You’d have to do it quickly. Compressed helium cylinders are smooth and often quite heavy, which means they have a high terminal velocity. You’ll have only a few minutes to use up all the cylinders. (As soon as you emptied one, you could drop it.)
You can’t get around this problem by moving your starting point higher. As we learned from the steak incident, since the upper atmosphere is pretty thin, anything dropped from the stratosphere or higher will accelerate to very high speeds until it hits the lower atmosphere, then fall slowly the rest of the way. This is true of everything from small meteors1 to Felix Baumgartner.
But if you inflated the balloons quickly, possibly by connecting many canisters to it at once, you’d be able to slow your fall. Just don’t use too much helium, or you’ll end up floating at 16,000 feet like Larry Walters.
While researching this answer, I managed to lock up my copy of Mathematica several times on balloon-related differential equations, and subsequently got my IP address banned from Wolfram|Alpha for making too many requests. The ban-appeal form asked me to explain what task I was performing that necessitated so many queries. I wrote, “Calculating how many rental helium tanks you’d have to carry with you in order to inflate a balloon large enough to act as a parachute and slow your fall from a jet aircraft.”
Sorry, Wolfram.
1While researching impact speeds for this answer, I came across a discussion on the Straight Dope Message Board about survivable fall heights. One poster compared a fall from height to being hit by a bus. Another user, a medical examiner, replied that this was a bad comparison:“When hit by a car, the vast majority of people are not run over; they are run under. The lower legs break, sending them into the air. They usually strike the hood of the car, often with the back of the head impacting the windshield, ‘starring’ the windshield, possibly leaving a few hairs in the glass. They then go over the top of the car. They are still alive, although with broken legs, and maybe with head pain from the nonfatal windshield impact. They die when they hit the ground. They die from head injury.”
The lesson: Don’t mess with medical examiners. They’re apparently pretty hardcore.
Everybody Out
Q. Is there enough energy to move the entire current human population off-planet?
—Adam
A. There are a bunch of science fiction movies where, because of pollution, overpopulation, or nuclear war, humanity abandons Earth.
But lifting people into space is hard. Barring a massive reduction in the population, is launching the whole human race into space physically possible? Let’s not even worry about where we’re headed—we’ll assume we don’t have to find a new home, but we can’t stay here.
To figure out if this is plausible, we can start with an absolute baseline energy requirement: 4 gigajoules per person. No matter how we do it, whether we use rockets or a cannon or a space elevator or a ladder, moving a 65-kilogram person—or 65 kilograms of anything—out of the Earth’s gravity well requires at least this much energy.
How much is 4 gigajoules? It’s about a megawatt-hour, which is what a typical US household consumes in electricity in a month or two. It’s equal to the amount of stored energy in 90 kg of gasoline or a cargo van full of AA batteries.
Four gigajoules times seven billion people gives us 2.8×1018 joules, or 8 peta-watt-hours. This is about 5 percent of the world’s annual energy consumption. A lot, but not physically implausible.
However, 4 gigajoules is just a minimum. In practice, everything would depend on our means of transportation. If we were using rockets, for example, it would take a lot more energy than that. This is because of a fundamental problem with rockets: They have to lift their own fuel.
Let’s return for a moment to those 90 kilograms of gasoline (about 30 gallons), because they help illustrate this central problem in space travel.
If we want to launch a 65-kilogram spaceship, we need the energy of around 90 kilograms of fuel. We load that fuel on board—and now our spaceship weighs 155 kilograms. A 155-kilogram spaceship requires 215 kilograms of fuel, so we load another 125 kilograms on board . . .
Fortunately, we’re saved from an infinite loop—where we add 1.3 kilograms for every 1 kilogram we add—by the fact that we don’t have to carry that fuel all the way up. We burn it as we go, so we get lighter and lighter, which means we need less and less fuel. But we do have to lift the fuel partway. The formula for how much propellant we need to burn to get moving at a given speed is given by the Tsiolkovsky Rocket equation:
and are the total mass of the ship plus the fuel before and after the burn, and is the “exhaust velocity” of the fuel, a number that’s between 2.5 and 4.5 km/s for rocket fuels.
What’s important is the ratio between , the speed we want to be going, and , the speed that the propellant exits our rocket. For leaving Earth, we need a of upward of 13 km/s, and is limited to about 4.5 km/s, which gives a fuel-to-ship ratio of at least . If that ratio is x, then to launch a kilogram of ship, we need ex kilograms of fuel.
As x grows, this amount gets very large.
The upshot is that to overcome Earth’s gravity using traditional rocket fuels, a 1-ton craft needs 20 to 50 tons of fuel. Launching all of humanity (total weight: around 400 million tons) would therefore take tens of trillions of tons of fuel. That’s a lot; if we were using hydrocarbon-based fuels, it would represent a decent chunk of the world’s remaining oil reserves. And that’s not even worrying about the weight of the ship itself, food, water, or our pets.1 We’d also need fuel to produce all these ships, to transport people to the launch sites, and so forth. It’s not necessarily completely impossible, but it’s certainly outside the realm of plausibility.
But rockets aren’t our only option. As crazy as it sounds, we might be better off trying to (1) literally climb into space on a rope, or (2) blow ourselves off the planet with nuclear weapons. These are actually serious—if audacious—ideas for launch systems, both of which have been bouncing around since the start of the Space Age.
The first approach is the “space elevator” concept, a favorite of science fiction authors. The idea is that we connect a tether to a satellite orbiting far enough out that the tether is held taut by centrifugal force. Then we can send climbers up the rope using ordinary electricity and motors, powered by solar power, nuclear generators, or whatever works best. The biggest engineering hurdle is that the tether would have to be several times stronger than anything we can currently build. There are hopes that carbon nanotube-based materials could provide the required strength—adding this to the long list of engineering problems that can be waved away by tacking on the prefix “nano-.”
The second approach is nuclear pulse propulsion, a surprisingly plausible method for getting huge amounts of material moving really fast. The basic idea is that you toss a nuclear bomb behind you and ride the shockwave. You’d think the spacecraft would be vaporized, but it turns out that if it has a well-designed shield, the blast would fling away before it has a chance to disintegrate. If it could be made reliable enough, this system would in theory be capable of lifting entire city blocks into orbit, and could—potentially—accomplish our goal.
The engineering principles behind this were thought to be solid enough that in the 1960s, under the guidance of Freeman Dyson, the US government actually tried to build one of these spaceships. The story of that effort, dubbed Project Orion, is detailed in the excellent book of the same name by Freeman’s son, George. Advocates for nuclear pulse propulsion are still disappointed that the project was cancelled before any prototypes were built. Others argue that when you think about what they were trying to do—put a gigantic nuclear arsenal in a box, hurl it high into the atmosphere, and bomb it repeatedly—it’s terrifying that it got as far as it did.
So the answer is that while sending one person into space is easy, getting all of us there would tax our resourc
es to the limit and possibly destroy the planet. It’s a small step for a man, but a giant leap for mankind.
1There are probably around a million tons of pet dog in the US alone.
weird (and worrying) questions from the what if? inbox, #7
Q. In Thor the main character is at one point spinning his hammer so fast that he creates a strong tornado. Would this be possible in real life?
—Davor
Q. If you saved a whole life’s worth of kissing and used all that suction power on one single kiss, how much suction force would that single kiss have?
—Jonatan Lindström
Q. How many nuclear missiles would have to be launched at the United States to turn it into a complete wasteland?
—Anonymous
Self-Fertilization
Q. I read about some researchers who were trying to produce sperm from bone marrow stem cells. If a woman were to have sperm cells made from her own stem cells and impregnate herself, what would be her relationship to her daughter?
—R Scott LaMorte
A. To make a human, you need to put together two sets of DNA.
In humans, these two sets are held in a sperm cell and an egg cell, each of which holds a random sample of the parents’ DNA. (More on how that randomization works in a moment.) In humans, these cells are from two different people. However, that doesn’t necessarily have to be the case. Stem cells, which can form any type of tissue, could in principle be used to produce sperm (or eggs).
So far, nobody has been able to produce complete sperm from stem cells. In 2007, a group of researchers succeeded in turning bone marrow stem cells into spermatogonial stem cells. These cells are the predecessors to sperm. The researchers couldn’t get the cells to fully develop into sperm, but it was a step. In 2009, the same group published a paper that seemed to claim they’d made the final step and produced functioning sperm cells.
There were two problems.
First, they didn’t actually say they had produced sperm cells. They said they produced sperm-like cells, but the media generally glossed over this. Second, the paper was retracted by the journal that published it. It turns out the authors had plagiarized two paragraphs of their article from another paper.
Despite these problems, the fundamental idea here is not that far-fetched, and the answer to R. Scott’s question turns out to be a little bit unsettling.
Keeping track of the flow of genetic information can be pretty tricky. To help illustrate it, let’s take a look at a highly simplified model that may be familiar to fans of role-playing games.
Chromosomes: D&D edition
Human DNA is organized into 23 segments, called chromosomes, and each person has two versions of each chromosome—one from their mother and one from their father.
In our simplified version of DNA, instead of 23 chromosomes, there will be just seven. In humans, each chromosome contains a huge amount of genetic code, but in our model each chromosome will control only one thing.
We’ll use a version of of D&D’s “d20” system of character stats in which each piece of DNA contains seven chromosomes:
Six of these are the classic character stats from role-playing games: strength, constitution, dexterity, charisma, wisdom, and intelligence. The last one is the sex-determining chromosome.
Here’s an example DNA “strand”:
In our model, each chromosome contains one piece of information. This piece of information is either a stat (a number, usually between 1 and 18) or a multiplier. The last one, SEX, is the sex-determining chromosome, which, as with real human genetics, can be “X” or “Y.”
Just like in real life, each person has two sets of chromosomes—one from their mother and one from their father. Imagine that your genes looked like this:
The combination of these two sets of stats determines a person’s characteristics. Here’s the simple rule for combining stats in our system:
If you have a number for both versions of a chromosome, you get the bigger number as your stat. If you have a number on one chromosome and a multiplier on the other, your stat is the number times the multiplier. If you have a multiplier on both sides, you get a stat of 1.1
Here’s how our hypothetical character from earlier would turn out:
When one parent contributes a multiplier and the other contributes a number, the result can be very good! This character’s constitution is a superhuman 24. In fact, other than a low score in wisdom, this character has great stats all around.
Now, let’s say this character (call her “Alice”) meets someone else (“Bob”):
Bob also has stellar stats:
If they have a child, each one will contribute a strand of DNA. But the strand they contribute will be a random mix of their mother and father strands. Every sperm cell—and every egg cell—contains a random combination of chromosomes from each strand. So let’s say Bob and Alice make the following sperm and egg:
If these sperm and egg combine, the child’s stats will look like this:
Alice has her mother’s strength and her father’s wisdom. She also has superhuman intelligence, thanks to the very good 14 contributed by Alice and the multiplier contributed by Bob. Her constitution, on the other hand, is much weaker than either of her parents, since her mother’s 2x multiplier could only do so much with the “5” contributed by her father.
Alice and Bob both had a multiplier on their paternal “charisma” chromosome. Since two multipliers together result in a stat of 1, if Alice and Bob had both contributed their multiplier, the child would have a rock-bottom CHR. Fortunately, the odds of this happening were only 1 in 4.
If the child had multipliers on both strands, the stat would have been reduced to 1. Fortunately, since multipliers are relatively rare, the odds of them lining up in two random people are low.
Now let’s look at what would happen if Alice had a child with herself.
First, she’d produce a pair of sex cells, which would run the random selection process twice:
Then the selected strands would be contributed to the child:
The child is guaranteed to be female, since there’s nobody to contribute a Y chromosome.
The child also has a problem: For three of her seven stats—INT, DEX, and CON—she inherited the same chromosome on both sides. This isn’t a problem for DEX and CON, since Alice had a high score in those two categories, but in CON,she inherited a multiplier from both sides, giving her a constitution score of 1.
If someone produces a child on their own, it dramatically increases the likelihood that the child will inherit the same chromosome on both sides, and thus a double multiplier. The odds of Alice’s child having a double multiplier are 58 percent—compared to the 25 percent chance for a child with Bob.
In general, if you have a child with yourself, 50 percent of your chromosomes will have the same stat on both sides. If that stat is a 1—or if it’s a multiplier—the child will be in trouble, even though you might not have been. This condition, having the same genetic code on both copies of a chromosome, is called homozygosity.
Humans
In humans, probably the most common genetic disorder caused by inbreeding is spinal muscular atrophy (SMA). SMA causes the death of the cells in the spinal cord, and is often fatal or severely disabling.
SMA is caused by an abnormal version of a gene on chromosome 5. About 1 in 50 people have this abnormality, which means 1 in 100 people will contribute it to their children . . . and, therefore, 1 in 10,000 people (100 times 100) will inherit the defective gene from both parents.2
If a parent has a child with his- or herself, on the other hand, the chance of SMA is 1 in 400—since if he or she has a copy of the defective gene (1 in 100), there’s a 1 in 4 chance it will be the child’s only copy.
> One in 400 may not sound so bad, but SMA is only the start.
DNA is complicated
DNA is source code for the most complex machine in the known universe. Each chromosome contains a staggering amount of information, and the interaction between DNA and the cell machinery around it is incredibly complicated, with countless moving parts and Mousetrap-style feedback loops. Even calling DNA “source code” sells it short—compared to DNA, our most complex programming projects are like pocket calculators.
In humans, each chromosome affects many things through a variety of mutations and variations. Some of these mutations, like the one responsible for SMA, seem to be entirely negative; the mutation responsible has no benefit. In our D&D system, it’s like a chromosome having an STR of 1. If your other chromosome is normal, you’ll have a normal character stat; you’ll be a silent “carrier.”
Other mutations, like the sickle-cell gene on chromosome 11, can provide a mix of benefit and harm. People who have the sickle-cell gene on both their copies of the chromosome suffer from sickle-cell anemia. However, if they have the gene on just one of their chromosomes, they get a surprise benefit: extra resistance to malaria.
In the D&D system, this is like a “2x” multiplier. One copy of the gene can make you stronger, but two copies—double multipliers—lead to a serious disorder.
These two diseases illustrate one reason that genetic diversity is important. Mutations pop up all over the place, but our redundant chromosomes help blunt this effect. By avoiding inbreeding, a population reduces the odds that rare and harmful mutations will pop up at the same place on both sides of the chromosome.
Inbreeding coefficient
What If? Page 12