centers. Solar arrays could power electrolysis plants to crack water into bi-propellent, move elevator cars up and down, and occasionally power Hall Thrusters to adjust the tether's orbit.


A rotovator is good for catches and throws when it's vertically aligned. But it's well aligned only for a very brief time during its spin. The rotovator must be correctly positioned when a launch window occurs. Ditto for catching from a Hohmann orbit.

Catches are harder. As a payload approaches a rotovator, the tip remains on its path for a brief time and then either zooms up or down (from the payload's point of view). In contrast, the tips of a vertical elevator remain at constant altitude.

Attitude Adjustment

A vertical tether stays vertical due to tidal acceleration gradient. It seems to me a rotovators attitude would need to be adjusted from time to time.

Possible Imports to Earth and LEO


I believe the most important import from asteroids will be water.

My last two blog posts are more or less in response to Jon Goff's The Slings and Arrows of Outrageous Lunar Transportation Schemes: Part 1 - Gear Ratios. Goff pointed out that only a small fraction of propellent mined at the lunar poles could be delivered to LEO.

Ever since Goff wrote that, I've been trying to think of ways to deliver extraterrestrial propellent to where it's needed.

"Wait a minute," you might be thinking, "This guy has just described a transportation system using momentum exchange and ion engines. Why is he still stuck on stone age chemical propellent?"

I believe the biggest obstacle to fully reusable spacecraft is an upper stage's 8 km/s re-entry. Hall Thrusters decelerate too slowly to help with that plunge.

A tether foot low enough to drop payloads into low velocity suborbital paths would be vulnerable to collision. Below 1000 km, space is full of sats and debris.

What the Super LEO tether could do is deliver propellent to LEO. An ellipse from the Super LEO foot would take .5 km/s to circularize at perigee. And some of that .5 km/s might be accomplished by aerobraking.

An upper stage refueled at LEO could do a healthy burn to lose most the 8 km/s. The upper stage might also beef up it's dry mass with structural support and TPS, also from asteroids. Given these options, upper stage re-use is very doable. If upper as well as booster stages can be economically re-used, the dream of cheap space access is realized.


Many asteroids have high concentrations of the platinum group metals. Right now these are precious due to rarity. But should they become more available, there are numerous ways they can be used.

Rare earth metals have many uses. Rare earths actually aren't rare. But they're hard to mine in an eco-friendly way. I would much rather see them mined on a lifeless, barren rock than in earth's biosphere. An asteroid in lunar orbit would make the moon's KREEP more accessible. Besides rare earths, KREEP also contains uranium and thorium, possible sources of energy.


As mentioned earlier, imported uranium and thorium could fuel terrestrial power plants.

The earth intercepts only .45 billionths of the sun's light. We're using only a tiny fraction of possible solar energy.

If we use solar energy to refine extra-terrestrial ore and import commodities to earth and LEO, in a sense we're importing energy.

Besides energy for refining, imported commodities would also require energy for transportation. To get a kilogram from just above C3 to LEO takes about 512 mega-watts. Tether momentum exchange could accomplish most of this. But it's energy used, regardless of source.


A source of up momentum would be a major game changer. A first step towards acquiring this source would have been the early version of the Asteroid Redirect Mission (ARM). This is doable. But for now it looks like popular opinion will keep this from ever being funded.

I'll continue singing the praises of asteroids and ARM. God willing, my small voice will have some influence.