Cassini and Me

Early this morning, while I lay sleeping, the Cassini spacecraft plunged into Saturn’s upper atmosphere. It had been in space over twenty years, and at Saturn for over thirteen.


The last image transmitted by Cassini, about 15 hours before impact.

Like the Galileo spacecraft before it, mission controllers intentionally configured Cassini to deorbit when it reached the end of operational life. Once the supply of maneuvering fuel is exhausted, there’s no way to control a spacecraft’s trajectory. Cassini would, eventually, crash into one of Saturn’s moons.

This isn’t a problem in-and-of itself—we don’t have much compunction about littering the Solar System with terrestrial debris. But we wouldn’t have any control over which moon, and that is a problem.

One of Cassini’s big discoveries was that Enceladus has liquid water plumes coming from its south pole. Both Enceladus and Titan (which is loaded with organic and pre-biotic molecules) could be potential abodes for life. Given that we can’t perfectly guarantee our spacecraft are sterilized of Terran microbes, disposing of Cassini in Saturn’s atmosphere is better than risking planetary contamination. The final, ring-grazing orbits concluded with a fly-by of Titan, a distant interaction robbing Cassini of just enough mechanical energy to send it into clouds below.

I agree with that logic, but the decision is bittersweet. In many ways, Cassini was the first probe that I followed through its entire science mission. I was paying attention when Cassini arrived, when the Huygens lander touched down on Titan, when the plumes were found on Enceladus, when the hexagonal storm was spotted at Saturn’s north pole. The other real contender for that title would be the Mars Exploration Rovers, Spirit and Opportunity, but the latter is still operating. Cassini, as of this morning, is over.

chesley bonestell titan

This famous Chesley Bonestell painting of Saturn from the surface of Titan greatly anchored my expectations for the Huygens probe.

The joy of discovery, however, is far from finished. Unless NASA finally comes around to nuclear propulsion or outer-planet aerocapture there won’t be many more Saturn probes in my lifetime, but most of Cassini’s results still haven’t reached my eyes. Even the Huygens probe, which only operated for a few hours during descent and landing, probably has some pictures I haven’t yet laid my hands on. And that’s before we consider the data from other instruments.

Space science, you see, isn’t just about snapping the prettiest photographs—though that’s certainly a part of it. We’re gathering information to test existing models of reality, and to begin the process of building new ones. Some will be refinements to our existing ideas, and some will be new notions entirely. When a scientist in twenty years has an idea about the way gas planets or their moons behave, they’ll have a wealth of information waiting to validate it with.


Artist’s rending of Cassini at Saturn. Note the high orbital inclination.

But there’s a lot of scientific value in those beautiful images Cassini sent back to us, too. Every kilo counts, and we didn’t cart those cameras to Saturn for the hell of it. We sent them to understand the planet’s atmosphere, its rings, its moons. We’ve learned so much we didn’t even know to ask about in 1997. Those pictures give us a better perspective on the science, and on our place in the universe.

Even the picture I’ve been using a personal motif the last couple years, of Earth seen from under the rings, is a Cassini image. Space exploration lets us see the view from a height, of our planet, of ourselves, and of our universe. This mission had to end, but it didn’t have to happen in the first place. We didn’t have to expand our knowledge and perception, because there’s no cosmic law dictating humanity choose wisdom over ignorance. We chose to send Cassini-Huygens to Saturn, and I’m very glad that we did.


Wrong, Wrong, Wrong


[You’ll have to take my word that this post was planned before Megan left a comment to much the same effect on Facebook. I still wrote it, because I love sharing space trivia, but because it took several more days to finish, my summer schlamperei cannot be questioned.]

Last week I discussed a mistaken explanation in one of my engineering textbooks. The specific explanation was wrong, but there’s two other issues with that innocuous caption in chapter one.

Hibbeler attributes a NASA image to a PhotoShop artist on Shutterstock. Now there’s nothing wrong with getting images from there. I find them a bit overthetop personally, but when has that ever been an impediment to the authors of textbooks on serious topics? Never, that’s when.

But why would you go through a stock image company when NASA images are public domain? It seems unnecessary, strongly suggests that author or graphic artist did a quick Google search, rather than having a clear image in mind. That brings us to the other problem.

This is a very specific, very recognizable image of Bruce McCandless operating the Manned Maneuvering Unit during STS-41B. You even see the same cloud patterns on the NASA website. And the last I heard, Bruce McCandless is a man.

Misgendering McCandless was obviously done for inclusion, but would it have been so hard to get a picture of a woman astronaut? It certainly would go further toward fixing the image of female spacetravelers in the reader’s mind.

Unfortunately for Hibbeler, there aren’t any pictures of women EVAing without a tether because no woman has ever operated the MMU. If that seems problematic, consider that the MMU was an experimental piece of technology that was really too dangerous to use regularly. Only six astronauts ever wore them, the last in November of 19841. After the Challenger disaster, NASA cancelled all the potential flights which might use the MMU and unofficially retired them from use.

But, we don’t really need a picture of a female astronaut operating untethered, because Hibbeler got it wrong. Astronauts aren’t weightless in orbit because they’re far removed from Earth’s gravity; they’re weightless because they’re falling around Earth at the same rate as their spacecraft and everything in it2. We can use an interior photo, this classic picture of Mae Jemison aboard Endeavour being a logical choice:


Now was that really so hard?

1To be explicit about it: including that final mission, only four American women had reached orbit, and only one had performed an EVA. The Soviet Union, meanwhile, only ever sent up two female cosmonauts. Tereshkova, in 1963, was basically a publicity stunt: she never returned to space and the Soviets resumed all-male crews until Svetlana Savitskaya launched in 1982, by which point it was clear that American women were going to space to stay. Nevertheless, Savitskaya beat Kathryn Sullivan to the first female spacewalk by 78 days.

2If we wanted pictures where the inverse squared law has kicked in, our selection is limited to the deep space EVAs during Apollo 15, 16, and 17. Performed during the coast back to Earth, terrestrial gravity had lessened, though their weightlessness was still attributable to free fall. However, our selection is all-male: Al Worden, Ken Mattingly, and Ron Evans.

What Constitutes Space?

I’ve been writing about the assorted difficulties faced in astronautical engineering, but this presupposes a certain amount of background knowledge and was quickly getting out of hand. So let’s start with a simpler question: what is space, anyway?

Generally speaking, space is the zone beyond Earth’s atmosphere. This definition is problematic, however, because there’s no clean boundary between air and space. The US Standard Atmosphere goes up to 1000 km. The exosphere extends as high as 10,000 km. Yet many satellites (including the International Space Station) orbit much lower, and the conventional altitude considered to set the edge of space is only 100 km, or 62.1 miles.

This figure comes from the Hungarian engineer Theodore von Kármán. Among his considerable aerodynamic work, he performed a rough calculation of the altitude at which an airplane would need to travel orbital velocity to generate sufficient lift to counteract gravity, i.e. the transition from aeronautics to astronautics. It will vary moderately due to atmospheric conditions and usually lies slightly above 100 km, but that number has been widely accepted as a useful definition for the edge of space.

To better understand this value, we need to understand just what an orbit is.

Objects don’t stay in space because they’re high up. (It’s relatively easy to reach space, but considerably harder to stay there.) The gravity of any planet, Earth included, varies with an inverse square law, that is, the force which Earth exerts on an object is proportional to the reciprocal of the distance squared. This principle is known as Newton’s Law of Universal Gravitation. Its significance for the astronautical engineer is that moving a few hundred kilometers off the surface of Earth results in only a modest reduction of downward acceleration due to gravity.

To stay at altitude, a spacecraft does not counteract gravity, as an aircraft does. Instead, it travels laterally at sufficient speed that the arc of its curve is equal to the curvature of Earth itself. An orbit is a path to fall around an entire planet.

The classic example to illustrate this concept, which also comes from Newton, is a tremendous cannon placed atop a tall mountain (Everest’s height was not computed until the 1850s). As you can verify at home, an object thrown faster will land further away from the launch point, despite the downward acceleration being identical. In the case of our cannon, a projectile shot faster will land further from the foot of the mountain. Fire the projectile faster enough, and it will travel around a significant fraction of the Earth’s curvature. Firing it fast enough1 and after awhile it will swing back around to shatter the cannon from behind.

Newton_s_cannon_large.gif (313×242)

Source: European Space Agency

In this light, von Kármán’s definition is genius. While there is no theoretical lower bound on orbital altitude2, below about 100 km travelling at orbital velocity will result in a net upwards acceleration due to aerodynamic lift. Vehicles travelling below this altitude will essentially behave as airplanes, balancing the forces of thrust, lift, weight and drag—whereas vehicles above it will travel like satellites, relying entirely on their momentum to stay aloft indefinitely.

But we should really give consideration to aerodynamic drag in our analysis, because it poses a more practical limit on the altitude at which spacecraft can operate. Drag is the reason you won’t find airplanes flying at orbital speeds in the mesosphere, and the reason satellites don’t orbit just above the Kármán line. Even in the upper atmosphere, drag reduce a spacecraft’s forward velocity and therefore its kinetic energy, forcing it to orbit at a lower altitude.

This applies to all satellites, but above a few hundred kilometers is largely negligible. Spacecraft in low Earth orbit will generally decay after a number of years without repositioning; the International Space Station requires regular burns to maintain altitude. At a certain point, this drag will deorbit a satellite within a matter of days or even hours.

The precise altitude will depend on atmospheric conditions, orbital eccentricity, and the size, shape, and orientation of the satellite, but generally we state that stable orbits are not possible below 130 kilometers. This assumes a much higher apoapsis: a circular orbit below 150 km will decay just as quick. To stay aloft indefinitely, either frequent propulsion or a much higher orbit will be necessary3.

On the other hand, it is exceedingly difficult to fly a conventional airplane above the stratosphere, and even the rocket-powered X-15 had trouble breaking 50 miles, which is the US Air Force’s chosen definition. Only two X-15 flights crossed the Kármán line.

Ultimately, then, what constitutes the edge of space? From a strict scientific standpoint, there is no explicit boundary, but there are many practical ones. Which one to chose will depend on what purposes your definition needs to address. However, von Kármán’s suggestion of 100 km has been widely accepted by most major organizations, including the Fédération Aéronautique Internationale and NASA. Aircraft will rarely climb this high and spacecraft will rarely orbit so low, but perhaps having few flights through the ambiguous zone helps keep things less confusing.

1For most manned spaceflights, this works out to about 7,700 meters per second. The precise value will depend on altitude: higher spacecraft orbit slower, and lower spacecraft must orbit faster4. In our cannon example, it would be a fair bit higher, neglecting air resistance.

2The practical lower bound, of course, is the planet’s surface. The Newtonian view of orbits, however, works on the assumption that each planet can be approximated as a single point. This isn’t precisely true—a planet’s gravitation force will vary with the internal distribution of its mass, which astrodynamicists exploit to maintenance the orbits of satellites. That, however, goes beyond the scope of this introduction.

3The International Space Station orbits so low in part because most debris below 500 km reenters the atmosphere within a few years, reducing the risk of collision. This is no trivial concern—later shuttle missions to service the Hubble Space Telescope, which orbits at about 540 km, were orchestrated around the dangers posed by space junk.

4Paradoxically, we burn forward to raise an orbit, speeding up to eventually slow down. This makes perfect sense when we consider the reciprocal relationship between kinetic and potential energy, but that’s another post.

Book Review: Ignition!

Subtitled “An Informal History of Liquid Rocket Propellants”, Ignition! is John D. Clark’s personal account of working with rocket fuels from 1949 until his retirement in 1970.

Dr. Clark is introduced to us by Isaac Asimov. Clark was roommates with L. Sprague de Camp during his undergrad years at Caltech, and wrote a pair of science fiction stories before deciding the market wasn’t for him, though he remained active in the community. Dr. Asimov met him during the war, when he came to work with de Camp and Heinlein at the Philadelphia Naval Yard.

John Clark, like Asimov, was a chemist, working on the problem of chemical rockets for the majority of his career. He writes this book, he tells us, both “for the interested layman” and for:

[T]he professional engineer in the rocket business. For I have discovered that he is frequently abysmally ignorant of the history of his own profession, and, unless forcibly restrained, is almost certain to do something which, as we learned fifteen years ago, is not only stupid but is likely to result in catastrophe.

For the layman, he attempts (and, I think, succeeds) at writing in a manner which is nevertheless very accessible. The sections with heavy technical content can be skimmed over without losing too much of the overall picture, though a little background knowledge certainly helps. I’m not sure you could use this book as a reference without a basic understanding of engineering thermodynamics, but if you haven’t studied that what business do you have designing rocket engines?

Unfortunately, Dr. Clark gives relatively little in the way of citations or suggestions for further reading. This is both an artifact of the era—when technical reports and journal articles were essentially inaccessible to the general public if your local library didn’t have a copy—and a consequence of the fact that much of the source material was at the time still officially classified. At several points the discussion is cut short because he’s not at liberty to discuss the matter. He acknowledges these difficulties and makes not pretense of this being an authoritative textbook.

On a related note, the content is heavily focused on the research done in America and the United Kingdom, with a chapter devoted to what information came out of the Soviet Union in later years. Due to the date of publication, this book does not cover modern developments (though the final chapter makes a series of predictions I might come back and grade).

Nor does Clark address solid propellants or hybrid combinations in any significant detail, which is slightly disappointing given my current studies, but would have made for a much longer and more complicated read. Not that I would have particularly minded; Dr. Clark is an engaging storyteller, frequently giving us various background information on the scientists and organizations trying to develop early rockets, first for abstract research, later for the military, and finally for the National Aeronautics and Space Administration.

These anecdotes keep the reading fun even through the most tedious of minutiae on monoprops and halogen fuels. Clark frequently (if unpredictably) goes into detail on the chemistry of a particular propellant and how the molecules interact with one another. Such interludes eventually rekindled my interest in chemistry as a subject, which is fortunate since I need another credit hour of it to graduate. Hopefully some of the material I learn this summer will be relevant to aerospace propulsion work.

Overall, I found this to be a good introduction to rocket fuels and the history of that field. While useful for beginners such as myself and as a refresher, it probably shouldn’t be treated as any sort of reference guide or definitive citation.

ignition back cover

An engraving by Dr. Clark’s wife, Inga Pratt, presented to NARTS in 1959.

Hopefully one day Ignition! will be in print again, but for now most of us are stuck reading it from PDFs found online. Hard copies went for hundreds of dollars before the likes of Elon Musk and Scott Manley began publicly praising the book.

Book Review: How to Live on Mars

I first read this book in high school, flushed on newly-found philosophy and bristling with plans for life as a commercial astronaut. SpaceX was just ramping up their ISS resupply program; Bigelow Aerospace was planning to launch another module before 2014. The possibilities seemed limitless.

That’s not the world we ended up living in. Astronauts haven’t launched from the United States in over five years. Virgin Galactic experienced LCOV during a 2014 test flight and put space tourism plans on hold while fixing the spacecraft’s control system. The biggest leaps forward has been landing Falcon 9 first stages, but it’s only in the last week that a used stage flew again. Falcon Heavy  still hasn’t been tested flown.

As such, the overall mood of Zubrin’s book feels….overconfident. Misplaced. Premature.

Our narrator is a congenial Martian colonist, giving us the down-low on what it takes to survive on Mars. It’s quite easy, he informs us, provided your follow his advice.

From choosing the correct transfer method to how to start a family, Zubrin (the Martian, not the 20th century astronautical engineer) walks us through the steps of becoming an economic and social success on the red planet. While many of the specifics are tailored to a fictional future history, the basic science is strictly factual.

It ranges from the mundane to the transcendental. At the more everyday end of things, we learn how to make plastics and almost every other raw material from the Martian soil and atmosphere. Through this avatar, Dr. Zubrin is making the case that living on Mars is entirely feasible. Steel and cement for construction, oxygen for breathing, nitrates for food—it’s all there. A few things would be a challenge (fictional Zubrin recommends stealing rocket parts as the best way to obtain aluminum), but the low-gravity environment greatly reduces the difficulty imposed by all sorts of engineering projects.

On the other end of the scale, we’re explained the general process of terraforming Mars into a habitable planet (and how to profit off it in the meantime). Now quite a few of these suggestions rely on a fairly specific potential architecture for the project, but the technical information holds.

This future history is amusing, though evokes a more cynical reaction from me after the last few years. I’m less optimistic about the odds of us reaching Mars before 2040, and less skeptical of NASA’s ability to get things done. To me, the issue seems to be less one of organizational competence and more of insufficient dedication at the highest levels (mostly Congress). While I’d like to believe that the private sector can fill that gap, it seems increasingly unlikely that they can achieve those ends at a plausible cost as the march of 21st century politics continues.

One thing he’ll probably have gotten right: the decay of terrestrial society into atomized, post-modern nihilism. I hope he’ll be proven wrong but there’s no strong signals to suggest that that trend is slowing.

On the whole, though, an optimistic book about the capacity for human ingenuity to conquer new frontiers and expand our understanding of the universe. Those interested in the project of space colonization, but unsure where to begin learning about, would be well advised to start with How to Live on Mars.


German Researcher Discovers Most Efficient Path to Mars

A civil engineer in Essen, Germany has determined the transfer orbit which will get astronauts to Mars the quickest.

Walter Hohmann, a civil engineer, spent several years studying physics and astronomy before publishing his book The Attainability of the Celestial Bodies. It may become required reading for NASA mission planners.

Fuel requirements will be central to the architecture of interplanetary spaceflights, Dr. Hohmann expects. To account for this, he solved for the trajectory which requires the least amount of velocity change, or what scientists call “delta V”. Spacecraft produce this acceleration by firing rocket engines.

The most efficient orbit between two planets turned out to be an ellipse that lies tangent to the planets’ orbital paths.


Source: University of Arizona

Such an orbit requires the least amount of energy to achieve when starting from Earth, but has a serious drawback. Least-energy trajectories are also the slowest. For a crewed mission, taking along enough food and oxygen could make a less efficient path ultimately cheaper.

Another problem is waiting for planets to be in the right place for launch. Because Earth orbits the sun faster than the outer planets and slower than the inner planets, the possible alignment for such a transfer trajectory only occurs occasionally. The window to leave for Mars only opens every two years, for example. Launching interplanetary spacecraft at other times would require vastly more fuel.

Nevertheless, astronomers and aerospace engineers find Dr. Hohmann’s discovery extremely useful for designing space missions.

Happy Amazing Breakthrough Day!

The Worst Week of American Spaceflight

On January 27th, 1967,the crew of Apollo 1 was undergoing a simulated countdown when an electrical fire started within the spacecraft. The hatch was bolted tightly onto the capsule. Escape was impossible and the blaze quickly grew in a pure oxygen atmosphere. Astronauts Gus Grissom, Ed White, and Roger Chaffee died on the pad.

On January 28th, 1986, the space shuttle Challenger was destroyed was destroyed 73 seconds after lift off for the STS-51L mission. Cold weather in the days before launch had weakened the rubber o-rings sealing sections of the solid rocket boosters. Flames escaped and penetrated the external fuel tank, igniting an explosion of liquid hydrogen and oxygen that disintegrated the orbiter vehicle. The crew was not killed in the explosion—forensic investigation revealed that pilot Michael Smith’s emergency oxygen supply had been activated, and consumed for two and a half minutes: the amount of time between the break-up to when the remains of Challenger landed in the Atlantic Ocean.

On February 1, 2003, the space shuttle Columbia disintegrated during re-entry over the southern United States after sixteen days in orbit. During launch, a piece of cryogenic insulation foam fell from the external fuel tank and struck the left wing of the orbiter, damaging the thermal protection system. As Columbia streaked across the southern sky, atmospheric gases heated by its hypersonic flight entered the wing and melted critical structural members. Ground observers in Texas could see the shuttle breaking apart over their heads. Rapid cabin depressurization incapacitated the crew.

This is the worst week in the history of American spaceflight. These three disasters are not the only dark spots on that record, by they are by far the worst. We remember them, and vow not to repeat the mistakes that led to their deaths.

After Apollo 1, Flight Director Gene Kranz gave the following address to his mission controllers:

Spaceflight will never tolerate carelessness, incapacity, and neglect. Somewhere, somehow, we screwed up. It could have been in design, build, or test. Whatever it was, we should have caught it.

We were too gung ho about the schedule and we locked out all of the problems we saw each day in our work. Every element of the program was in trouble and so were we. The simulators were not working, Mission Control was behind in virtually every area, and the flight and test procedures changed daily. Nothing we did had any shelf life. Not one of us stood up and said, “Dammit, stop!”

I don’t know what Thompson’s committee will find as the cause, but I know what I find. We are the cause! We were not ready! We did not do our job. We were rolling the dice, hoping that things would come together by launch day, when in our hearts we knew it would take a miracle. We were pushing the schedule and betting that the Cape would slip before we did.

From this day forward, Flight Control will be known by two words: “Tough and Competent.” Tough means we are forever accountable for what we do or what we fail to do. We will never again compromise our responsibilities. Every time we walk into Mission Control we will know what we stand for.

Competent means we will never take anything for granted. We will never be found short in our knowledge and in our skills. Mission Control will be perfect.

When you leave this meeting today you will go to your office and the first thing you will do there is to write “Tough and Competent” on your blackboards. It will never be erased. Each day when you enter the room these words will remind you of the price paid by Grissom, White, and Chaffee.

Gene Kranz is right. Tough competence is what those of us in the space business must strive to be, every day, for lives are on the line, and the future of manned exploration of the cosmos is at stake.

These seventeen are not the only space travelers to die in the line of their work, and undoubtedly more astronauts and cosmonauts will perish in our conquest of the universe. That is no excuse for sloppiness. The Apollo 1 fire could have been prevented. STS-51L should not have launched. STS-107 could have been saved on-orbit. It’s the job of engineers, technicians, flight controllers, and fellow astronauts to see accidents before they occur and prevent them from happening.