More about lightsails

The deployed solar sail from an on-board camera (The Planetary Society)

A few weeks ago I blogged about lightsails, and in particular mentioned the Planetary Society’s spaceship LightSail 2, which was launched specifically in order to test this technology. The idea was relatively simple – get a small satellite, about the size of a cereal box, into earth orbit, then deploy the sail and see whether the orbit can be controlled using solar radiation alone.

Now, this isn’t really the sphere of operations that you would generally consider a lightsail – they function at their best when on a long journey and can build up momentum second by second. Here in Earth orbit, the overall effect is to make the orbit more elliptical – one part of the orbit is raised in altitude, but another part is lowered, and at some point the little satellite will encounter too much resistance from the atmosphere and will come down, burning up as it does so. The advantage of doing it so close to home is that there is hardly any signal lag, so controlling the sail’s angle, and tracking the consequences of changes, is very much easier.

Light sail control data (Purdue University)

To cut a long story short, the experiment worked. After a couple of weeks, the orbit had been raised around 3km. That doesn’t sound much, but it’s enough to show that the whole thing is controllable. A lot of analysis has been carried out on the orbital changes – you can imagine that as the satellite goes around the Earth, the angle relative to the sun is constantly changing. It was important to show that the observed changes were the result of ground commands, not just the random effects of sunlight shining at odd angles. So the orbital data has been heavily scrutinised, and came out successfully at the end.

Colour-corrected image of Earth partly obscured by the sail from the onboard camera (Planetary Society)

The extended mission period also gave the ground control team experience in how to best use the constantly changing angle. By the end of those two weeks of deployment, they had learned what worked well and what didn’t. It’s good experience for this kind of mission, but as I said earlier, a more realistic use-case would be to go on a transfer trajectory to a more remote destination – say Mars – and on such a journey. the angle between sail and sun would not vary anywhere near so much.

The experiment will continue through the rest of August, maybe a bit longer, and anyone who wants to see the current status can go to http://www.planetary.org/explore/projects/lightsail-solar-sailing/lightsail-mission-control.html which gibves all kinds of geeky information as well as a neat map showing the current location of LightSail 2.

While talking about space news, it’s certainly worth mentioning India’s Chandrayaan 2 mission. That has just left Earth orbit, and aims to soft-land about 600km from the Moon’s south pole in about a week. The approach used is similar to that of Israel’s Beresheet, in which a series of gradually elongated elliptical orbits around the Earth is eventually traded at a transfer point to a series of gradually diminishing orbits around the Moon. The lunar south pole is thought to be the most promising location for water ice, lurking on the surface in deep shadow areas and hence available very rapidly for human use. Proving that this really is – or maybe is not – the case is an important step towards building a permanent settlement on the Moon. The landing itself is scheduled for early September. The main mission web site is at https://www.isro.gov.in/chandrayaan2-home-0 and here’s a short video describing it.

Hopefully I shall be saying some more about that in September. But inevitably at present, the question for this blog is what these events have to do with fiction. My own vision of the future exploration of the solar system has spaceships using an ion drive rather than lightsails, since I expect these to be faster, and more effective in the volume outside the asteroid belt, as solar radiation drops off. But I can easily image automated lightsail ships being used for cargo which is not time-critical – not unlike how we send some freight by air and some by water today.

But the lunar south pole has been suggested many times as a good place to build a base, going back at least to Buzz Aldrin’s Encounter with Tiber. I makes perfect sense to me, and it would be great if Chandrayaan 2 was able to directly confirm that water ice is there.

Concrete and Low Gravity

An early stage…

Every now and again I have cause to get involved in one or other building project up here in Cumbria – not exactly something I reckon to have much aptitude in, but there’s always need for spare pairs of hands. And as the job gets moving around me, I always start thinking about how much more difficult the job would be in the micro-gravity of orbit, or indeed on some planet where the atmosphere is different to our own. Mars maybe. So many of our current practices and presumptions about building and making things derive from working on a planet with a decent level of gravity, and where the ambient temperature and air pressure are conducive to helping the project moving along. Of course, there’s something of a circular argument buried in that, since we have had to work with Earth’s conditions for a very many years. Presumably if we had evolved and grown up on Mars we would work things differently, and wonder to ourselves how anyone could possibly construct buildings in three times the surface gravity and a hundred times the air pressure!

Now the particular job this week was laying a concrete floor – as you can see from the pictures, it was making a new layer to even up the various levels of an existing floor. What may not be so obvious is that it also slopes gradually from back to front (to provide some drainage), so there was some nifty preparatory work with wooden beams to provide the necessary angle to smooth off against. You can see some of these in the next picture. The whole floor will – in a few weeks – support a canning machine for several of our beers, so there’ll be other installation stages as time goes by.

A bit later…

The concrete itself came ready-mixed, in one of those neat little lorries that do the mixing as they are driving along to you, and then pour it out in smaller or larger dollops as the need arises. With the confined space we had to work in (confined as regards a truck, not a human) this meant lots of smallish dollops into wheelbarrows which were then tipped in whatever place was necessary. So the lorry itself exercised some of my low gravity pondering. The mixer relies on gravity to thoroughly muddle all the different components up as the barrel turns – no gravity, then no mixing. The water, sand, shingle, cement and what have you would all just gloop around and not combine into a single substance which will set hard. In orbit, or on an asteroid, you’d have to design and build a different way to mix things up. Then the act of pouring relies on gravity to pull the stuff down a chute into a waiting wheelbarrow. I guess you’d have to have something like a toothpaste tube, or the gadgets you use to apply icing to cakes.

Finished product (1)

Laying concrete basically consists of a couple of stages: first you plonk barrowloads or shovelfuls where you want them, and then you smooth it down, broadly by means of a wooden plank laid across two guide beams, and in fine by means of a trowel or similar instrument. So you need a definite sense of what’s down, you need to be able to press down onto the initially rough and lumpy surface, and you need inertia and friction to help you, and . In micro-gravity you have none of these things. Any direction can be down, it’s impossible to press without first bracing yourself on some convenient opposing support, and although inertia and friction are still present, they don’t necessarily operate in the ways or directions you expect. There are not many concrete floors on the ISS, nor wil there be if the space station were to remain up there a long time.

After that you wait for the concrete to set – part of that is just water evaporating, and part is chemical reactions between the various constituents. And it’s kind of important that it sets at a sensible rate, neither too fast nor too slow. Now, if you poured out that same floor on Mars, I’m not sure the end result would be the same. Certainly the water would evaporate, but in all probability this would happen rather too quickly for comfort. What about the chemistry? The average Martian surface temperature is about -63° Centigrade, compared with say 14° C on Earth as an overall average. I don’t know if the necessary chemical reactions would happen at that temperature, but I have a suspicion that they might not. You could end up with a floor that was weak or brittle.

In short, a task that took five of us a few hours of a morning, without too much frustration or difficulty, could well become profoundly difficult or even impossible elsewhere in the solar system. So when I write about near future space habitats – the “domes” of my various stories – I always assume that they are made by very large versions of 3D printers. The technology to print buildings has been demonstrated on an Earth scale for disaster relief and similar occasions, and it makes a whole lot more sense to send a large printer to another planet and use local materials, rather than to send sacks of sand, cement etc across space, and then hope that the end result will be acceptable! Meanwhile, here on Earth I dare say we will be laying concrete floors for a long time yet.

Finished product (2)

A basic introduction to the Solar System

Cover - Far from the Spaceports
Cover – Far from the Spaceports

I needed to write a sort of general introduction to the solar system assumed by Far from the Spaceports and its various sequels – the exact reason for this must wait for another day to reveal, but I found the exercise interesting in its own right. Most of the future facts are pretty obvious when you are immersed in the books, but it may be helpful to have them all summed up in a neat way.

So here it is: the future history of the solar system – or at least edited highlights thereof – spanning the next century or so.

The solar system of the Far from the Spaceports series

The great breakthrough that allowed widespread human colonisation of the solar system was the development of a reliable high-performance ion drive for spaceship propulsion. The first successful deployment of this technology in experimental form was in 1998, and successive improvements led to near-complete adoption by around 2050. By the time of Far from the Spaceports and the sequels, old-style chemical rockets are now only used for shuttle service between a planet’s surface and orbital docks, with the ion drive taking over from orbit.

NEXT ion drive in operation (NASA)

The great virtue of the ion drive is that it provides continual acceleration over a long period of time, rather than big delta-v changes at start and end of the journey followed by a long weightless coast period. Thus, although the acceleration rate is very low, the end result is a much faster trip than when using chemical rockets. With the kinds of engine available in the stories, a journey from Earth to the asteroid belt takes an average of three weeks, the exact time depending on the relative orbital position of the target as compared to Earth. Longer journeys are more efficient if you avoid making interim stops – breaking a journey half way makes the travel time nearly half as long again as just going direct, because of the time wasted slowing down and then speeding up again. As a result, trade or passenger routes typically go straight from origin to destination, avoiding intermediate stopovers.

At around the same time, artificially intelligent software reached a stage where the systems were generally accepted as authentic individuals, with similar rights and opportunities to humans. Known as personas, they are distinguished from simpler AI devices which are simply machines without personality. Personas have gender and emotion as well as logic and algorithms. Slate is the persona who features most prominently in the early stories in the series. In terms of early 21st century AI development, Slate is a closer relative to digital assistants such as Alexa, Siri or Cortana, than she is to humanoid robots. As a result, she can – with effort and care – be transferred into any sufficiently capable computer system if the need arises.

Amazon Dot - Active
Amazon Dot – Active

The first generation of personas to go out on general release were called the Stele class – Slate is one of these. About a decade later, around the time of The Authentication Key (in progress), the Sapling class was released, and after another decade the Scribe class appeared. Steles are regarded as solid and reliable, while Saplings are more flighty, being prone to acting on impulse. Scribes are stricter and more literal. They first appear in The Liminal Zone (in progress). There are plenty of sub-persona machines around, serving specific tasks which do not require high levels of flexibility of intelligence or awareness.

Solar system colonisation has proceeded in a series of waves, and at any time some habitats are flourishing while others have been left behind the crest of the wave. The original motivation for settlement was typically mining – bulk extraction of metals and minerals could be done more cheaply and with fewer political constraints away from Earth’s surface. However, there are many places which appeared at first sight to be profitable, but which subsequently proved to be uncompetitive. Many settlements have had to rethink their purpose of being, and the kinds of industry or service they can offer. Very often, as you get to know a new place, you see the signs of this rethink – perhaps an old warehouse or chemical extraction factory has been converted to a new function such as accommodation or finance.

Phobos, NASA/JPL
Phobos, NASA/JPL

A habitat is routinely called a dome, even though few are actually dome-shaped. Very often several units will be loosely connected by passageways or flexible tubes, as well as delving underground if the surface rocks permit. The first stage of settlement was usually to deploy one or more giant three-D printers to construct the habitat shells from native material. After that, individual customisations have been added according to need, taste or whimsy. The biggest single threat to a dome is typically some kind of fault or crack exposing the occupants to the surface environment of the planet, asteroid or moon – normally this is quickly fatal. Hence each dome has its own set of rules for managing this risk, which are very strictly enforced.

There is no unified solar system political or economic authority. Each habitat manages its own internal affairs in broad alignment with its current purpose for existence. Some are essentially puppet offices for large corporations, others are scholarly or academic research stations, but most have achieved a degree of economic independence and are self-governing. It is generally believed that travel lags of a few weeks or months prevent effective government from elsewhere. Notions of political control are usually set aside because of the constant need to cope with the many external hazards faced by anyone in a spaceship, or on the surface of an inhospitable planet or moon. Each habitat, then, protects its own interests as it sees fit, including monitoring the volume of space immediately nearby, and adopts a laissez-faire attitude to other habitats.

Alexa Timing logo
Alexa Timing logo

Most habitats are culturally and racially mixed, and people’s names are often the most obvious memories of the Earthly heritage of their family. A few places, depending on the circumstances of their foundation, reflect a particular single culture group. It can be difficult for outsiders to integrate into these. But generally speaking, a person gets the reaction that their conduct deserves, regardless of their place of family origin. It can be very difficult to recover from a bad impression created on first meeting. Conversely, a person who shows that they are respectful of local customs, and have particular skills that contribute to the life of the habitat, will find no difficulty fitting in.

Welcome to the world of Far from the Spaceports!

Artist’s impression – Dawn’s ion drive (NASA)

Lightsails

NASA illustration showing how the sail might be supported by struts

Lightsails, or solar sails, are an idea which has cropped up as a speculative way to propel spaceships many times since (at least) the early 17th century. In 1610, Johannes Kepler wrote to Galileo “Provide ships or sails adapted to the heavenly breezes, and there will be some who will brave even that void” – this seems to have been inspired by noticing that the tails of comets always faced away from the sun, rather than pointing back along their direction of motion. The analogy with sailing ships was powerful and persuasive, and many people over the years embellished on it in both fiction and mathematical exploration. I should mention that, at least at present, I am not planning to use lightsails in my own science fiction series, though it is tempting just because of the elegance of the idea!

The science of the solar sail (Wiki)

It was soon obvious that in order to work, the sail must have a huge area compared to the relatively small payload or cabin space – writers talked about “tremendous mirrors of very thin sheets”, or “wings of metallic foil of a square kilometre or more in area”, or “large, metallic wings, acres in extent”. The huge advantage over a conventional spacecraft is that it carries no fuel, except possibly for something to power small attitude-correction thrusters. The fuel source is the sun itself, and provided that the angle of the sail is kept accurately maintained, acceleration goes on every second of every day, allowing quite remarkable speeds to be attained in time. The downside, of course, is that the further out you travel, the less light falls on you, and hence the less acceleration can be achieved.

Model of Japanese IKAROS lightsail spaceship (Wiki)

A number of proposals have been made to address this. One is to build an array of giant lasers at some suitable way-station, which would supplement the waning light received from the sun. Another is to adopt a trajectory which dips close in to the sun, talking maximum advantage of the intense light there, before heading out towards the real target. And a third approach, which has only been made possible as technology has become extremely miniaturised, is to make the payload tiny. For example, something the size of a fair-sized mobile phone can carry a lot of instrumentation, but weighs a tiny fraction of a vessel able to carry humans and their cargo.

Travel times to the inner planets (out as far as Mars) take something like six months to a year to complete. If you wanted to go to the outer planets (Jupiter and on) then you’re talking a few years – a couple to Jupiter itself, and less than ten to get to Neptune or Pluto. And – assuming you have already built suitable acceleration lasers – you could get to nearby stars in a few decades. And all without the need to take large quantities of fuel. It’s not fast, but then neither are conventional methods – it took the Juno probe about five years to reach Jupiter, and the Cassini probe nearly seven to get to Saturn, using the current standard method of using a big burn at the start, followed by a long coasting period with occasional course corrections.

LightSail 2, artist’s impression (The Planetary Society)

So there’s a lot of interest in exploring this technology, and my immediate trigger for writing this was the Planetary Society’s LightSail 2 spacecraft , which was launched on top of one of SpaceX’s Falcon rockets. Over the next few weeks and months it wil carry out a series of proof of concept maneuvers. Several years ago, the Japanese IKAROS project showed that solar radiation could indeed be used in a live spaceship to adjust course and speed – no great surprise, but actually getting engineering proof was a great achievement. Perhaps the most ambitious currently planned mission is the Breakthrough Starshot project, which hopes to send a fleet of about a thousand miniature spaceships to Alpha Centauri, the nearest star, in order to fly by its planets and return information. This journey, presupposing the planned laser propulsion array can be built, should take 20 or 30 years, and the current plan is to launch in 2036. I might still be alive to see the outcome!

Finally, I would be remiss if I failed to mention the (fictional) solar sail ship which features in an episode of Star Trek Deep Space, in which it is called a lightship. Here, our intrepid captain and his son recapitulate a traditional journey taken from the planet Bajor in a rather steampunk-looking vessel – the trip is successful, though they are boosted not just by solar radiation but also by unusual space conditions… presumably so the journey can take weeks rather than decades!

Bajoran lightship (Star Trek Deep Space 9, image from Memory Alpha)

“The Immortal Yew” – Some Thoughts

Cover, The Immortal Yew (Amazon)

As a digression from my recent science fiction posts, here’s one about the natural world, and its intersection with history. I have been reading through the Royal Botanic Gardens’ book The Immortal Yew, written by Tony Hall, and finding it fascinating.

The first part of the book covers, in a kind of whistle-stop tour, various snippets of curious facts and suppositions about yew trees, while the remaining 4/5 lists a total of 76 particularly impressive yews around the country. Most of these are in England and Wales, with a few in Ireland and one in Scotland.

I guess most of us encounter yew trees in churchyards – the jury is out as to whether the origin of this custom was spiritual (yew trees have symbolised immortality and resurrection in more faiths than just Christianity) or practical (it stopped farm animals from grazing their way through the graves). Either way, this location has meant that the trees were protected from casual lopping, and so have survived. And indeed the majority of the showcased yews are in churchyards.

Martindale Yew branches
Martindale Yew branches working their way across the soil

It is surprisingly hard to determine how old a yew tree is – the main trunk hollows out after a few centuries, losing all the heartwood and almost all the associated tree rings. To add confusion, a few centuries later still, the tree puts down another central shaft which, as it were, grows in place of the original trunk. All the while, the original bark keeps growing around the outside like a kind of shell. Low branches drape along the ground and frequently put down their own roots, resulting in a cluster of rooted trunks. It is often hard to tell whether we are looking at a single tree or several grouped closely together. Historical records can help, and typically tell us that some of these yews are well over a millennium old. How much over a millennium? We just don’t know, but there is circumstantial evidence that yews can live for perhaps 3000 years. Such trees considerably predate the churchyards they find around them. It is likely that yews are the oldest living things in Europe. The Martindale Yew (close to Ullswater lake) may well be 1500 years old. The church building (known as Old Martindale church, to distinguish it from the new one up the road) dates back to 1220 – a respectable age, but dwarfed by the tree it nestles beside.

With such antiquity, and a whole slew of medicinal and military associations, yews have a firm place in European folklore. One snippet I particularly liked related to Yggdrasil, the Norse tree of life connecting the various worlds. Normally reckoned to be an ash tree (Wikipedia certainly thinks so), the references in the Poetic Edda suggest it is both evergreen and needle-bearing… neither of which applies to ash trees. Was Yggdrasil a gigantic yew tree? Seeing some of the magnificent specimens photographed for The Immortal Yew, it is easy to think so.

So next time you are near a churchyard, drop in to say hello to the yew tree which will almost certainly be growing there, and think about what it has witnessed during its lifetime. Each and every yew has quite a story wrapped up in its substance, and could be woven equally into history or fantasy.

The Martindale Yew with the church in the background

Orbits 3

Theatrical poster for Chinese release (Wiki)

The third and last of this short series on orbits was inspired by a Chinese science fiction film I have been watching on Netflix, together with an analysis I read of the science proposed. The film is The Wandering Earth, and is based on a novella by Liu Cixin, who is perhaps better known as the author of the splendid book The Three-Body Problem. Chinese science fiction is interesting to read – of course it is based on the same sorts of postulated scientific breakthroughs as European or American books, but the perception of recent history is very different, as is the kind of future society that we might expect to live in. There is an assumption that international cooperation will happen naturally in a crisis, led by Chinese technology and expertise. It’s a refreshing change (for a European) than the typical assumption many American authors make, that a world government would be bound to be based on American soil and largely staffed by US citizens!

Now, The Wandering Earth is set some forty years in the future, in which the sun is rapidly expanding, and humans are forced to try to migrate the entire planet Earth to a new home in the neighbouring star system, Alpha Centauri. The entire journey is expected to take something like 2500 years. It’s a bold twist on the idea of the generation ship, in which a group of a few hundred or thousand individuals live on board a large but conventional spaceship, expecting to pass through many generations before arriving at their destination. Here, the ship is the whole planet, and the hopeful survivors are numbered in billions. To achieve this, the Earth is equipped with a huge number of fusion-powered thrusters which slowly propel the planet away from our sun. The crisis of the film occurs as they attempt to use the gravity of the planet Jupiter as a slingshot to get more speed.

Now, the basic scenario of the sun expanding is not actually expected to occur for another five billion years or so, so a forty year time horizon is a bit crazy… but it does allow us to witness near-contemporary technology and human attitudes at work in a crisis, and I am enthusiastic about books set in this near time horizon! But could the thruster idea actually work? Could the Earth’s orbit be altered by such a means?

Poster – Armageddon film (Wiki)

The perennial threat of a meteor or asteroid on collision course with Earth has triggered a few suggestions as to how we would shift the orbit of that incoming body. A large bomb, for example, or a long series of smaller ones, detonated on the surface of the meteor so as to deflect irts course. These methods only really work on something that is small to start with, and any explosion large enough to shift the Earth’s orbit is probably going to do something catastrophic to the land, sea, atmosphere, or all of them together! So we can forget that one, or similar (but less explosive) variations such as docking a spaceship and pushing the body to one side using the main engines.

You could imagine launching a long series of rockets all from (roughly) the same place, which would tend to push the Earth in the opposite direction. And they would also carry up material from the Earth in the form of body and fuel weight. If you wanted to get our Earth to the orbit of Mars doing this, you’d use up around 85% of the Earth’s mass to do so – in other words you would get there, but with only 15% of the planet left. Doesn’t sound appealing.

NASA xenon ion thruster under test (NASA)

A more effective solution is an ion drive thruster (which I am keen on for other reasons as well, and which features in my own science fiction series). This, indeed, is the solution adopted in The Wandering Earth – large thruster drives are located on the Earth’s surface at major cities, while the population move underground to keep warm and avoid pollution. You keep more of the Earth’s native material this way – getting to Mars only uses up 13% of the Earth. Indeed, lots of the outdoor shots in the film show colossal excavation and earth-moving machinery tirelessly at work to feed the engines.

To avoid swallowing up the Earth as fuel, you have to use some external source. Two come to mind. The first is the light of the sun,. captured with some sort of mirror or solar sail. It’s slow, but you could achieve the desired effect in about a billion years. Not good enough for the film’s plot, but actually it would easily suffice for the real situation our remote descendants will need to tackle. The second is to exploit the huge amount of matter drifting around our solar system in the form of asteroids, and deflect these into new orbits which graze past our Earth. As they do this, the same slingshot effect that we normally use to accelerate small spacecraft can be exploited to move the Earth. Each interaction achieves a minuscule change, but a huge number of them eventually gets the job done. Of course, you’d have to have extreme trust in the orbital calculations, since the method relies totally on getting this long stream of asteroids as close as possible to the Earth without actually colliding with us! This, bizarre as it sounds, seems to be the most effective way to solve the problem. Each asteroid or comet can be used multiple times (until their own orbit degrades so much that they fall into the sun), so you just keep the asteroid train going round and round, pulling the Earth a little at a time. Again, it wouldn’t do for the immediate crisis presumed by the film, but it could work in the real-life long-term scenario.

So here is the conclusion of the series – we have migrated from the problems of getting into Earth orbit, to moving around between orbits, to the vastly bigger goal of moving the entire Earth in its own orbit around the sun. The common feature – you’d better do the calculations right in order to get where you want, and your intuition about how to go from A to B is not always right!

From The Wandering Earth (BBC)

More about Doggerland

Brown Bank, North Sea (BBC)

I was going to do my third post in the series about orbits, but that intention was derailed by reading a rather fascinating preliminary report from the research vessel Belgica, which spent 11 days dredging parts of the North Sea between England and the Netherlands, in the Brown Bank region where, so far as we can tell, the current sea floor is not so very different from the ground surface not long after the last ice age, when the ice had receded north, and Doggerland flourished as an inhabited part of northern Europe. Further south and west, layers of silt from the Thames and Rhine have tended to cover over the ancient layer, so better results are found by exploring north of a line from Great Yarmouth to Rotterdam.

Carved bison bone with zigzag decorations typical of the late Paleolithic (National Museum of Antiquities, Netherlands)

Anyway, the science team on the Belgica found evidence for a fossilised forest, together with peat residues suggesting adjacent wetlands. Both of these terrain kinds are associated with human settlements in this era (around 10-12,000 years ago), and the team are hopeful that a return visit later this year will – quite literally – uncover decisive signs of human settlement. Slowly over recent years, largely through cores drilled out for quite different reasons such as oil prospecting, both human remains and ancient artefacts have been found, but the picture so far is scattered and hard to interpret.

As well as dredging operations, other surveys have been carried out to try to map the original land surface below today’s water, in order to get a sense of the overall topography. This has helped shift the perception of Doggerland from being “just” a land bridge joining what is now the United Kingdom to continental Europe, towards the sense of a huge area containing many different human hunter-gatherer groups, each probably specialising in one particular terrain type. The total population would have been in the thousands. I guess we all know the end – over a period of many years, but probably punctuated by sudden crises every so often, the land

One day, hopefully, I shall get to write a book about Doggerland, probably to be set in the final years of its decline. Meanwhile, I shall be following news of its rediscovery with interest…

Doggerland as the ice retreated (Sonja Grimm)

Orbits 2

Gravity film release poster (Wiki)

Last time I talked a bit about orbital paths for getting from the Earth to the Moon, and how there are several alternatives, each using different amounts of time and fuel. Today I want to talk a bit about orbits around Earth – the kind of trajectories used by a multitude of satellites today. This post was partly motivated by a conversation in the film Gravity – a film which I quite enjoyed for the human interactions, but was seriously disappointed by the cavalier attitude to orbital mechanics! There’s a point fairly late on where George Clooney’s character points out to Sandra Bullock’s character an orbital space station where she can find additional supplies, and directs her to fire her own capsule engines while keeping the other station central in her forward window.

It sounds good – and it would sort of work on Earth if you translate Sandra Bullock’s plight into moving cars around a car park – but there is absolutely no way that such a strategy would get her to her target up in orbit. Quite apart from the matter of getting the two space vehicles to meet up in space, there’s also the problem of matching speeds so that she could easily move across to the second capsule. Earth orbits simply don’t work like that – it is certainly possible (with sufficient fuel) to transition from one orbit to another, but getting the engine burn right is a difficult task, and one which needs to be carried out by computer program rather than optimistic guesswork.

Hohmann transfer orbit (Wiki)

The most energy-efficient way to transition from one circular orbit to another is by means of what is called a Hohmann transfer orbit – an elliptical path that touches both circles at a tangent. Other transfers can be used, and may well take less time to complete, but they will use more fuel. The simple picture here assumes that both orbits are in the same plane – in a more general case it will be necessary to shift orbital plane from the old angle to the new one. But that aside, just looking at the two objects shows that the engine burn required is not at all directed straight at the target – indeed in this case it is almost diametrically opposite. (Hohmann transfers are also used on a larger scale for trajectories between planets, and exactly the same principles apply).

Now, the great majority of satellites are in Low Earth Orbit (LEO) – not a single trajectory, but a zone of space from about 200 to 2000km altitude, or equivalently with an orbital period of under two hours. LEO is easy to get to, and hence economical of fuel, but especially at the lower end of the range, orbits will slowly decay because of the very tenuous atmosphere still present there – hence either the lifetime is limited, or occasional actions must be taken to lift the orbital height. A consequence of such a low orbit is that the spacecraft transits the sky quickly – or equivalently, any one part of Earth stays in view for only a short time. Satellites providing the various GPS systems orbit much higher, around 20-30000km (12-15 hours). And higher still, a little over 40000km, are the geostationary orbits, in which a satellite takes 24 hours to orbit the planet, and so appears to be stationary over a particular place.

Cover - Far from the Spaceports
Cover – Far from the Spaceports

What does this have to do with writing? Well, maneuvering around a planet in low orbit is a tricky thing to get right, and not something you want to leave to intuition. In Far from the Spaceports, Mitnash gets a shuttle up to his spaceship, The Harbour Porpoise, which the AI persona Slate has been keeping in LEO waiting a decision on where they were heading off for. They end up going off to the asteroid belt, but their first move would be to get out of LEO and leave Earth’s gravity well behind. That’s not something where Mitnash would glance out of the window, push a couple of buttons, and hope for the best – Slate, or possibly the ship’s own onboard controller, would do some real calculations before firing up the main engines. And then off they go…

Next time, a zany idea I just read about concerning moving the orbit of the Earth itself…

Orbits 1

Artist’s impression of Beresheet probe mid-flight (Israel Space Agency)

A few weeks ago now, an Israeli space probe called Beresheet (“Beginning”) reached the moon’s surface – sadly a system glitch in the last few seconds of approach meant that it crashed rather than soft-landed, but nevertheless it was a remarkable achievement. A particularly interesting feature of the trajectory that the ground crew chose is that it is quite unlike the method adopted by pretty much every probe before now, including the Apollo spacecraft.

Successive orbit paths (Israel Space Agency)

The typical way to reach the moon has been in three phases – up to Earth orbit first, then a substantial burn of the main engines to escape Earth orbit and head towards the moon, then another burn to enter lunar orbit. The long leg of this is traversed with no engine activity at all, barring some trivial course corrections. Beresheet, however, adopted quite a different approach, as shown in the accompanying picture. A series of much smaller engine burns shifted the orbital path round Earth into ever-longer ellipses, until the path was close enough to the moon to be captured by the gravity there. The probe never attained escape velocity from Earth, and the trip took rather longer to arrive – a couple of months rather than a few days. However, it used less fuel and had the advantage that the successive changes in orbit allowed for lots of checking and fine-tuning.

Almost all probes to date, including Beresheet, have been driven by chemical rockets. They can exert huge acceleration at need, but have the disadvantage that the fuel tanks empty comparatively quickly. The times when the engines are used have to be rationed and carefully calculated, in order to have enough left over for critical events late on in the spacecraft’s journey. About the only exception to this is the Dawn probe, which visited the asteroid belt and returned remarkable new details about Vesta and Ceres. Dawn used an ion drive, which yields comparatively little acceleration but can be run continuously for weeks or months at a time. It’s a neat way to pile up substantial velocity without using much fuel at a time. It’s also the spaceship drive that I presuppose for Far from the Spaceports and successive books in that series – from a human-traveller point of view it has the huge advantage that you don’t have to put up with long periods of weightless travel, as well as considerably reducing travel times. The fact that the drive runs continually for all that time hugely offsets the fact that the actual propulsive power is much smaller.

That’s it for this week – next week there’ll be a few more thoughts on orbits. Meanwhile, here’s the last picture Beresheet took before crashing into the lunar surface…

Beresheet’s final image (Israel Space Agency)

About a podcast

Absolute Business MIndset podcast logo

A short blog today as I get back into blog writing after a very busy Easter. And it’s something a little bit different for me – a friend and former work colleague interviewed me for his podcast series over the weekend, and it has now gone live.

Now, I’ve never really got into podcasts, and Marks’ normal focus for his series is to do with business (as you can tell from his series title, Absolute Business Mindset), but we both managed to make something of the interaction.

Different people use different podcast software, but this site
https://gopod.me/1340548096 gives you a list of different options through which you can access the interview. Alternatively, search for Mark’s series by its title, Absolute Business Mindset.

In it, you can hear me talking with Mark about all kinds of stuff, largely focused around maths, artificial intelligence, Alexa and so on, ultimately touching on science fiction. The whole thing takes about an hour, and Alexa takes more of a central role in the second half. Enjoy!

Writing, both historical and speculative