Based on the 2001–2018 average, 1 out of every 1.5 billion humans is in space at any given time, most of them on board the International Space Station.
ISS crew members ferry packages down from the station by putting them in the spacecraft carrying crew back to Earth. But if there’s no planned departure for Earth any time soon—or if NASA gets sick of delivering your internet shopping returns—you might have to take matters into your own hands.
Getting an object down to Earth from the International Space Station is easy: you can just toss it out the door and wait. Eventually, it will fall to Earth.
Excerpted from How To. A former NASA roboticist, Randall Munroe is also the author of What If? and Thing Explainer. Buy on Amazon.
There’s a very small amount of atmosphere at the ISS’s altitude. It’s not much, but it’s enough to produce a tiny but measurable amount of drag. This drag sooner or later causes objects to slow down, fall into a lower and lower orbit, and eventually hit the atmosphere and (usually) burn up. The ISS also feels this drag; it uses thrusters to compensate, periodically boosting itself up into a higher orbit to make up for lost altitude. If it didn’t, its orbit would gradually decay until it fell back to Earth.
Astronauts accidentally deliver packages to Earth this way all the time. While working on the ISS, spacewalkers have accidentally dropped a variety of random objects, including a pair of pliers, a camera, a tool bag, and a spatula an astronaut was using to apply a repair adhesive for testing. Each of these inadvertently created satellites circled the Earth for a few months or years before its orbit decayed.
A package you toss out the door will suffer the same fate as all the lost parts, bags, and random pieces of equipment that have drifted away from the station over the years: it’ll deorbit and enter the atmosphere.
This shipping method has two big problems: First, your package will burn up in the atmosphere before it ever reaches the ground. And second, if it does survive, you’ll have no way to know where it will land. To deliver your package, you’ll have to solve both these problems.
First, let’s look at how to get your package to the ground intact.
Reentry Heating
When stuff enters the atmosphere, it often burns up. This isn’t because of some weird property of space. It’s because everything in orbit is going so fast. When objects hit the air at those speeds, the air doesn’t have time to flow out of the way. It compresses, heats up, turns to plasma, and often melts or vaporizes the object in the process.
To keep our spacecraft from being destroyed, we attach heat shields to the front, to absorb the heat from reentry and protect the rest of the craft.1 We also give them special shapes, which helps create a cushion of air between the shock wave and the surface of the spacecraft, keeping the hottest plasma from touching the hull.
The fate of an object hitting the atmosphere depends on its size.
The Earth’s atmosphere weighs as much as a layer of water 10 meters thick. To figure out whether a meteor is likely to make it through, you can imagine that it’s literally hitting a 10 meter layer of water. If the object weighs more than the water it would have to push aside to reach the surface, it will probably make it through. This works pretty well for a rough approximation!
Very large objects—house-size or larger—have enough inertia to punch through the atmosphere and hit the ground without losing much speed. These are the objects that leave craters in the ground.
Small objects—anything from pebble-size to car-size—are too small to smash through the atmosphere. When they hit it, they heat up until they break apart, evaporate, or both. Sometimes, pieces of these objects survive entry into the atmosphere, either because other pieces absorb the heat and shield them, or because they’re made of a material that can withstand the reentry conditions. But when they do, they lose their orbital speed and then fall at terminal velocity straight down to the ground. After the brief pulse of heat during breakup, this free fall through the cold upper atmosphere takes several minutes, which is why meteorites are often very cold when they’re found.
These surviving bits of debris hit the ground at relatively low speeds. If they land in soft dirt or mud, they can splash a little, but they don’t leave much of a crater. This is why all impact craters on Earth are large: only large, heavy objects keep their orbital kinetic energy all the way to the ground. There are impact “craters” a few feet across—barely larger than the objects that made them—and impact craters a few thousand feet across, but nothing in between.
Without shielding, spacecraft break up in the atmosphere. When large spacecraft enter the atmosphere without a heat shield, between 10 percent and 40 percent of their mass usually makes it to the surface, and the rest melts or evaporates. This is why heat shields are so popular.
To protect your package on the way down, you can use a heat shield, too. The easiest kind is an ablative heat shield, one which burns away as it goes. It’s not reusable, like the heat-resistant tiles on the Space Shuttle, but it’s simpler and can handle a wider range of conditions. Then, you just need to shape the capsule so that it points in the right direction—heat shield in front, package in back—and send it on its way.
You may also want to add a parachute, for the final drop, but if your package is something lightweight or durable, like socks, paper towels, or a letter, it might be able to survive the final terminal velocity fall relatively undamaged.
Every human-built object which has been designed to survive reentry has used a curved protective heat shield—with a few exceptions.
The Apollo Suitcases
The Apollo program sent seven teams of astronauts to land on the Moon. Each crew carried, among other things, a suitcase-size “experiments package” which would be left on the Moon’s surface to take measurements and transmit information to Earth. Six of the seven were powered by radioactivity from plutonium. (The first experiments package, on Apollo 11, was simpler. It had solar power for its electronics, but still used plutonium heaters to keep it warm.)
Six of the Apollo teams landed on the Moon and deployed their suitcases. One of them, Apollo 13, famously did not. After part of their spaceship exploded,2 they aborted the mission and flew back to Earth. Everyone was ok, it was very heroic, etc. But let’s talk about that suitcase.
Since the astronauts didn’t make it to the Moon, they couldn’t leave the plutonium-filled suitcase there, and it came back with them to Earth. That created a problem.
Only the command module, with the astronauts inside, was designed to safely return to the Earth’s surface. The other parts of the spacecraft, including the lunar lander, were designed to burn up in the atmosphere. The command module only had enough room for the astronauts and their samples. The suitcase—and the plutonium core, which was stored separately—would have to stay behind in the doomed lander. But if the container holding the plutonium broke apart, it would scatter the radioactive material into the atmosphere.3
Luckily, the engineers behind the suitcase had anticipated this possibility. The plutonium was contained within a high-strength cask, about the size and shape of a small fire extinguisher, shielded by layers of graphite, beryllium, and titanium. The protective shell would allow it to survive reentry, even as the rest of the discarded lunar module broke apart violently around it.
When the Apollo astronauts climbed into the command module as they approached Earth, they left the suitcase behind in the lunar module; then they fired the lunar module’s engines to divert it to the area over the Tonga Trench, one of the deepest parts of the Pacific—so the cask would fall into the sea and sink to the bottom. In the decades since, no excess radioactivity was ever detected, which means the protective shell did its job. The cask of plutonium lies on the floor of the Pacific to this day. The plutonium is about half-decayed by now, but it’s still producing over 800 watts of heat as of 2019. Maybe some deep-sea critter looking for warmth is cuddled up to it right now.
Send a Letter
One of the best ways to get around the engineering challenges of reentry might be to ditch the heat shield entirely in favor of a simpler solution: a manila envelope.
Lightweight objects that experience more drag start slowing down at a higher altitude, where the air density is lower. Since the air is so thin, it doesn’t heat the object as efficiently, and although the reentry takes longer, peak temperatures can be much lower. In fact, calculations by Justin Atchison and Mason Peck have shown that an object shaped like a sheet of paper, curved to fall flat side first, could in theory enter the atmosphere “softly” without ever reaching especially high temperatures.
If you print your message on a sheet of baking parchment paper, aluminum foil, or some other thin and lightweight material which can survive being warmed up, you might just be able to toss it out the door as is. As long as it’s shaped right, it could make it to the ground intact. In fact, a team of Japanese researchers planned to try this by launching paper airplanes from the ISS. They designed the planes to survive the heat and pressure of reentry, but, sadly, the project never went through.
A package tossed by hand from the ISS will descend gradually over the course of many orbits, with little control over the eventual landing point. Controlling where the package will land is much harder than simply delivering it to Earth.
Returning spacecraft generally try to control where they land. Some do this with more precision than others. SpaceX’s spent rocket boosters can guide themselves precisely enough to land directly on a target on the deck of a boat, while the older Apollo and Soyuz spacecraft have generally missed their targets by a few miles.4 Spacecraft undergoing uncontrolled reentry—like your package—can miss their intended landing site by hundreds or thousands of miles.
You can improve the precision of your package delivery by throwing the package really hard. A fast throw can get the package down into the atmosphere more directly, without a long delay as atmospheric drag causes its orbit to slowly decay in a hard-to-predict way. Surprisingly, the way to do this isn’t to throw the package downward, toward Earth. Instead, you should throw it backward. If you throw it downward, it will still have enough forward speed to stay in orbit—it will just be a slightly different orbit. You want it to lose speed instead.
The faster you throw the package, the more precise its landing. The ISS is traveling at almost 8 kilometers per second, but luckily, you don’t need to throw your package that fast. Shaving off just 100 meters per second from the orbital speed at the ISS’s altitude is enough to deliver your package to the atmosphere. Unfortunately, throwing something at 100 m/s is difficult. Even the fastest pitchers don’t break 50 m/s. Golf balls, on the other hand, travel fast enough. A golfer floating next to the ISS could conceivably hit a golf ball out of orbit in a single stroke. If your package is the size of a golf ball, you can try that delivery method.
If you launch the package at 100 m/s, it will enter the atmosphere traveling at a downward angle of about 1°, which will give you a debris footprint—the area where your package might land—over 2,000 miles long. If you’re aiming for St. Louis, it could land anywhere between Montana and South Carolina. If you can throw it harder—250 or 300 m/s—you can enter the atmosphere at a proportionally steeper angle and cut the debris footprint down to a few hundred miles. However, no matter how fast and precise your throw, the randomness of turbulence and wind will keep you from hitting a target with a precision of better than a few miles.
MIR
In March of 2001, the space station Mir was about to reenter the atmosphere. Most of it was expected to burn up, but some of the larger modules had a chance of making it to the surface. The Russian Mission Control planners tried to time its reentry so it would come down over an uninhabited region of the Pacific, but no one knew exactly where it would land.
Capitalizing on this, Taco Bell came up with a unique promotion: they floated a giant sheet on the Pacific with a bull’s-eye painted on it, and offered a free taco to everyone in America if any piece of Mir hit the target.
Sadly, no debris ultimately hit the bulls-eye.5 Most of the larger pieces hit the surface of the ocean in the vicinity of 40°S 160°W—the “spaceship graveyard,” a region far from land where the wreckage of over 100 spacecraft has splashed down—and sunk to the bottom.
Despite the claims of many eBay auctions, no confirmed Mir debris was ever recovered. If you do find some, you could always try bringing it to Taco Bell headquarters in Irvine, California. Maybe they’ll let you exchange it for a taco.
Addressing
You may not be able to aim your package very carefully, but don’t despair—that doesn’t mean it can’t be delivered! You just need to figure out what address to write on it. But, as the US government learned in the 1960s, figuring out what to write on space packages can be tricky.
The first US spy satellites used film cameras. After they had taken their photos, the capsules containing the film were dropped back to Earth. If all went well, they would be tracked on the way down, and an Air Force aircraft would literally catch them out of the air using a long hook.
Things didn’t always work as planned. Several capsules returned to Earth uncontrolled; one, which came down in the Arctic near Svalbard, was never found. In early 1964, a Corona reconnaissance satellite—after taking a few hundred photos—broke down in orbit, stopped responding, and headed toward an uncontrolled reentry. Government officials watched anxiously, trying to determine where it would enter the atmosphere. Eventually, it became clear that it was going to land somewhere in the vicinity of Venezuela.
Observers in the area were told to watch the skies, and on May 26, 1964, debris was seen streaking over the Venezuelan coast.
The officials thought it had probably landed in the ocean, but it had in fact fallen on the border between Venezuela and Colombia. It was found by some farmers, who took it apart, removed the gold discs they found inside,6 and tried to put the rest of it up for sale. A farmer used the parachute lines to make a harness for his horses. When no one wanted to buy it, the capsule was handed over to Venezuelan authorities, who contacted the United States.
Up until 1964, the returning capsules were labeled UNITED STATES and SECRET in threatening letters, intended to dissuade people from opening them and accessing their highly classified cargo. After the 1964 incident, the United States changed their labeling strategy. Instead of a stern warning, they simply stamped them with a message—in eight languages—promising a reward for bringing the capsule to the nearest US consulate or embassy.
If you want to maximize the chance that the person who finds your package helps deliver it to its intended recipient, bribery might be the way to go.
Illustrations by Randall Munroe
1 Why don’t spacecraft slow down using rockets, then enter the atmosphere at a low speed, to avoid the need for a bulky heat shield? The answer is simple: it would take way too much fuel. The spacecraft that use rockets for landing, like the Curiosity rover or SpaceX’s reusable launchers, do most of their decelerating using atmospheric drag, and only use rockets for the last bit of the landing.
Getting a spacecraft going against gravity fast enough to get into orbit takes dozens of times the spacecraft’s own weight in fuel, which is why rockets are so big. Slowing down would take roughly the same amount. Which means that instead of launching a 1-ton spacecraft to orbit using 20 tons of fuel, you’d need to launch a 1-ton spacecraft and 20 tons of fuel to slow it back down. But now instead of a 1-ton spacecraft, you’re effectively launching a 21-ton spacecraft, which means you’ll need 420 tons of fuel. Compared to 420 tons of fuel, a 100-pound heat shield is a way more efficient solution.
2 It’s not as bad as it sounds. Ok, it was roughly as bad as it sounds.
3 On the other hand, this was the mid-20th century—you’d think if they were so worried about radioactive particles in the atmosphere, perhaps they should have considered not setting off so many nuclear bombs. But what do I know; I wasn’t there.
4 The Apollo command modules landed in the ocean. Soyuz spacecraft land in a large open area in Kazakhstan where they’re not likely to hit anything.
5 Was Taco Bell serious about this? Well, sort of. They bought a $10 million insurance policy to cover the demand for free tacos in the unlikely event of a “win.” This policy was purchased from SCA Promotions, a company which provides coverage for promotional contest wins. When a company wants to promise a large prize to anyone who completes a difficult task, they pay a fixed amount to SCA Promotions, and SCA pays out if they succeed. However, the premiums Taco Bell paid for the policy probably weren’t too high—since they placed the target near the Australian coast, thousands of miles west of the reentry path.
6 The gold discs were part of a science experiment. The science experiment was part of the cover story, in case anyone asked what the satellite was doing up there.
From HOW TO: Absurd Scientific Advice for Common Real-World Problems by RANDALL MUNROE, published by RIVERHEAD, an imprint of Penguin Publishing Group, a division of Penguin Random House LLC. Copyright © 2019 by xkcd inc.
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