How to Spot a Psychopath

Further fizzing

7 August 2012 — dan

A reader writes:

After reading your post about vinegar and bicarbonate of soda as cleaners, I've been using bicarb more to clean and deodorize things (a bicarb and water paste is pretty good for spills on fabrics and carpets - just massage it in then leave it overnight and vacuum it up the next day. Clogs the vacuum filter something fierce, though).

In the course of my experiments, I've found that if you add bicarb to near-boiling water, it fizzes. This is with plain water fresh out of the electric kettle, not water plus vinegar or anything else acidic. Add bicarb to the water, it fizzes and dissolves. Add more bicarb, more fizz. Add more hot WATER to the existing bicarb-and-water solution, and it fizzes again!

What's going on, here? I know dissolving stuff in water can change the boiling point, but I think it usually INCREASES it, and the difference isn't usually very large. Is the bicarb providing nucleation sites for boiling? Why's it still happen when the bicarb's dissolved, though? And how can it boil water that's not hot enough to boil naturally any more?

Suze

The Wikipedia article actually explains this; above 70 °C, sodium bicarbonate and various other bicarbonates decompose. In sodium bicarbonate's case, it goes from NaHCO₃ to sodium carbonate (Na₂CO₃), water and carbon dioxide. The hotter it is, the faster this happens, and it happens in solution too.

So the fizz is still carbon dioxide bubbles, just as if you'd added bicarb to vinegar, but the source of the CO₂ bubbles is different.

Psycho Science is a regular feature here. Ask me your science questions, and I'll answer them. Probably.

And then commenters will, I hope, correct at least the most obvious flaws in my answer.

Posted in Psycho Science, Science. Leave a Comment »

Megatons and milligrams

4 August 2012 — dan

A reader writes:

In one of Alastair Reynolds' books, someone sets off a "pinhead-sized" antimatter bomb, and it explodes with a yield of about two kilotons of TNT. Is that accurate? Would you really only need that much?

Jamila

I think you're talking about Revelation Space, the first book in that series, written slightly before Futurama debuted and so forgivable for its inclusion of a captain named Brannigan.

First, note that in a matter-antimatter explosion, you're not just converting the mass of the antimatter into energy. You're also converting an equal mass of matter, because if that matter was not around there'd be no annihilation and no explosion.

The energy yield of matter annihilation is a simple case of mass-energy equivalence, and thus subject to the famous e equals mc squared. Which is to say, energy in joules equals the mass being annihilated in kilograms times the square of the speed of light in metres per second.

The dominant number there is obviously c-squared; the speed of light in vacuum is 299,792,458m/s, and squaring that gives you 89,875,517,873,681,800. Or, in less-cumbersome scientific notation about 8.99E+16 - 8.99 times ten to the power of 16.

"TNT equivalent" bomb-yield numbers are tightly defined, too; one ton of TNT is defined as 4.184 gigajoules.

Now, what's a pinhead weigh?

I just grabbed some ordinary one-inch dressmakers' pins and found there were about fifteen whole pins to the gram. I'm not about to snip off enough pinheads to get them to add up to the minimum resolution of my triple-beam balance, but I'd guess the mass of these pins' heads to be ten milligrams, at most.

Fortunately, the mass of the Revelation Space bomb is mentioned in the book; it's described as containing "only a twentieth of a gramme of antilithium". That's fifty milligrams, but that doesn't sound like a crazy weight for the head of a stouter pin than the ones I weighed.

Plugging fifty milligrams, 0.00005 kilograms, into e=mc^2 gives

e = 0.00005 * c^2

= 4.49378E+12 joules

= 4494 gigajoules

...which at 4.184 gigajoules per ton of TNT, adds up to 1.074 kilotons. Double that to take into account the matter that's annihilating with the antimatter, and you get 2.148 kilotons. Which is indeed close enough to two kilotons for horseshoes, hand grenades and tactical nuclear weapons.

The biggest thermonuclear explosion ever created by humans, the immense and impractical Soviet "Tsar Bomba", had a possible yield of about 100 megatons, but was dialled down to 50. 50 megatons at 4.184 gigajoules per ton is 2.092E+17 joules. Turning e=mc^2 around to solve for mass, m = e/c^2, gives:

m = 2.092E+17 / c^2

= 2.33 kilograms of matter converted into energy, for the biggest bomb we've ever made, and possibly the biggest bomb we ever will make.

Around the weight of a healthy adult chihuahua.

(See also solid blocks of electrons, which knock antimatter energy density into a cocked hat and which may be a technology within the reach of some entities in the Revelation Space universe. Oh, and see also, also, the fun you could have whacking lumps of plutonium together by hand.)

Psycho Science is a regular feature here. Ask me your science questions, and I'll answer them. Probably.

And then commenters will, I hope, correct at least the most obvious flaws in my answer.

Posted in Books, Psycho Science, Science. 2 Comments »

"Small boy on bridge, this is Ghost Rider requesting a flyby..."

30 July 2012 — dan

Some more of my friend Mark's FPV quadcopter, this time buzzing around Brisbane, where he lives.

(The funny-looking tower is the Skyneedle.)

Posted in Photography, Toys. 3 Comments »

Insert drunk farmer here

27 July 2012 — dan

A regular reader took this picture...

...of an impressive piece of antique finger-grabbing agricultural equipment, and suggested it as another illustration for the "pulley paradox". Again, if you don't know about crowned pulleys, contraptions like this look offensively impossible.

As a weekend project, I suggest commenters point to the most belt-and-pulley-infested machinery they can find on the Web.

I am also pleased to note that unless the corporate copyright enthusiasts manage to extend terms yet again, the works of W. Heath Robinson should pass into the public domain at the end of 2014.

(A few works illustrated by Robinson are already in the public domain, but I don't think any of the stuff for which he's most famous is, yet.)

Posted in Machinery. 5 Comments »

Green in, red out

26 July 2012 — dan

A reader writes:

why do some things glow brightly in colours OTHER THAN BLUE when illuminated by a blue LED flashlight? Is it fluorescence? But doesn't that only happen under ultraviolet light?

Does this mean my blue LED flashlight has UV output? it's incredibly bright, but is it actually even brighter and more dangerous than it looks?

Ant

First up: I highly recommend coloured LED flashlights. They let you do this!

LED-flashlight fluorescence demonstration

The above animation accurately reproduces what it was like for me selecting the images to use to illustrate this post, except I was doing it fullscreen on a 30-inch monitor, and so almost neutralised my neurons.

(If you're using Chrome and are now hammering away on the escape key in a desperate attempt to make this brain-slapping animation stop, allow me to suggest the GIF Stopper extension.)

In the olden days, the only coloured portable lights normal humans could afford used an incandescent bulb, with a coloured filter over it. This was incredibly inefficient, and usually didn't even give you one tightly-defined wavelength of light. Your green-filtered flashlight probably still emitted some red and blue.

Today, you can get high-intensity coloured LEDs with a very tight band of output frequencies; no blue in your green, no green in your red. I think the best-value options are the coloured variants of the Ultrafire 501B lights.

I reviewed a white 501B years ago here, but this line of lights still sells well today, because they're basically just SureFire knockoffs with standardised lamps and battery compartments. So you can today buy a white 501B that's quite a bit brighter than the one I reviewed, or upgrade your old 501B with a newer interchangeable lamp, or stick a cheap coloured Ultrafire lamp in your old SureFire incandescent flashlight, et cetera. As long as you stick with a single 18650 lithium rechargeable or two rechargeable or non-rechargeable 123-size cells. Any cheap LED module that's meant to fit in a a flashlight like this should work.

(As Fallingwater points out in the comments, there are also lamps this same shape that want a very different input voltage, and the dirt-cheap lamps may not work very well for various reasons. I think all of the cheap coloured lamps are for one or two lithium cells, though, and they're low-powered by "tactical flashlight" standards so don't have heat problems either. These lamps work from one or two cells because they have a multi-voltage driver. Incandescent bulbs are not this tolerant. Standard small two-123-cell SureFire-type lights with incandescent bulbs will produce a dim orange light from a single 18650. If you somehow manage to drive an incandescent bulb from twice as many cells as it expects, it will die immediately.)

Here's an eBay search that finds a bunch of coloured Ultrafire flashlights and lamps. The lamps start at $US9.99 delivered, but a whole flashlight (without batteries) is under $US15 delivered.

A red, a green and a blue Ultrafire 501B, plus three 18650s and a charger from eBay will only cost you about $US50 all told. The cheapest dealers all have free shipping, too, so you can buy the lights one at a time and not lose any money.

I'd really get all of them, though, and I don't even go to raves. It's just so much fun chucking large amounts of coloured light around. And yes, you do get a pretty decent white-ish light if you shine them all at the same thing.

(See also the positively antiquated Technology Associates "Rave'n 2", which I reviewed more than ten years ago and which I think they still sell. It's still fun, too.)

So. Where was I? Oh yes, fluorescence.

Fluorescence happens when a substance absorbs some kind of radiation, usually light, and then emits light of its own.

It happens when the incoming energy, usually a photon, "excites" an electron to a higher quantum state. When the electron then "relaxes" back to its ground state, it loses some energy to heat and emits the rest as a new photon.

Since the energy and frequency of a photon are directly related, and the outgoing photon is less energetic than the incoming one was, one-photon fluorescence like this only works "downward" in the ROYGBIV spectrum. You'll only see visible-light fluorescence when you're illuminating a fluorescent object with light closer to the blue end of the spectrum than the colour the object fluoresces.

("Upwards" fluorescence is actually possible, when two photons are absorbed but only one emitted. I think this is pretty much unknown in everyday, visible-light fluorescence, though.)

Ultraviolet light is beyond the blue end of the visible spectrum, so it can cause fluorescence in any visible colour. But there's no rule that says the incoming light can't be visible; it just has to be further up the spectrum than the colour of fluorescence it creates.

So here are some brightly-coloured objects from around my house, illuminated by tungsten-filament bulbs. Some of the dyes used to colour many modern polymers are highly fluorescent; shining an ultraviolet light around your house is the best way to find them, but a blue LED flashlight will do a good job too.

A red flashlight's no use, though. It's probably possible for red light to cause visible fluorescence that's even deeper into the red, but you'd probably need a spectrometer to distinguish it from simple reflection of the illuminating light.

Here, we see what basic colour theory says we should. All we're seeing is the red light that bounces off the scene, so everything is shades of red, and the less red there is in the colour of an object, the less of the incoming light will bounce off it and the closer to black it will look.

Go to green light, though - not even blue! - and suddenly fluorescence is happening. The red Gakken mini theremin (as hard to play as a full-sized theremin, but with the mellow, soothing tone of a Stylophone! Buy one today!), and the red rubber Escher's solid (sold as a dog chew toy, of all things, at my local discount shop), and the red crooked dice, are behaving as basic colour theory says they should. There's no green in them, so they look black.

The orange parts of the Nerf guns, though, are cheerfully fluorescing under the bright green light.

(Actually, only the little "Secret Strike" is a Nerf product; the double-barrelled gun is a Buzz Bee Double Shot, which ejects the empty shells when you break it open!)

I think the yellow parts of the toy guns may be fluorescing a bit under green as well. They mainly look yellow only in comparison with the fluorescing orange plastic (as per this amazing optical illusion), and my digital camera certainly isn't a calibrated colourimeter, but there's still a significant amount of red in there with the bouncing green. That adds up to at least a yellow-ish green.

The length of red paracord (useful for all sorts of things, and also the only flexible string I've found that Joey's little razor teeth don't go straight through) and the carapace of the crab Hexbug, aren't as fluorescent as the plastic, but they're having a go.

Oh, and check out the two Hoberman Switch Pitch balls. One is green and orange and is fluorescing a little and reflecting rather more in the green light; the other is blue and magenta, and is hardly fluorescing at all.

(The Switch Pitch is, I think, one of the greatest fiddle-toys ever invented. I know this post's littered with affiliate links, but seriously, buy a Switch Pitch, if you can. Not everything Hoberman make is a classic; the Brain Twist, for instance, is a worthy attempt at Hoberman-ifying a Rubik's Cube, but I reckon it's more of an ornament than a toy. But the Switch Pitch and the tougher, hard-to-find Switch Kick, are brilliant.)

OK, on to the blue light that started this interminable thing.

Now the lower-fluorescers from the green-lit shot are fluorescing with more enthusiasm, the things that never fluoresced in the green are still sticking to pre-quantum-physics colour theory, and the orange plastic has gone nuts. There's a pretty sizeable energy gap between LED-blue and that orange, so it's sucking up and spitting out ~~electrons~~ photons with great enthusiasm.

My own store of quantum energy ran out before I made an actual UV-lit version of the picture, but I could pretty much just Photoshop one up in less time. All of the fluorescing things in the blue-lit image would look much the same under UV, and everything else would be invisible. Or, more realistically, you'd see everything else in faint blue, because the ultraviolet compact-fluorescent lamps I've got here emit a fair bit of visible blue-violet light along with the UV.

You can get UV LEDs that emit proper near-UV light (not the more dangerous UV-B or even more dangerous "germicidal" UV-C) with very little visible output. Most "UV" LED flashlights use cheaper purple LEDs, though, which may have a bit of near-UV output but basically just do what a blue LED light does, only more so.

And yes, you can get UV Ultrafires, too, but I don't know which flavour of "UV" LED they contain.

Psycho Science is a regular feature here. Ask me your science questions, and I'll answer them. Probably.

And then commenters will, I hope, correct at least the most obvious flaws in my answer.

Posted in Psycho Science, Science, Toys. 4 Comments »

Unexpectedly pretty thing of the day

19 July 2012 — dan

If you see a welder marking out a piece of metal with what looks like chalk, or a tailor doing the same to cloth, they're likely to not be using standard blackboard chalk.

Plain chalk is calcite, one of the several forms calcium carbonate can take. Welders' and tailors' chalk, on the other hand, is "French chalk", a stick of solid talc, magnesium silicate. Ground up, talc is the base for talcum powder.

This was just another of the pieces of vaguely interlinked data that float around in my mind, until I discovered I could buy ten 125-by-12-by-5mm (about 5 by 0.5 by 0.2 inches) sticks of French chalk, plus a sliding metal holder with a pocket clip, for a grand total of 4.8 Euros including delivery to me here in Australia.

(As I write this, that's about $US5.90, £3.80, or $AU5.70. Here's the eBay listing, here's the seller's store, here it is on eBay Australia, and here on eBay UK.)

So I had to buy the darn things, of course, in order to hasten the day on which my flattened corpse will be discovered beneath a fallen pile of scientific, electrical, medical and engineering toys and curios.

The talc sticks are unexpectedly beautiful objects. They're very smooth, despite visible sawblade marks on the sides...

...and they have the slippery feel of soapstone. Or, more accurately, soapstone has the slippery feel of talc, because soapstone is a metamorphic talc-schist.

They're moderately fragile, of course, but quite dense, and much harder-wearing than calcite chalk. And I think they've been cut from solid mined blocks of natural talc, because they all have slight marks and veins and other inconsistencies, which become more apparent...

...if you shine a light through them.

(The backlight is my possibly-actually-antique flashlight.)

I think there are two reasons why you'd want to use talc rather than calcite for marking out. First, the mark can be more accurate, because although talc is the definitive soft material (scoring one on the Mohs hardness scale), it's actually quite a bit harder and sturdier than a stick of blackboard chalk, and thus won't wear much in the course of one line across metal or cloth. Calcite itself is much harder than talc, but calcite chalk is deliberately made porous and weak; French-chalk sticks are solid and waterproof. A stick of solid non-porous white calcite would rip the paint straight off your blackboard.

The second and probably more important reason to prefer French chalk for marking steel or cloth is that when you draw with a talc stick, you get a line of freshly-created talcum powder. I think this will stick better to a surface than a normal chalk mark, and resist being rubbed or shaken off as you join and cut and otherwise handle your metal or cloth.

(There could be chemical reasons for the choice too, for welders at least. Magnesium silicate is used in some high-temperature pottery glazes, and it's also used as a welding flux, for gas welding at least.)

The ability to precisely draw talcum powder onto a surface could be mechanically useful, too. When I was a kid I used talcum powder to lubricate Technic Lego contraptions, because it doesn't make much of a mess and doesn't attack plastic. Graphite powder, which you can similarly topically apply with a soft pencil or artists' graphite stick, is a better dry lubricant - but it turns everything black and conducts electricity, which may or may not be desirable.

Talc is also a high-temperature electrical insulator. You could easily carve and drill small custom insulators out of French-chalk sticks, or use them unmodified as formers for heating elements or what-have-you.

What I'm actually likely to do with my sticks of talc, of course, is just fiddle with them aimlessly and admire them for their surprising beauty.

I reckon I got value for money, just for that.

Posted in Nerdery, Toys. 5 Comments »

Stab your steak!

14 July 2012 — dan

"And now, Mister Bond..."

Yes, these are ranks of little pointy blades with angled chisel tips.

They're all about five centimetres long, and when you use the implement of which they are a part they protrude about 19mm (3/4 of an inch) into some flesh.

Which you later eat.

I don't like cooking. But I can cook a steak. High heat, short time, remember to turn off the smoke detector nearest the kitchen, job done. The less you muck around with it, the better.

(Actually, those annoying scientific-cooking people suggest that frequent turning of a steak is desirable. A religious war will clearly result. The losers get eaten.)

I'm not made of money, though. So the cheaper my steak-meat can be, the better.

I can get a kilo of thick-cut boneless chuck from the local Aldi for eight bucks Australian. That's good for two large steak dinners, or four more reasonable ones. And there's nothing wrong with the flavour of chuck, or what's locally known as "gravy beef" (boned shin), or any number of other cuts from less fashionable parts of the cow. The problem, of course, is that they're full of gristle and connective tissue.

It's surprising how tender even cheap steak can be if you don't overcook it, but the really cheap stuff goes way over the "just needs a lot of chewing" line up into the unacceptable realm where it seems that no amount of chewing is sufficient to actually disintegrate the stuff.

The traditional solution to this problem is, of course, the tenderiser. Which, according to most people, is some kind of mallet, generally resembling a miniaturised version of Kannuki the Giant's signature weapon.

Beating a steak senseless will indeed make it much less chewy, but squashing is not actually a very good way of breaking up gristle; it can take a surprisingly long time, and invariably leaves you with a mutilated beef pancake. That's perfectly acceptable for some dishes, but pulverised beef is pretty close to just being mincemeat ("ground beef", in US parlance). You might as well buy mince in the first place and make meatloaf or rissoles or something, if you ask me.

You can also tenderise meat chemically, with an enzyme or just by letting it go a bit rotten. I haven't tried enzyme tenderisation, but dry-aging my own beef and then shaving off the mould isn't my idea of an appetising activity.

A while ago, though, this Cool Tools post alerted me to the existence of tenderisers that use blades, instead of brute force.

Intrigued by the idea, I tried just laying a cheap steak out on a cutting board and stabbing the hell out of it, all over on both sides, with a couple of little paring knives.

I highly recommend any penny-pinching carnivore try this. It doesn't take very long, and the results are excellent. The meat looks, and cooks, much the same as it did before, but all the stringy stuff has been pre-separated into short pieces. And if you want to marinate your steak in something, the stab-wounds get a lot more flavour into the meat. (I also tried pouring marinade on the steak and then stabbing it, which worked even better but was somewhat messy.)

Satisfied that the technique worked, it was clearly time for me to purchase a kitchen gadget that does the stabbing in a more organised way. The Cool Tools post recommends a "Jaccard SuperTendermatic", with 48 blades in three ranks of 16, which lists on Amazon for $US23.76 ex delivery (cheap to free within the USA, expensive everywhere else), at time of writing.

I'll betcha one of the swish shops in the next suburb over from me has name-brand blade tenderisers too, and I'll also betcha they charge at least a hundred bucks for one.

Instead, I hit eBay and bought a brandless 48-blade unit for a princely $AU17.98 including delivery to Australia, from a Hong Kong eBay seller. That was almost two years ago now; I didn't want to write anything about it before I was sure that the cheap brandless version wouldn't fall apart, maim the user, commit the signature kitchen-gadget failure of being impossible to clean, et cetera.

It doesn't, and I can't imagine that the more expensive brand-name ones work any better.

The current eBay going rate for 48-blade units is less than $AU20 delivered (about $US20 or £13).

(That eBay search doesn't seem to be geo-targeting very well for me here in Australia; here's one that ought to only turn up items that can be shipped here.)

UPDATE: As mentioned in the comments below, there are rotary blade tenderisers as well, that roll like a pizza cutter. Here's a search that I think finds them a bit more effectively than the above searches.

UPDATE 2: Renowned crapvendors DealExtreme also now have 48-blade tenderisers similar to the one I've got. There's a black one and a white one, each for $US17.40 including delivery to anywhere, which I think undercuts the eBay dealers by a little.

It's easy enough to use a blade tenderiser: You just put it on the meat and press down. The blades slide out of slots in a guard on the bottom, and when you release the pressure springs retract the blades again.

The tougher the gristle you're tenderising, the harder it'll grip the blades and resist them retracting. Basically, the more resistance to the blades a given location on the meat has, and the more impressive the crunching sound when you stab it, the more times that area should be stabbed.

The springs are the only weak point of this design, I think. The standard springs in the tenderiser I got were very stiff and heavily pre-loaded, which meant they retracted the blades out of the meat very well, but forced you to push down on top of the meat too hard in the first place, squashing the steak.

I removed the blades and took the handle apart (four simple screws), removed the standard internal springs, and added the natty external coil-over replacements you see in the picture:

They're shock springs for a model car, and they aren't strong enough to retract the blades by themselves, so I have to push the guard and handle apart a bit myself, but the steak is minimally squashed. I think that's a good trade-off.

The very cheapest blade tenderisers found by that eBay search have only 16 blades, and the spring setup might be better for those. The standard springs don't totally squish the life out of the meat, either; I am unsure how much of my motivation to modify the thing came from an actual need to do so and how much was just my desire to tinker with things.

Apart from that, though, I've had no problems with this thing at all. It works, and it keeps working. I've deliberately bought the toughest cuts of beef I can find - even when they're not actually any cheaper than a slab of chuck - and it's worked, quite quickly, on all of them.

The blade cartridge is removable for cleaning. You push the blue button on the handle to one side and press the tenderiser down on a breadboard, and the blade cartridge pops out the top. The guard at the bottom slides out for cleaning, too. Both of these parts can go in the dishwasher.

Actually, you can put the handle assembly, or the whole assembled tenderiser, in the dishwasher if you like. If you do, though, water will get into the handle, and not want to come out.

The only parts that contact the meat are the blades, the slotted guard and the edge of the guard-holding frame, though, so you can dishwash the removable parts and quickly scrub the frame by hand. As I said in my old review of the AeroPress coffee maker, "impossible to clean" is right up there with "does not actually work" in the list of Mortal Kitchen-Gadget Sins. My blade tenderiser does not have that problem.

Even if you don't have any trouble affording fancy naturally-tender steak, a blade tenderiser could come in handy to make meat more marinatable, or any other time you need a lot of little slits cut in something or someone.

If your grocery budget is tighter, though, one of these things can pay for itself the first time you use it for a family meal. You can even use it after you cook a steak, if there's a gristly bit you missed.

At not much more than twenty bucks delivered for the brand-name one in the States, or for only about twenty bucks delivered on eBay, it comes highly recommended from me.

Posted in MiniReviews. 17 Comments »

Avoiding the blaze of glory

12 July 2012 — dan

A reader writes:

How come NASA spacecraft need all that heat shielding, but SpaceShipOne and Two don't? Does this have something to do with escape velocity - they don't go that fast, so they fall back down when the engines stop and don't have to re-enter. But they do get outside the atmosphere, right? Is there more than one kind of re-entry?

Cherie

There's no clear line where "the atmosphere" stops. By convention, the Kármán line at an altitude of 100 kilometres is treated as the end of the atmosphere; SpaceShipOne made it to 112 kilometres, and SpaceShipTwo is intended to do the same, but with more people on board. But satellites in low orbit well above the hundred-kilometre line need periodic re-boosting to compensate for the drag of the tenuous outer reaches of the atmosphere. Take the International Space Station, for instance; it orbits from 330 to 410 kilometres up, but still needs periodic re-boosting to prevent its orbit decaying. This goes for anything else delivered or serviced by the Space Shuttle, too; inability to reach high orbit was one of the Shuttle's numerous shortcomings.

(The Shuttle carried some satellites that ended up in high orbits, and even space probes that left earth behind entirely, but those payloads needed their own booster rockets for the second part of the trip.)

(Oh, and orbital decay also shows up in umpteen Star Trek episodes as another Acme Mechanically-Assisted Plot-Tensioner, even when the Enterprise seems to be orbiting way above the conceivable atmosphere of any earth-like planet. Presumably they deliberately keep themselves in a super-slow pseudo-orbit by use of engine power, because... tech tech tech.)

You could say that "re-entry" means any trip from orbital altitude back into the atmosphere, but what most people mean when they use the term is a trip from an actual orbit back into the atmosphere. That's where the big difference lies, because orbital velocity is high.

Low-orbital velocity is particularly high, because the closer an orbiting object is to the thing that it's orbiting, the stronger will be the gravitational pull on it, and the higher its orbital speed must be, for it to actually be in orbit and not just fall back down.

The earth, orbiting approximately 150 million kilometres from the (very large mass of the) sun, takes a year to go around it once, travelling at about thirty kilometres per second.

The moon orbits approximately 385,000 kilometres from the earth; if the earth had the mass of the sun then the moon's orbit would be extremely fast at that relatively small distance - Mercury, orbits the sun at an average distance of about 58 million kilometres, and travels at about 48 kilometres per second. But because the earth is much less massive than the sun, the moon takes 27.3 days to go around us once, travelling at only about one kilometre per second relative to us.

The International Space Station's low orbit takes it around the planet in only about ninety minutes; it therefore travels at about 7.7 kilometres per second, more than 22 times the speed of sound at sea level.

Re-entry is still a problem even if you're out in a very distant and slow orbit, though, because you can't just teleport from that distant orbit to the edge of the atmosphere. You have to use something - rockets, or some gravity-assist trick around some other body - to reduce your orbital velocity, putting you on a new orbit that intersects the planet's atmosphere, preferably in a survivable way.

That orbital adjustment reduces your speed relative to the planet, but then your new elliptical path means you fall toward the planet at greater and greater speed. Five hours before splashdown at the end of the Apollo 11 mission, the spacecraft was about 76,000 kilometres from the earth and approaching the planet at less than three kilometres per second. Five and a half hours later, as the spacecraft started to catch some real atmosphere and lose radio contact, they were still about 3,000 kilometres from their splashdown point (including a large diagonal component, since they weren't plunging straight down toward the planet), and were now moving at eleven kilometres per second.

(There's a lot more complexity to orbits and de-orbiting in the real world, of course, not least because many orbits are far from circular, with a slow portion further from the planet and a fast portion closer to it. Such orbits can be rather useful, and various advanced and less-advanced simulators exist to help you get a feel for them.)

So one way or another, a return to the earth from orbit or from a trip to some other part of the solar system involves very high speeds. Such high speeds, in fact, that friction with the air contributes little to the heating effect; it's air piling up in front of you and trying to get out of your way, and being heated by hypersonic compression, that creates the glowing plasma halo and glowing-hot heat shields on re-entering spacecraft.

You can avoid all of this if, like SpaceShipOne and Two and other "sub-orbital" vessels, you never get anywhere near orbital velocity, and just fly up until the sky is black and the earth is curved, then fall back down. When you start to fall there's little air resistance and almost as much gravity as at the surface of the earth (even the International Space Station is close enough to the earth that it's subject to gravity about nine-tenths as strong as at sea level), so you can get up to some moderately impressive speeds by aeroplane standards. But you're a long way from true re-entry speed.

For comparison, the fastest aircraft humans have ever managed to make that truly qualifies as an aircraft - takes off and lands under its own power, can be refuelled and re-used, has enough fuel to fly a reasonable distance, carries living humans and usually keeps them that way - is the Lockheed SR-71 spy plane. Most of the SR-71's technology remains impressive today and was nearly miraculous in 1964, but the thing was such a nuisance to operate (and was largely superseded by satellites and drones) that it's now been retired in favour of its 1950s predecessor, the glider-like U-2, maximum speed only impressive by World War II standards.

Flat out, with its skin hot enough to melt lead and five kilograms of fuel going into the engines per second, the SR-71 could manage about one kilometre per second.

That's nine times the speed of the fastest production car, three times the land speed record, and quite close to the muzzle velocity of the most outrageously fast rifle bullets. But any random piece of dead-satellite or rocket-casing space junk that fireballs its way to destruction in the atmosphere is pretty much certain to beat the SR-71 by a factor of at least ten. Space Shuttle re-entry was carefully controlled to get it under nine kilometres per second before it started really heating up, but you can see why it was such a big deal when Columbia had a hole the size of a saucer in one leading edge.

You can avoid all this, too, if you've got a lot more engine power to play with. Come up with a sci-fi drive that can deliver lots of thrust for long periods of time with little vehicle mass (in technical terms, both large thrust and very high specific impulse; the closest we've managed to come to these goals has been strangely unpopular...), and you can leave the atmosphere as slowly as you like, accelerate to orbital velocity as slowly as you like, and generally Superman your way around the solar system without having to endlessly account for every joule and newton lest you end up drifting to Neptune while your air runs out, or turn into an array of orange streaks across the sky.

This is where "escape velocity" comes in, too. Escape velocity (more correctly, in physics terms, escape speed, since direction is irrelevant) is how fast you need to be going, from wherever you currently are, to break free of the gravity of a given body. If you're at sea level on an earth with a magic spaceship that is not subject to air resistance, then 11.2 kilometres per second is the speed you need. If you shoot off in any direction (even, theoretically, through the planet, if your magic spaceship is also not subject to ground resistance...) at 11.2 kilometres per second, you're not going to come back down.

Escape velocity on the moon (where air resistance really isn't a problem) is only 2.4 kilometres per second, but Alan Shepard's golf balls definitely did come back down. They probably wouldn't have on Phobos or Deimos, though, because those tiny bodies' escape velocities are only 11.3 and 5.6 metres per second, respectively.

Escape velocity isn't of much direct relevance to Earth-launched spacecraft, though, because something shot out of an 11.2-kilometre-per-second cannon at sea level will definitely come back down after atmospheric drag eats most of that speed. The great problem of getting things up out of our atmosphere and gravity well when all you have to propel them are poxy chemical rockets is finding a way to strike a balance between having lots of rocket power, and using most of that power just to launch the fuel and engines that you need to launch the fuel and engines that you need to launch... You get the idea.

A particularly good simulator of this conundrum also exists!