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Human Earth explorers land on planet X. They are there to survey it. They are unaware that there is life on X until ...

The locals are very aggressive and fire on them from a distance. The thing is that the projectiles (which could be bullets or arrows or whatever) travel very slowly. So slowly that the humans can see them coming and dodge provided they are looking in the right direction.

If the humans are hit by a projectile then it penetrates them and can be fatal.

Question

How can I scientifically (and not magically) reconcile the slow movement of the projectiles with the fact that they can reach a distant target and be fatal?

Assumptions

The action takes place away from civilisation. You can imagine scrub-land with low hills.

The weapons are hand-held by aliens that can be considered similar to humans.

The projectiles are fired from the weapon, not thrown by hand.

Once a projectile leaves the weapon it is not powered (i.e. it's a bullet, an arrow, etc.).

Any reasonable science-based assumptions may be made about the atmosphere or other physical characteristics of planet X.

The range of the weapons must be at least 50 metres. They must be able to work anywhere on the planet.

Humans must be capable of seeing and dodging an incoming projectile from 20 or so metres away before it reaches them. The projectiles must therefore be slow-moving and big enough to be seen.

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    $\begingroup$ does it have to travel by air? or ground projectiles are ok? $\endgroup$ – Kepotx Mar 1 at 15:15
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    $\begingroup$ How easy does it need to be to dodge? Is this like stepping out of the way of a bubble floating past, or something that can be done if you're alert and nimble, but still poses a legitimate threat? I ask because you don't actually need to slow regular arrows down by very much to make it possible for them to be dodged, but that still doesn't make it easy. $\endgroup$ – Nuclear Wang Mar 1 at 15:24
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    $\begingroup$ "unaware that there is life" - the fact that a sterile Mars-like environment is a home to large humanoids is scientifically unlikely... but this is just a side point. $\endgroup$ – Alexander Mar 1 at 17:25
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    $\begingroup$ most weapons tend to favor reliability, I'm confused why the aliens would bother to use a weapon that an alert person could avoid $\endgroup$ – BKlassen Mar 1 at 17:59
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    $\begingroup$ You not only need to explain why the projectiles are slow but also why the aliens would use them in that case. There can be biological explanations but make it believable. $\endgroup$ – Sebastiaan van den Broek Mar 2 at 7:36

16 Answers 16

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A few years ago my group and I experimented with staff slings, a sling attached to the end of a quarter staff which is used as a lever extension to increase the power input. A quarter staff is just a stick or pole that is your own height plus a quarter. Throwing stones out to sea (from mainland Britain to the Isle of Anglesay) we found we could get around half way to the opposite coast, 1600ft distant, with the the local glacial deposited pebbles. They traveled fairly slowly and could easily be tracked but the most interesting projectiles were the bar shaped slate stones we threw. These were about 1x1 inch and around 2 inches long and made a loud buzzing noise when thrown. We also found that if thrown at the cliff face behind (this was discovered by accidentally releasing the stone too early) it would shatter into razor sharp spinning shards). Throwing small grenadillos or even over short distances, grenades would be devastating for such a primitive weapon. Reactive metal projectiles like phosphor or perhaps a gas warhead like chlorine similar but reduced in size to those used against the city of Leicester in the 1400's

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    $\begingroup$ Thanks. I like that answer. I hadn't thought of sound alerting the targets but that would make an interesting twist. It would help them know where to look but would cause increasing confusion the more shooters there were. Also the shattering into sharp pieces sounds plausible and presumably they would have experimented with all kinds of sharp but noisy stones to both intimidate and enforce retreat. $\endgroup$ – chasly from UK Mar 2 at 12:25
  • $\begingroup$ If the ammo was suitably aerofoil shaped, like a flying disk, then it would both help extend the range, and also allow the natives to perform curved "trick shots" such as boomeranging the projectiles around to attach from behind. $\endgroup$ – Chronocidal Aug 4 at 12:38
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    $\begingroup$ If they had the technology to build grenade-like devices, wouldn't they realistically be able to develop some technology to launch them faster, like cannons or at least something like a crossbow? $\endgroup$ – Hypnosifl Aug 4 at 22:44
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Lower gravity on the planet

Projectiles start falling to the ground as soon as they are launched. On Earth this means if you want to lob a boulder onto your enemy, you need your catapault/trebuchet to get the boulder going fast so it can cover the distance between you and your opponent before it finishes falling to the ground. On an alien planet with lower gravity, the projectile will fall to the ground more slowly so it doesn't need to go as fast. If it's going slower it will still reach your opponent. So your aliens pick up big boulders and throw them, but they move slowly through the air like astronauts on the moon:

https://youtu.be/x2adl6LszcE?t=95

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  • $\begingroup$ Arrows on the moon!! $\endgroup$ – Joe Bloggs Mar 1 at 21:58
  • $\begingroup$ Lower gravity would indicate thinner atmosphere so less air resistance on the projectile too. $\endgroup$ – Anketam Mar 2 at 3:42
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    $\begingroup$ @Anketam Is that generally true? Titan has a thicker atmosphere than Earth but lower gravity. $\endgroup$ – Tashus Mar 2 at 4:17
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    $\begingroup$ @Tashus One other major factor for atmosphere is the temperature, lower temperature gasses move slower, and thus can be contained with lower gravity, hydrogen under a temperature of 1k will have enough gravity on its own to collect together and form the base of a star $\endgroup$ – Ferrybig Mar 2 at 13:43
  • $\begingroup$ Hmm just learned something new $\endgroup$ – Anketam Mar 2 at 18:40
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Frisbees, with explosives. The thrower presses a switch, or pulls a tag, or does something else which does not affect the weight distribution of the disc before throwing in order to arm the device for real use. Practice can be done with unarmed discs, or more frequently, with imitations made to have the same weight and mass distribution.

A well-thrown frisbee can travel quite a distance. They are sufficiently slow moving and big enough to be seen that Ultimate Frisbee players can often effectively run to dodge (or more commonly, intercept) the flying object.

I have seen humans sent to the hospital with serious injuries from even non-exploding hard plastic frisbees they weren't paying sufficient attention to striking them the wrong way in the head.

A literal American football, rest-of-the-world football, baseball, etc. could also work, with launching mechanisms traditionally used in those sports. The arming device could be set so that it won't explode on the person launching it, and the exploding part only becomes active after the launching shock and a certain amount of flight.

While only the most skilled local warriors might have accuracy at that range with a football type weapon, unless they have some special biology, a moderately skilled trained humanoid could probably get a frisbee or baseball reasonably well targeted at that range.

You can even assume the locals have a sport in which they use something very much like the object which is used as a weapon when manufactured with explosives. Then, they have a well-trained military capable of defending the peaceful tribe without having to dedicate a lot of specific time/resources/etc. to defensive war preparations.


If you need to have a launcher, I'd go an exploding baseball type object (think about pop fly ball trajectories for ease of dodging if needed) combined with something like a water balloon launcher, or a traditional catapult/trebuchet with a blunt stone, burning ball of wood, etc. We humans had & used those centuries ago. Also consider Lacrosse-style basket-on-stick ball launchers and/or disc launchers like hand held traps, used to launch clay "pigeons" in shotgun shooting when non-handhelds aren't available.


Semi-related: A very fast baseball can also be very dangerous, but not as readily dodgeable and therefore not as good a fit to this question as a slower baseball that merely explodes at the end.

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    $\begingroup$ I'm not sure if the explosives are necessary, blades around the perimeter could be fatal if they hit the right spot $\endgroup$ – rtpax Mar 1 at 19:17
  • $\begingroup$ @rtpax I started out thinking about blades too, but they would probably make it harder to throw. If the blades came out mid-flight, they would likely affect the flight path and reduce accuracy. Playing catch or other sport to practice, with blades coming out mid-flight, would not be a lot of fun on the receiving end. So I think explosives would likely fit the criteria better. $\endgroup$ – WBT Mar 1 at 19:19
  • $\begingroup$ This is looking good. You may not have noticed, "weapons are hand-held" and "Once a projectile leaves the weapon". This of course implies that the aliens don't throw the Frisbees by hand. Is it possible to fix this with a hand-held 'weapon' projecting the Frisbee? (preferably aimable) $\endgroup$ – chasly from UK Mar 1 at 19:34
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    $\begingroup$ @chaslyfromUK Sure there are, just less common. $\endgroup$ – WBT Mar 1 at 19:51
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    $\begingroup$ I'm picturing something like the Disc Blade Gun from Ratchet & Clank when you mentioned that it has to be something like a frisbee shooter. $\endgroup$ – Phlucious Mar 1 at 22:12
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Your 'projectile' is a swarm of poisonous insects

Or, some sort of living creature. Or, like, nanobots. The aliens keep them in some sort of handheld storage device. When they want to fire, they release the swarm. Tiny, extremely poisonous insects/creatures/bots fly through the air, and, if they encounter humans, sting them to death.

The swarm isn't really 'guided' so if you get out of the way, they sort of buzz on to wherever they were going. Easily dodged, if you are paying attention, but quite fatal if you end up in the middle of them.

Like this, except with more bees and less Doberman.

enter image description here

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    $\begingroup$ I initially misread that as "Doobie-o-matic", and wondered how spliff rolling fitted with burglars and a dog. $\endgroup$ – Graham Mar 1 at 23:57
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    $\begingroup$ Hive hand from Half-Life! $\endgroup$ – n0rd Mar 2 at 0:40
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    $\begingroup$ @Graham the spliff rolling was obviously in the hands of the cartoonist to come up with this in the first place! $\endgroup$ – gbjbaanb Mar 3 at 0:52
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We've had those for millenia in our own history. They are called spears. The launcher is your own arm.

Depending on the spear and who is throwing it, the range you ask for is more than feasible. And if you see a piece of wood with a very sharp point coming your way, thrown by someone sixty or more feet away, you have enough time to duck or cartwheel out of the way.

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    $\begingroup$ If you can dodge a spear, you can dodge a ball! $\endgroup$ – kingledion Mar 2 at 14:14
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The locals perform some entomological warfare by lobbing nests of deadly alien bees at the explorers, just like some humans may have done in the past.

Using slings or other such tool the locals (presumably wearing some protective clothes) throw the nests in a high arc at groups of explorers. The explorers can see the nests coming and jog away from the landing site before the inevitable angry cloud of alien bees form.

However, if the explorers don't get out of the area quick enough then they face death by a swarm of projectiles who just had their home turned into a grenade.

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    $\begingroup$ or my idea: native plants whose reproductive cycle involves an explosive tube base that shoots a stem into the distance, tipped with an explosive pod that scatters sharp little seeds on impact. Not enough explosive to make it go fast or far, but dangerous enough if one hits you. The natives cultivate and harvest these for weapons. $\endgroup$ – gbjbaanb Mar 3 at 1:17
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The energy behind a projectile depends on mv2. Bullets and similar projectiles maximize v because energy increases as the square.

Your aliens maximize m.

giant throws buffalo

A very large mass projectile can be moving quite slowly and still do a lot of damage when it hits. The humans can see these things coming and dodge to some degree as long as there are not too may at once.

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    $\begingroup$ A 9mm bullet with a muzzle velocity of ~1180 m/s has about the same amount of kinetic energy as a 1350 kg Volkswagen Beetle going ~39 km/hr. Different damage profile, sure, but I definitely wouldn't want to be hit by either. $\endgroup$ – bta Mar 1 at 22:17
  • $\begingroup$ @bta A 9mm bullet is ~7.5g. 0.0075 kg * 1180 (m/s)^2 = 10 kJ. 1350 kg * 11 (m/s)^2 = 163 kJ. The car has to be going 2.8 m/s -> 10 km/hr for this to be true. $\endgroup$ – kingledion Mar 2 at 14:19
  • $\begingroup$ In addition - not many 9 mm bullets leave a muzzle travelling at three and a half times the speed of sound. $\endgroup$ – Penguino Mar 3 at 21:04
  • $\begingroup$ @kingledion I was using the data for the Browning rounds as listed in this chart, which might not be accurate. Either way, the numbers confirm that slow moving can still be quite deadly. Particularly if that velocity is applied in a mostly vertical direction. $\endgroup$ – bta Mar 4 at 17:52
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    $\begingroup$ @bta Note that link is all in imperial units. Velocity in ft/sec, not m/sec; energy in ft-lbs; mass in grains. Those are the standard measurements for discussing small arms in the US, but you need unit conversions to compare to a metric Volkswagen :) $\endgroup$ – kingledion Mar 4 at 18:03
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grenade launcher

we are thinking so hard about this answer. a grenade launcher. you said it needs to still be lethal and it seems everyone is trying to find a way to make an instakilling slow moving object. why not make the slow moving projectile explode when it reaches its target. It could even be rocket propelled and be slow enough for people to react. or we could go with plasma rounds. a chain reaction of creating plasma out of the very atmosphere focused in the direction of the intended target.

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Everyone seems to be going fairly high tech or require some special requirements of the environment such as low gravity for such a system to work. But there are real-world options.

So you want a slow moving ballistic (= unpowered) projectile weapon with a range of at least 50 meters that can inflict fatal wounds. Let's explore some options.

The Math

I'm using [this tool][1] to calculate distances, angles and timing.

You said that at 20 meters, a human should be able to dodge. Let's say it takes about 1 second to recognize the projectile and move out of the way. Let's also assume that the humanoid aliens are about our size and fire their weapons from about 1.7 meters in height.

If you were to fire straight at the target - so at a 0 degree angle in respect to the ground, to reach a distance of about 20 meters you would need to fire the projectile at 35 meters per second and you would hit the targets ankle at 0.58 seconds after firing.

Through googling, we know that to maximize distance we need to fire at 45 degrees and we can then calculate that the slowest projectile that can hit at 20 meters needs to travel at about 14 meters per second. This takes 2.177 seconds and since the projectile is coming from above, it might hit more than just the ankle.

For 50 meters, we need at least 22 m/s and it takes 3.3 seconds. So just to be safe, let's say the maximum firing velocity is 25 m/s, giving us a maximum range of 65 meters. Since the aliens are out for blood, they always throw at this velocity and adjust the angle for closer targets.

So at 50 meters with a 25 m/s projectile the aliens can fire at an angle of 24 degrees and it takes 2.23 seconds to reach the target. At 20 meters distance the alien can fire at 5 degrees with a flight time of 0.85 seconds. 0.85 seconds might not be enough to reliably dodge, but someone with fast reflexes can probably still do it.

Regular arrows at about 70 or more m/s are too fast, hitting something at 20 meters away in 0.25 seconds. A baseball thrown by hand by a professional player can travel at 40 m/s giving us 0.58 seconds to dodge at 20 meters (incidentally, the batter is standing even closer and can react well enough to hit the ball with his bat. That's a good sign that your humans can dodge within 1 second).

A baseball isn't very deadly, but it shows that the speeds necessary can be achieved even without having assisting tools. For example, a throwing knife, while not being the deadliest weapon, already fulfills your requirements, although they loses effectiveness at range.

Deadliness

To make a projectile deadly you need force, if there's enough force it will even penetrate the target. A bullet gets its force from its speed, a thrown rock gets its force from it's mass. Something like an arrow is neither very fast (at 70 m/s) nor massive, but by using a pointy arrow head, most of the force can be focused on the very tip of the arrowhead which can then penetrate the target. If you use something like a blunt arrowhead, it won't be able to do much damage.

Since you don't want speed, as long as it's more than 25 m/s, we have to focus on mass and being pointy.

A Javelin, or throwing spear, is a weapon that fits your demands perfectly. The current world record in javelin throw is 98 meters. Using the previously calculated formulas, we can guess that this was thrown at about 31 m/s. These javelins are optimized for throwing distance, weight 800 grams and are only pointy enough to stick in the ground on landing. Even unoptimized, they occasionally hit one of the judges (as searchable on youtube), and occasionally penetrate and stick in them - deaths happen. Incidentally, apparently it's considered save enough for judges to stand on the field - I assume a testament to their dodgeability.

If they're made more pointy and their weight is increased, they should become both more deadly and even slower, so they better fit in your dodgeable at 20 meter criterion again.

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The projectiles are white-hot (or caustic or very sharply pointed or covered in some kind of a contact poison/toxin). If you fail to dodge they can do some major damage even without the piercing force behind them that comes from velocity in the case of bullets on earth.

You would need either some form of levitation, or lower gravity, to get past the fact that a slow projectile would generally fall to the ground before hitting the target.

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Extremely dense projectiles in low gravity

Works great on a planet with massive Osmium reserves!

As stated in another answer, the equation for the energy behind a projectile is $mv^2$. This means that (aside from physical hazards such as explosives, heat, and sharpness) there are two ways to go about increasing the power of the projectile:

  • Make it heavier: A truck at 40 MPH is much more likely to kill you than a piece of paper at the same speed
  • Make it faster: Although much more effective at increasing the energy (because it is squared), this projectile is slow

So, if we go with heavier, we'll need it to be dense. I'll assume the target energy should be that of a .230 grain, .45 caliber bullet (252 M/s, 28.35 grams). This equates to a $mv^2$ of 7,813,968. I'll assume "slow" is two seconds to cross the 20-meter distance mentioned...10 meters per second. This gives us a $v^2$ of 100, meaning that the projectile must weigh around 78 kilograms (172 lbs). That gives us around 3,473 cubic centimeters of projectile, assuming it's made of Osmium (the densest metal that exists). This could be formed into a cylinder with a diameter of 10 cm and a length of 45 cm. Of course, this would be quite hard to make into a handheld weapon, and would drop to the ground almost instantly. The solution? Lower the gravity! On a Moon-sized planet, this would be only 13 kg. Add a new explosive to propel it at the right speed, and make a few tweaks to the gravity, and voila!

Why would these be used in the first place?

The land is covered in hills, and the gravity fluctuates enormously depending on altitude. Bullets and arrows are hard to aim, since you have to be able to tell how they drop depending on where you are. Problem is, they travel so fast you can't see where they went. Imagine some sort of fog, or really tall grass. However, this slow projectile is very easy to see, allowing the attacker to gauge their aim after one shot. As an additional bonus, the durable and easy-to-find projectiles would be easy to scavenge after the battle is over, allowing them to be reused.

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Everyone is offering different methods without any justification why that method exists. Without a plausible explanation, a slow moving launched projectile will break suspension of disbelief. Here is a system that can work...

Your aliens are slow moving, strong, and have exterior shells. Thus they need tools to speed weapons to be able to penetrate shells of their enemies. With their strength, they use huge arrows (made out of steel) that move slower than human arrow to kill their enemies. They do not need it to be faster as their enemies are not too fast either. But a human is fast enough to evade these arrows. However, the size and sharpness of these shell destroying arrows is very deadly to humans.

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  • $\begingroup$ I was looking for this answer. The projectiles may be slow from a human perspective, but if they're not effective for that people they wouldn't be used...but the reason why can vary. Slow movement, thick shells, and heavy projectiles makes one setup for this. Might not even need slow movement otherwise if certain kinds of movement are constrained so they can't dodge these projectiles well. And thick shelled creatures may be less able (or likely) to dodge well, anyway. Maybe they can't jump or fall easily, or have a hard time looking up high enough to accurately dodge missiles from above $\endgroup$ – Megha Aug 6 at 5:09
  • $\begingroup$ Erik Flint's Mother of Demons has a similar setup, where one of the advantages humans had was certain kinds of motion unnatural to the other species - though it did keep the peoples roughly balanced, with positives and negatives for the physicality of both folk. $\endgroup$ – Megha Aug 6 at 5:13
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I'd lean toward a small ball-like projectile fired from a gun taking advantage of the Magnus effect by launching it with a lot of bottom-spin. (You can see the Magnus effect by comparing the basketball dropped off a dam without spin and with spin.)

The ball would need to have a skin with a design on it to magnify the effect, but thin enough that it bursts on impact with the target, releasing an acid that "burns" through clothing and flesh. It could be an acid vapor under pressure, for instance. You could even have the ball's skin only partially resistant to the acid so that it's eating at the ball from the inside, making it more fragile and prone to bursting when it hits its target. (The weapon would fill the ball when you cocked it just prior to firing it.) That also has the advantage of them bursting sometimes in the air before reaching the target, which may still be useful to the attacker sometimes.

In terms of justifying it vs. arrows or metal bullets, perhaps there isn't that much ore and wood around, but there are abundant sources of this acid and the material for the skin of the ball. (Perhaps they both come from the same abundant plant, for instance.)

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    $\begingroup$ Perhaps the slow speed is necessary to prevent it from bursting midair from air resistance $\endgroup$ – Redwolf Programs Aug 6 at 12:35
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Well if the projectiles are fired upwards they would take a long time to fall on the target, making it easy to predict where it's going to land if you can see it being fired.

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Projectiles that are accelerated over the course of their flight

Such examples of this would be the real-world GyroJet ammo, in which each caseless projectile had a small "rocket motor" which accelerated the projectile over the course of its flight, as opposed to a traditional bullet where the acceleration occurs in one very short event. These rounds sped up over the course of their flight until their propellant ran out, so it could theoretically start slower yet reach lethal speeds easily after traveling a long distance.

EDIT: I reread the originally posted question after posting this answer. While GyroJet ammo does make the projectile powered after leaving the weapon, these were technically their own, self-contained bullet

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Snakes. On a Plane.

Any projectile travelling through air will have resistance and gravity to defeat. Gravity will pull the projectile into a parabolic arc, air resistance will stunt that arc. Faster throwing means a flatter arc. This seems to exclude slow, easy to dodge projectiles.

But. If said projectile has lift (like an airplane) it will have a force to resist gravity. Problem: This increases resistance, slowing the projectile even faster. Also, a wing that is slowing down increasingly loses lift, so the arc would be flatter, but not much. (Or the glidepath has a steady downward angle (maybe the aliens always attack from higher ground, with some kind of stone frisbee?))

But. If the plane has a way to manipulate it's coefficient of lift CoL, it can produce the same lifting force from different airspeeds, thus travelling horizontally in a straight line until all forward energy is spent. This is finicky, and requires constant adjustments. No easy mechanism will do that, without handwaving.

But. There are tree-snakes on earth that can a) glide, and b) have a way of manipulating their CoL (they use it for soft landings, exchanging much of their forward momentum for lift shortly before touchdown) - they have the necessary sensory equipment and motor-coordination to pull it off.

Your martians trained highly venomous snakes to fly on a flat trajectory after being flung towards someone (bonus: some snakes have true heat-vision!). The snakes will strike on impact. The farther you are away, the slower the snake will already be, at some point dropping from the air like a stone.

btw. the sum of all straight horizontal trajectories could be called a plane! That' s where the snakes would be.

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