Space Lasers Fired at the Moon!

Space lasers fired at the Moon! It sounds like something from an Austin Powers movie – do you mean a “Space Laser” <air quotes> 🙂

The truth is even more interesting. Astronomers at observatories in new Mexico, Italy and Germany have been firing lasers at the Moon for 50 years as part of a long-ranging experiment that has yielded data on the tidal behaviour of Earth’s oceans, the surprising flex of the elastic lunar surface (up to 15 cms), the gradual movement of the Moon away from the Earth, and confirmation of Einstein’s gravitational theories.

Mercury's Tidally Locked Orbit
Apollo legacy lives on – through prisms

Arrays of hundreds of prisms left on the lunar surface by Apollo missions receive the incoming laser beams and bounce them back to Earth. The Apollo 11 and 14 arrays have 100 quartz glass prisms each, while the array left by Apollo 15’s astronauts has 300! The accuracy in measurement these prism arrays allow is stunning — and the experiment just keeps yielding data year after year because the arrays require no power or maintenance.

The returning signals have allowed the orbit, rotation and orientation of the Moon to be very accurately determined, and have confirmed that he the distance between the Earth and Moon is increasing by around 4 cm a year.

The experiment has highlighted the behaviour of Earth’s ocean tides, but also has shown that the lunar crust also rises and falls in a solid lunar “tide”. It has also confirmed that the Moon has a fluid core! This really surprised me, having thought (like many others) that the Moon was a “dead” rock. In fact the prevailing theory, even among scientists, was that the core would be cool and solid. The Moon’s fluid core affects the position of its north and south poles, which the experiment was sensitive to pinpoint.

The experiment has also confirmed Einstein’s theory of gravity, which assumes that the attraction between bodies is independent of their composition – proven true for the gravitational affects between the Sun and Moon, and Sun and Earth, despite the higher iron content of the Earth.

And that’s not the end for lunar reflectors. NASA has recently approved a new generation of reflectors to be positioned within the next ten years. These would be spread over a larger area, allowing more extensive analysis of lunar geography and further verification of Einstein’s gravitational theory.

Cool, huh?

Studies like this are invaluable in understanding new worlds. As a SF writer, they provide invaluable insights when it comes to building your own planets. Check out my own world-building in my SF novel, The Tau Ceti Diversion.

With the crew dead, and the starship’s jury-rigged fusion threatening a lethal explosion, Karic and the surviving officers finally reach a habitable planet. It’s a miracle, but the last thing they expected was to find that planet already occupied . . .

Get it now!

Near Future SF

Try some Near Future SF! With the crew dead, and the starship’s fusion drive held back from a lethal explosion, Karic and the surviving officers reach a habitable planet – the last thing they expected was to find it already occupied . . . #TheTauCetiDiversion @ChrisMcMahon111 #ScienceFiction #NearFuture Check it out on Amazon!

Near Future SF

Try some Near Future SF! With the crew dead, and the starship’s fusion drive held back from a lethal explosion, Karic and the surviving officers reach a habitable planet – the last thing they expected was to find it already occupied . . . #TheTauCetiDiversion @ChrisMcMahon111 #ScienceFiction #NearFuture https://amzn.to/2k8k1Vx

Atmosphere on a Fictional Planet

So you’ve got your story working, but how do you sketch out the atmosphere on a fictional planet? Maybe you have some idea of the mass, radius and gravity and you’ve got the orbit in the ‘sweet spot’ goldilocks zone where liquid water can be present on the surface, but what will conditions on the surface actually be like?

What sort of factors go into whether that planet, presumably an Earth-like rocky world, will have an atmosphere that can support terrestrial life?

Planets above a blue planet

The gravity of the planet is one key variable, along with surface temperature, and the strength of the planet’s magnetosphere, which can protect against atmospheric stripping due to solar wind.

The surface temperature of a planet will determine how much kinetic energy, and so velocity, the gas particles will have. If that temperature, and velocity, is high enough it will exceed the planet’s escape velocity and the molecules will fly off into space like tiny spaceship explorers. Earth has lost most of its very light gases like hydrogen and helium in this way, whereas the gas giants have enough gravity to retain them. We kept our water, and we’ve got a lot of it! If Earth was sitting where Venus is things would be different, the additional temperature would give those lighter gases like water vapour enough energy to escape, and also prevent any being trapped on the planet’s surface itself (whereas some is ‘sequestered’ on Earth as water and ice at our lower surface temperature). But beyond the early, settling down period where the lighter gases are lost, any world larger than Earth, orbiting in that goldilocks zone, will not continue to lose a significant proportion of its atmosphere through thermal processes.

Here’s a cool pictorial on thermal escape (source: Wikipedia).

Solar_system_escape_velocity_vs_surface_temperature.svg

Beyond that thermal stripping process, is where the magnetosphere comes into its own, deflecting the solar wind – one of the main non-thermal processes leading to atmospheric loss. The very thickness of a planet’s atmosphere (retained due to its gravity, and as a function of surface temperature), will also protect a planet from the solar wind, even in the absence of a magnetosphere. It’s thought that Venus’ thick atmosphere, ionized by solar radiation and the solar wind, produces magnetic moments that act out to 1.2-1.5 planetary radii away from the planet to deflect the solar wind, much like a magnetosphere (but an order of magnitude closer to the planet). In fact, it’s thought the dominant non-thermal atmospheric loss process on Venus is actually from a type of naturally induced electrical acceleration. On Venus, the stripping of the lighter electrons from the atmosphere causes an excess of positive charges, accelerating ions like H+ out of its atmosphere.

Our explorers need a breathable atmosphere, but they also need an atmospheric pressure like our own Earth’s.

My fictional planet of Cru, in the Tau Ceti Diversion, has comparable surface temperatures to Earth, but a higher surface gravity. The higher surface gravity, and its lower density, allowed me to assume a lighter atmospheric composition, and allow an atmospheric pressure, or weight of atmosphere, close to surface much like Earth’s. That atmospheric composition is crucial to having a reasonable atmospheric pressure – its not just the gravity of the planet. Venus, even though it has slighter lower gravity than Earth, has a crushing atmospheric pressure of 90 times Earth’s due to its heavier  atmosphere of CO2.

Check out what my my intrepid explorers found in my novel The Tau Ceti Diversion when they touched down on the planet!

Read it now on Amazon!

Tau-Ceti-Diversion-severed-ebook-cover (Medium)

Estimating Surface Gravity on a Fictional Planet

So you want to estimate surface gravity on a fictional planet? Easy!

One of the things I had to do as part of the rework of my novel The Tau Ceti Diversion, is to try and work out the surface gravity of my fictional planets. From the Kepler data, there are two exoplanets located in the Tau Ceti system that are likely to be in the system’s habitable zone, or where there is the possibility of liquid water on the surface, and perhaps life as we know it.

To play around with my estimates of gravity, I used ratioed rearrangements of Newton’s law of gravity (law of universal gravitation) and a simple formula relating the density of a spherical planet to its mass and radius (these are at the bottom of the post in the ADDENDUM).

WARNING: MATHS CONTENT!!!

Here’s Newtons famous law:)

law of gravity

The two planets thought to be in Tau Ceti’s habitable zone are denoted Tau Ceti e and Tau Ceti f. What is known about these two planets is their likely orbit, eccentricity, and their mass. All of these properties have been derived by calculation, based on observed data, so are all known to within appropriate error bounds, but I’m leaving the error off my scribblings so things don’t get too messy.

Tau Ceti e is thought to be around 4.3 Earth Masses, or Me (i.e. 4.3 times as heavy as Earth), while Tau Ceti f, the planet that orbits a bit further out, is thought to be around 6.67 Me. For the astronomically minded, these two planets orbit at around 0.55 and 1.35 AU from Tau Ceti respectively.

So, here’s where I cheated a bit, like any good engineer. I started with the answer I wanted and calculated backwards to see if the answer I wanted led to reasonable base assumptions. This is not as cheeky as it sounds, because when you have an insoluble problem (i.e. not enough data is known for an explicit result), an iterative approach is often used.

For my story to work, I needed a surface gravity on my planet of no more than 1.2g – that’s twenty percent higher than Earth’s. But how could I get a gravity that low on a planet that was over 4 times the mass of Earth? The answer is that surface gravity is a function of mass and radius, or going a step further along the calculation path, mass and density.

I used a ratioed form of Newton’s law that allowed me to relate the ratio of two planets gravitational forces to the ratios of their masses and radii. I already knew the ratio of the gravities ( assumed at gTCe/gE= 1.2) and the ratio of the masses (MTCe/ME =  4.3), so could calculate the ratio of radii (rE/rTCe) at 1.89.  Using another formula that related the ratio of the two planet’s densities to their ratioed mass and radii, I could then calculate their ratioed densities (dens TCe/ densE) at 0.63. So at the end of all that, to have a surface gravity of 1.2 g, Tau Ceti e would have to have a density of 63% of Earth’s. Is that reasonable?

The density of Earth is 5.514 g/cm3, not too much different from the density of a rocky planet like Mercury (5.427 g/cm3), but a lot higher than other solar system planets like Jupiter and Uranus (1.326 g/cm3 and 1.27 g/cm3 respectively), comprised of lighter materials. A surface gravity of 1.2g on Tau Ceti e would put its density at around 3.5 g/cm3, less dense than our own rocky planets, but certainly in a feasible range.

So what sort of densities would you expect for the Tau Ceti system? One clue is the metallicity of the system, which is a measure of the ratio of iron to hydrogen in the star’s makeup. In the case of Tau Ceti, this is estimated to be around one third of our own sun. This indicates the star is likely to be older than the Sun, made up of stellar remnants left over from less evolved stars that have not had time to form as much of the heavier elements in their internal fusion factories.

So Tau Ceti is made up of lighter elements. Based on this, it was reasonable to assume that the planets in the Tau Ceti system would also be made up of proportionally lighter elements, and quite possibly in the range I had estimated. Tau Ceti e and Tau Ceti f are also large planets – much larger than our own Earth – so having a density in between Earth and our own gas giants also made sense to me.

Using the same planetary density I had calculated for Tau Ceti e, for the larger Tau Ceti f, gave me a surface density of around 1.4g for the bigger planet – just a little too high for feasible human colonisation – and that fit nicely with my story as well.

It was a lot of fun playing with these calculations, and thankfully the known science fit with my story, at least with some comfortable wiggle room!

Check out what challenges that increased gravity provided for my intrepid explorers in my novel The Tau Ceti Diversion!

Read it now on Amazon!

 

 

 

 

 

 

 

 

ADDENDUM

For those interested in the maths. . .

Density formula:  densp= Mp / (4/3*pi()*rp^3)

Where:

densp= Density of Planet (kg/m3)

Mp = mass of planet (kg)

rp = radius of planet (m)

In ratio form: densp1/densp2= Mp1/Mp2 *(rp2/rp1)^3

 

Ratio of Newtons law relating gravity, mass and radius of two planets:

gp1/gp2= Mp1/Mp2 *(rp2/rp1)^2

 

Tau Ceti – One of Our Celestial Neighbours

The Tau Ceti system is indeed one of our close cosmic neighbours. At less than 12 lightyears away, it is one of the closest systems to Earth’s own solar system – along with others such as the Centauri system and Epsilon Eridani. Because of its nearness to our own solar system, it has been a favourite in science fiction for decades. A likely first or second step for any intrepid interstellar explorers.

I first started toying with the idea of a novel set in the Tau Ceti system more than twenty years ago. And as these things go, the story developed in fits and starts as I bounced between novel projects and other stories. One of the things about writing science fiction, particularly near-future SF, is that the science never stands still. And particularly, in the last few decades, the developments in astronomy and the identification of planets outside our solar system, called exoplanets, has been almost exponential!

When I wrote the first draft of The Tau Ceti Diversion, there was not a single confirmed planet identified outside Earth’s solar system. Now, thanks largely to the latest Kepler space-based telescope discoveries, there are more than 3000! Not only that, but there have been five identified in the Tau Ceti system itself, with one – and possibly two – in the habitable zone around that star.

What did this mean for me? It meant a ton of research, and lot of very careful rewriting!

In my very early drafts of The Tau Ceti Diversion, I was free to imagine an Earth-like solar system of planets and shape them as I saw fit for the story. But by the time the last draft was completed, only months ago, I had very specific information about what those planets might be. I knew their approximate mass, their orbits, even their eccentricity. I had to go back to the drawing board – and my excel spreadsheets – to try and work out how these known planets would fit within the very specific constraints of my story. Not the least of which was that my story included a tidally locked planet!

It’s no accident that the Tau Ceti system has been popular as a setting for science fiction. Even before the identification of its family of planets, Tau Ceti, in the constellation of Cetus, was known to be very similar to our own Sun. It is smaller, about 78% of the Sun’s mass, and is the closest solitary G-class star (the same spectral class as the Sun). That’s enough to make it seem like our cousin. Add to that Tau Ceti’s stability, and lack of stellar variation, and you already feel like moving in. The only hitch is the presence of a debris disk, which means that any planet orbiting Tau Ceti is likely to face more impact events than planets in our own solar system.

Seen from Tau Ceti, the Sun would appear much like Tau Ceti does to us – a third magnitude star visible to the naked eye.

The composition of Tau Ceti, as measured by the ratio of its iron to hydrogen content, or metallicity, is lower than our Sun, indicating that it is older: its makeup derived from earlier stars yet to manufacture the same amount of heavy elements in their internal fusion factories.

So similar to our own Sun, and so close, it’s no wonder that it is also a target for the SETI (Search for Extra-Terrestrial Intelligence) program.

As readers of my novel The Tau Ceti Diversion will discover, the explorers in my novel certainly find some intelligent life there!

Read it now on Amazon!

Tau-Ceti-Diversion-severed-ebook-cover (Medium)

 

Going Faster Than Light

Going faster than light is the Holy Grail of space travel, and is often depicted in science fiction. It seems as easy as flicking the switch to jolt the ship into Hyperspace. I mean, it worked for Han Solo, right?

It was Einstein who first postulated the idea that the speed of light is constant in any “frame of reference”. Basically, no matter how fast you were going, light would always be moving away from you at the same speed. As counter-intuitive as this was, his theories of special relativity and general relativity have been borne out by direct observation and experiment.

Just about all of us use GPS data on a daily basis, with signals pinging from our smart phones through our networks to global satellites. The clocks on those GPS satellites all run slower than those on Earth, a direct prediction of relativity, and corrections are used on a routine basis to bring them into line with their “stationary” counterparts. Astronomers also routinely use Einstein’s predicted ‘gravity lensing’ to make observations of the universe, and have used this technique to pin down the enigmatic ‘dark matter’ that makes up so much of our universe.

So if Einstein’s predictions tell us we can’t go faster than the speed of light, is that it for our desire to go speeding through the Universe in our faster-than-light spaceship? Interestingly enough, not necessarily. . .

There are two potential loopholes than emerge from Einstein’s work, and both of them have to do with the way spacetime can fold up. The ‘warp drive’ and the more familiar idea of wormholes.

The warp drive, originally a concept from science fiction, is familiar from just about every episode of Star Trek. The idea for the warp drive is that spacetime would be expanded behind the spaceship, and compressed in front of it, to such a degree that the ship would seem to flash through vast distances in moments. The ship itself would not actually be moving, but be inside a ‘warp bubble’. This is a pretty exotic solution of Einstein’s equations, but physicists have shown that it is possible – at least mathematically. Despite moving so fast, the astronauts would not be subject to any inertial effects because they are not actually moving. They would, however, be in a state of ‘free fall’, due to the angle of folded space in front of them. Some people have questioned whether our warp drive pilots would get cooked by intense, blue-shifted light, but the jury seems to be still out on that one.

The warp drive has been dubbed the Alcubierre drive, after the physicist who first proposed this solution. Believe it or not, the theory was evolved by Alcubierre in response to the use of the warp drive on Star Trek. The travellers on the warp drive capable ship would be cut off from the outside universe, riding on a ‘wave’ of compressed space, along a corridor or warped space-time that would probably have to be constructed in advance, like some sort of cosmic superhighway. Alcubierre himself muses “We would need a series of generators of exotic matter along the way, like a highway, that manipulates space for you in a synchronized way”.

The graphic below gives a 2-dimensional representation of the spacetime around the ‘warp bubble’, stretched to create a gradient pushing the ship forward. Just don’t try to leave the bubble – you would get ripped apart.

Alcubierre space time

To make the Alcubierre drive work we need a pretty exotic fuel – either negative matter or negative energy to be precise. Now that’s negative matter – as apposed to dark matter (which is invisible but has weight) or antimatter (positive energy but reversed charge). Both dark matter and anti-matter have been proven to exist. So far there is no proof that negative matter exists. If it did, it would fall up rather than down, and would have left any solar system long ago (being repelled by ordinary matter) and be drifting out in the middle of nowhere somewhere. So finding negative matter is going to be hard, but perhaps possible using gravity lensing techniques.

Negative energy, though – believe it or not – has been demonstrated by experiment.

In the experiment, two plates in a vacuum, positioned very close together, experience a net movement toward each other because of the ‘pressure’ difference of virtual particles being created at the quantum level around and between the plates. These are electron-antielectron pairs that burst out of nowhere for incredibly brief periods of time, then disappear as they collide (preserving the average energy stat). As brief as their appearance is, the particles create a real effect. That ‘pressure’ causes the predicted movement in the plates, and that equates to a net amount of energy. Since that energy is coming from ‘nowhere’ (and energy must be conserved) to make the whole system balance the plates have a net negative energy left between them. And how much? The effect, called the Casimir effect, was measured in the laboratory in 1996 at Los Alamos. The attractive force is the equivalent to 1/30,000 of the weight of an ant. We would need a lot more than that!

As a civilization, we are a long way from any faster than light travel, even if it is possible. It’s true that Einstein’s equations give solutions that show the possibility of both the warp drive and even wormhole travel, but are these real possibilities, or mere mathematical curiosities? If it is possible, we would need an awesome amount of energy. It’s estimated that to keep a transversable wormhole open wide enough to allow human travellers to pass through, you might need as much as a Jupiter mass of negative energy. That’s clearly well beyond us now.

That doesn’t mean we can’t reach the stars, just that we can’t get there quickly!

Fusion drives, or even antimatter drives, or a combination of the two, will enable us to construct starships that could travel at respectable fractions of the speed of light.

In my novel, The Tau Ceti Diversion, the starship Starburst uses a fusion drive, assisted by an antimatter ‘burst’ to reach a new solar system and look for planets to colonize. Much of the action in the book takes place on planet tidally locked to Tau Ceti, some 12 lightyears away.

The novel was officially launched on 1st September 2016, and is available in both electronic and print formats! Grab a copy!

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