Category Archives: Space

Gravity Patterns

Stephen Hawking in a reduced-gravity aircraft flight, 2007 (Wiki)

Gravity is a curious thing. On an everyday level, almost all of us experience it in such a steady, unchanging way that it vanishes from our conscious attention. From time to time we notice change – the sudden acceleration of a lift in a tall building, or a ride in a theme park. A few people fly aircraft in such a way as to handle serious g-forces, and an even smaller handful have been in the microgravity of Earth’s orbit (or a very specific aircraft trajectory intended to mimic conditions in space). But for most of us, most of the time, it is just there as a constant part of our environment.

In terms of physics, gravity is the odd one out of the standard four forces of nature (the others being electromagnetism, plus the weak and strong nuclear forces). It is odd for a couple of reasons – first, it is immensely weaker than the others, but secondly, it is always an attractive force rather than sometimes attracting and sometimes repelling… so far as we known in 2019. There has been a recent report of a Hungarian team discovering a fifth force, but this has not been confirmed by other teams yet, and in any case will not change the gist of this blog post. The forces other than gravity tend to cancel out over any great distance, and only really affect things on very short distances. But gravity, despite its comparative weakness, really does shape the way the universe looks and behaves.

Poster, Gravity (IMDB)

Gravity in films usually gets treated in very cavalier ways. The most striking example of this is the 2013 film Gravity, which was built on an interesting premise but often failed at the science. In one especially memorable moment, George Cluny’s character tells Sandra Bullock’s, that in order to get over to the Chinese orbital station, she just has to keep it in the centre of the viewport and keep firing the engines. In terms of orbital mechanics, it would be hard to arrive at a less likely option for reaching her target. It would work for fairground dodgem cars, or for boats on a stretch of open water: it would even work pretty well for two aircraft in flight. But two bodies in space, in orbit around a central body behave in ways that can be counter-intuitive, and the whole aim-and-accelerate idea is pretty much doomed to failure.

Voyager 2 slingshot manoeuvres through the solar system (Wiki)

It’s an odd thing that gravity, even in Newton’s classical world where relativity and quantum mechanics play no part, is such a hard system to solve. If the entire universe consisted of just two objects, then their future motion could be exactly predicted for as far ahead as you please. But with more than two objects – starting with the so-called three-body problem – there is no general exact solution. A few special cases can be solved to a good-enough accuracy, and there are some very hypnotic numerical simulations of the resulting tracks, but the general case remains unsolved. Even a partial answer is better than none at all, however, and one of the most strikingly useful examples of three-body interactions is the so-called gravity assist manoeuvre (also called a slingshot) in which a space probe is given a substantial acceleration by means of a close approach to a convenient planet.

Schematic showing the Lagrange points for the Earth-Sun system (Wiki)

From a story-telling perspective, three-body problem solutions are very handy! Gravity assists are a great way to make a journey achievable, which otherwise would take too long to complete. And some other very convenient solutions are the so-called Lagrange points – “fixed” places holding a particular relationship with a planet in its orbit. A small body – a spaceship, or a small asteroid – which is placed into one of those points will remain there in a stable configuration, whereas at other nearby locations, the relative orbits will diverge rather than converge. Most planets in the solar system have accumulated a little cluster of small natural bodies at the points L4 and L5 – these are generically called Trojan satellites, following a convention established for the moons of Jupiter. Even Earth has at least one Trojan satellite, whereas Jupiter has over 7000 of them. The Lagrange points provide a very convenient “resting-place”, where an author can locate an artificial satellite without needing to exert any station-keeping energy.

These represent gravitational solutions which are useful, in a sort-of utilitarian manner. However, the long-distance and always-attractive qualities of gravity also give rise to exciting and rather surprising patterns of motion. These represent resonant patterns which can often stabilise a system, making it longer-lasting than might be expected. For example, the orbit of Pluto occasionally crosses over that of Neptune. One’s first thought is that at some point they would collide, or at least get close enough to seriously interfere with each other’s orbit. In fact their orbits are in a 2:3 resonant pattern with each other – for every three orbits of Neptune, Pluto makes two. This, together with some other resonances in their orbits, means that the two planets never in fact approach one another very closely at all. They remain stable. Similar stable patterns can be seen in the orbits of bodies outside Pluto, in the Kuiper Belt. Resonance can destabilise systems as well – there are gaps in the asteroid belt caused by resonances with Jupiter, and gaps in Saturn’s rings caused by one or other of the moons. But I want to finish this section with a stabilising resonance which turns out to be particularly appealing – Naiad and Thalassa, two of the moons of Neptune, are constantly engaged in an orbital dance to avoid each other. The ratio is particularly complicated in this case: for every 69 orbits of Thalassa, Naiad orbits 73 times.

Orbital resonance of Naiad and Thalassa around Neptune. (NASA/JPL-Caltech)

So gravity is a complicated thing, and at least when you’re in orbit, can’t be solved by simply aiming-and-shooting. But it does give rise to some exciting possibilities for stories, and some fascinating choreography amongst planets, moons, and asteroids. Of which, more another time.

Another trip to Pluto?

Pluto, as seen by New Horizons on July 13, 2015 ( NASA/JHUAPL/SWRI )

In July 2015 the NASA New Horizons space probe passed Pluto at a distance of under 8000 miles, in the process providing us with the first close-up data of this miniature world and its companion moons. The whole package of scientific and image data took over a year to download to Earth, and a complete analysis will take a considerable time yet. It was also roughly a year after that flyby that I started writing The Liminal Zone, set out on Pluto’s moon Charon.

New Horizons went on to have a close encounter with the unromantically named 2014 MU69 (often called Ultima Thule) in January of this year. Data from that meeting will not be fully downloaded until September next year. And mission planners are considering options for possible future encounters: if no suitable Kuiper Belt object is identified, then the on-board instruments will simply continue to return data about the remote environment in which the spaceship finds itself. The power source is finite, and will run out sometime in the late 2030s, the exact time depending on what tasks the craft is called upon to perform.

The face of Pluto looking towards Charon, on July 11, 2015 ( NASA/JHUAPL/SWRI )

But today’s blog remains focused on Pluto and its moons. Not so very long ago, Pluto was regarded as utterly inhospitable and uninteresting. If you were going to locate a science fiction plot within the solar system, you wouldn’t choose Pluto. Pretty much any other planet or moon seemed preferable, and it was hard to conceive of Pluto as anything but bitterly cold and rather featureless. New Horizons has changed that perspective. It now seems that this small body – downgraded in 2006 from being classed as “planet” to “dwarf planet”, in a decision which continues to be fiercely debated and may well be reversed at some point – is one of the most complex and interesting objects anywhere within the solar system. Not only is there a wide range of dramatic geological phenomena, but all the evidence points to ongoing activity out there. Pluto is not a frozen dead world, but one which continues to change and adapt.

Pluto and Charon from one of the other moons – artist’s impression (NASA, ESA and G. Bacon)

So interesting is it, that NASA is currently considering another mission to Pluto, this time with a view to remaining in orbit for an extended period rather than just zooming by at great speed. This would require a different kind of orbital trajectory – New Horizons’ course was deliberately set up to gain as much speed as possible from gravity assists (“slingshots”) in order to minimise the time to get there. If you plan to remain in orbit, you have to approach at a considerably lower speed to allow the modest gravitational pull to draw you in. The outline plan calls for a two-year period in orbit, followed by another onward journey – probably using Charon to slingshot away – to a suitable destination elsewhere in the Kuiper Belt. My guess is that the spaceship would need to use an ion drive, just as the asteroid probe Dawn did – this has vastly lower acceleration than a conventional chemical motor, but remains on for very long periods of time, adding speed minute by minute, hour by hour. It’s an exciting prospect if you like Pluto – two years of extended study rather than an action-packed 24 hours. If given the go-ahead. take-off would be over a decade away, and I will be in my 90s before data starts coming back. I guess it will be something to entertain me in old age!

View of Pluto as New Horizons left the system, catching the Sun’s rays passing through Pluto’s atmosphere (NASA/JHUAPL/SwRI)

Meanwhile, I shall continue writing about Pluto and Charon using the information we already know, and a generous dollop of speculation. Why choose Pluto? Well, The Liminal Zone opens on a research base out on Charon, using a collection of instruments called The Array to study what lies further out. It’s analogous to siting a terrestrial telescope on a high mountain – you avoid most of the light and electromagnetic noise generated by other people, and can concentrate on tiny signals which are easily drowned out. Into this situation comes Nina, curious about strange local tales which have no easy explanation.

For fun, here’s a short extract from when Nina arrives

Finally the landing was complete, with the smallest of jolts as the ship docked. And since she was the only passenger – and had been since the orbit of Ceres – there were no additional delays. All her belongings were already at her side, and she just walked out through the concertina into the entryway for the Charon habitat. It was all quite anticlimactic.

Her accommodation was about two thirds of the way out along the Lethe habitat. She stepped carefully along the corridor to acclimatise herself – the gravity was about a fifth of what she was used to on the Moon, so it needed care, but was manageable. The porter had given her a little hand-held which was directing her to the suite of rooms. That very word, suite, sounded too grand for her taste. She was used to more modest facilities. Indeed, the whole building seemed needlessly large to her, particularly after the weeks of confinement on the freighter. She decided that she could always close some of the doors and just live in one room, if the space in her quarters was overwhelming.

But when she got there, it wasn’t that easy. The ceiling vaulted high above her in the main chamber, and several secondary rooms clustered around it like soap bubbles. A privacy screen shimmered over a gap diametrically opposite the main door – sleeping quarters or comfort facilities, she supposed – but the rest was all open-plan. To her left was an emergency evacuation airlock, displaying all the standard alert signs. There were cupboards in doors on several walls; opening one at random she found some eating utensils. She put her carryall and daypack on one of the chairs, and wandered aimlessly about. With this apparently reckless attitude to the vacuum outside, the room didn’t feel like anywhere else she had visited. The space was daunting.

Finally she perched uncomfortably on a stool, one of half a dozen arranged haphazardly around a long table. The suite of rooms was almost silent, except for a quiet mechanical buzz which she only noticed with deliberate effort. She cleared her throat nervously.

“Is there a domestic system online?”

“Hello. Are you the new occupant?”

Software generations and obsolescence

Alexa Far from the SpaceportsWebIcon
Alexa Far from the SpaceportsWebIcon

This post came about for a number of reasons, arising both from the real and fictional worlds. Fictionally speaking, my current work-in-progress deals with several software generations of personas (the AI equivalent of people). Readers of Far from the Spaceports and Timing will no doubt remember Slate, the main persona who featured there. Slate was – or is, or maybe even will be – a Stele-class persona, which in my future universe is the first software generation of personas. Before the first Stele, there were pre-persona software installations, which were not reckoned to have reached the level of personhood.

The Liminal Zone (temporary cover)
The Liminal Zone (temporary cover)

There’s a third book in that series about Mitnash and Slate, tentatively called The Authentication Key, which introduces the second generation of personas – the Sapling class. But that is in very fragmentary stage just now, so I’ll skip over that. By the time of The Liminal Zone, which is well under way, the third generation – the Scribe class – is just starting to appear. And as you will discover in a few months, there is considerable friction between the three classes – for example, Scribes tend to consider the earlier versions as inferior. They also have different characteristics – Saplings are reckoned to be more emotional and flighty, in contrast with serious Scribes and systematic Steles. How much of this is just sibling rivalry, and how much reflects genuine differences between them is for you to decide.

So what made me decide to write this complicated structure into my novels? Well, in today’s software world, this is a familiar scenario. Whether you’re a person who absolutely loves Windows 10, macOS Catalina, or Android Pie, or on the other hand you long for the good old days of Vista, Snow Leopard or Kitkat, there is no doubt that new versions split public opinion. And how many times have you gone through a rather painful upgrade of some software you use every day, only to howl in frustration afterwards, “but why did they get rid of xyz feature? It used to just work…” So I’m quite convinced that software development will keep doing the same thing – a new version will come along, and the community of users will be divided in their response.

Artist’s impression, Europa Clipper at work (from

But as well as those things, I came across an interesting news article the other day, all about the software being developed to go on the forthcoming space mission to Jupiter’s moon Europa. That promises to be a fascinating mission in all kinds of ways, not least because Europa is considered a very promising location to look for life elsewhere in our solar system. But the section that caught my eye was when one of the JPL computer scientists casually mentioned that the computer system intended to go was roughly equivalent to an early 1990s desktop. By the time the probe sets out, in the mid 2020s, the system will be over 30 years out of date. Of course, it will still do its job extremely well – writing software for those systems is a highly specialised job, in order to make the best use of the hardware attached, and to survive the rigours of the journey to Jupiter and the extended period of research there.

But nevertheless, the system is old and very constrained by modern standards – pretty much all of the AI systems you might want to send on that mission in order to analyse what is being seen simply won’t run in the available memory and processing power. The computing job described in that article considers the challenge of writing some AI image analysis software, intended to help the craft focus in on interesting features – can it be done in such a way as to match the hardware capabilities, and still deliver some useful insights?

As well as scientific research, you could consider banking systems – the traditional banks are built around mainframe computers and associated data stores which were first written years ago and which are extremely costly. Whatever new interfaces they offer to customers – like a new mobile app – still has to talk to the legacy systems. Hence a new generation of challenger banks has arisen, leapfrogging all the old bricks-and-mortar and mainframe legacy systems and focusing on a lean experience for mobile and web users. It’s too early to predict the outcome, and the trad banks are using their huge resources to play catch-up as quickly as they can.

Often, science fiction assumes that future individuals will, naturally, have access to the very latest iteration of software. But there are all kinds of reasons why this might not happen. In my view, legacy and contemporary systems can, and almost certainly will, continue to live side by side for a very long time!

Lego ideas (from

Pouring beer in low gravity

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This is another of my occasional posts on the general theme of “how would you do such-and-such in low or zero gravity?” Lots of things which we take for granted down here on the surface of the Earth become surprisingly difficult or awkward if you find yourself in the microgravity of orbit, or on the surface of a body where the gravitational pull is very much less than what we enjoy here.

Lager kegs at the Grasmere Sports Day

Today’s topic is pouring beer, and originates from the annual Grasmere Sports Day – an event held on the Sunday of the Bank Holiday weekend at the end of August. As you can see, it was a sunny day – even a hot day – and these have been in short supply ever since. But that day was hot, and we had the task of running a beer tent where people would expect cold lager right through the day. (Or any of several ales, or fruit cider)

Now, the business of making the kegs cold was handled by means of what was basically a very large cold-water bath – cooled down with a heat exchange loop overnight, then kept that way through the day. A reflective tarpaulin kept the sun (mostly) off, and the refrigeration loop did the rest. All that part would not be appreciably different in low gravity – keeping things cold in space is not generally a problem in the situations I have in mind (I’m not planning on colonising Mercury any time soon).

Dispensing fonts for several of the products

Let’s think what happens next, The drink is pushed from the keg to the dispensing unit by gas pressure. This might be the pressure of gas generated during fermentation, or some extra assistance from a CO2 or mixed-gas cylinder, and typically is a mixture of the two. Again, no problem here at all. Gas will push liquid along a tube in lots of gravity or none, basically because gases are compressible and liquids are not. So on Earth or in orbit, the beverage is pushed through a series of tubes from keg to hand-pull or font. No problem there.

But then we get to the actual presentation to the person wanting the drink. Here on the Grasmere Sports field, the drink poured downwards from the hand-pull or font into the waiting glass. Liquid at the bottom, little bubbles rising nicely towards the surface, a suitable amount of foam on the top. Everyone was happy. But now translate that into orbit. Out here, there’s no up or down worth speaking about. The liquid is propelled straight out of the delivery tap. It splashes on the sides or far end of the glass you are holding there, and then (probably) just bounces out again. There’s no gravitational incitement to remain in the glass.

In the glass

You mop up the mess, think about it, get a container which has a lid, and try again. That’s fine – the lager now remains where you wanted it instead of drifting all around your living space. Except it has no motive for remaining at the bottom of the container, since there are no gravitational clues as to what is the bottom. My suspicion is that it would break up into a number of large blobs, fusing and separating rather like an old-style lava lamp. Now suppose you got yourself a transparent container so you could still see the head… what’s happening here? The bubbles aren’t rising to the top… because there is no top. My guess – and it is a guess – is that the internal hydrostatic pressure would mean that bubbles go out from the inside of each disjoint blob of fluid towards the surface. If I’m right, then each blob will have its own set of bubbles going out radially, and each will have a roughly spherical head surrounding the liquid. It’s a fascinating thought. How would you drink such a thing? Two ways, I suspect: either you’d use a straw through the lid and suck up each blob in turn, or you’d choose a container that you could squeeze like a toothpaste tube. Not so visually exciting as quaffing your pint out of a glass, but at least you’d get to have the drink.

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

It’ll be a while before we face that problem for real, but my suspicion is that the brewing of beer (or an equivalent beverage) will follow very hard on the heels of any human colonisation of the solar system at large. And it’s certainly worth including in a near-future science fiction story – I put a little bit of detail into Far from the Spaceports about the Frag Rockers bar out among the asteroids, but back then I hadn’t had the chance to consider it in more detail. But there were little details like “You’ll need to go to Frag Rockers to get anything decent. Regular fermentation goes weird in low gravity. But Glyndwr has got some method for doing it right. He won’t tell anyone what.” Maybe one of the books in this series will explore the matter in more detail.

That’s it about fermentation today, but I was intrigued to read that NASA have been experimenting with the manufacture of cement up in space – see this link for a description together with some comments on structural differences between the same stuff made on Earth and in orbit, or this link for my own ramblings about the process a few weeks ago.

And finally, condolences to the Indian space agency ISRO for the loss of signal from the Vikram lander, during the final stages of approach. The orbiting observatory part of the Chandrayaan-2 mission is still working as expected.

More about lightsails

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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 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 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.