In the movie Gravity the driving force of the plot is a catastrophic cascade of space debris. An exploding satellite sends high speed debris into the path of other satellites, and the resulting collisions create more space debris until everything from a space shuttle to the International Space Station faces an eminent threat of destruction. Not unexpectedly, the movie portrayal of such a situation is not particularly accurate, but the risk of a debris cascade is very real. 

The projected level of debris in the future for various orbits. Credit: NASA

It’s known as the Kessler syndrome, after Donald Kessler, who first imagined the scenario in the 1970s. The problem comes down to the fact that small objects in Earth orbit can stay in orbit for a very long time. If an astronaut drops a bolt, it can stay in orbit for decades or centuries. Because the relative speed of two objects in orbit can be quite large, it doesn’t take a big object to pose a real threat to your spacecraft. On the highway a small pebble can chip your car windshield. In space it can be done by a chip of paint traveling at thousands of kilometers per hour. In the history of the space shuttle missions, there were more than 1,600 debris strikes. Because of such strikes, more than 90 space shuttle windows had to be replaced over the lifetime of shuttle missions.

While that might sound alarming, it’s actually quite manageable. Upgrades and maintenance were quite common on the shuttle missions, and we tend to err on the side of caution when it comes to replacing parts. Modern spacecraft also have ways to mitigate the risk of small impacts, such as Whipple shields made of thin layers of material spaced apart so that objects disintegrate when hitting the shield rather than the spacecraft itself. We also have a tracking system that currently tracks more than 300,000 objects bigger than 1 cm, so we can make sure that most spacecraft avoid these objects.

Map of known objects in Earth orbit (sizes exaggerated) Via Reddit.

But the risk of big collisions isn’t negligible. In 2009 the Iridium 33 and Kosmos-2251 satellites collided at high speed, destroying both spacecraft and creating more dangerous debris. It wouldn’t take many collisions like this for the debris numbers to rise dramatically, and more debris means a greater risk of collisions. In Gravity the cascade happens very quickly, triggered by a single event. The reality is not quite so grave. Instead of happening overnight, Kessler syndrome would occur gradually, raising collision risks to the point where certain orbits become logistically impractical. It could occur so gradually that we might not notice it early on, and there are some that argue it’s already underway.

The good news is that we’re aware of the threat. And, as the old saying goes, knowing is half the battle. Already we take steps to limit the amount of debris created. New spacecraft include end of life plans to remove them from orbit, either by sending them into Earths atmosphere to burn up, or sending them to a “graveyard orbit” that poses little risk to other spacecraft. There are also plans on the drawing board to clear orbits of debris, particularly in low-Earth orbit where the risk is greatest. The cascade effect is a real risk, but it’s also one we can likely manage with a bit of ingenuity.

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We often imagine space probes as having large solar panels in order to provide them with the electrical power necessary to do cool science. While many spacecraft do have solar panels, there are some cases where such panels either won’t work, or aren’t enough by themselves. In these cases, we typically rely on an alternative energy source: nuclear power.

This alternative is known as a radioisotope thermoelectric generator, or RTG. It works by using a radioactive material as a power source. Typically the isotope used is plutonium 238, which is created synthetically in nuclear reactors. It has a half-life of 87.7 years, which means it can provide fairly consistent power over several decades. The Voyager probes, launched in 1977 continue to operate on RTG power.

The warm glow of an RTG. Credit: Wikipedia

An RTG doesn’t generate electricity directly, but rather the radioactive decay is shielded so that the RTG provides a source of heat. The heat is then converted to electric power through a thermocouple. When a conductor is subjected to a thermal gradient (where one side is hot and the other side cold) it produces a potential voltage. A thermocouple relies upon this principle to create electric power. Although thermocouples aren’t very efficient (only about 10% of the energy is converted to electric power) the RTG produces more than enough heat to be useful. Sometimes the RTG heat is used directly to keep the components of a probe warm.

There are some disadvantages to RTGs that prevents us from using them all the time. Solar panels are easy to build, and there’s no risk of radioactive contamination. Although there has been some concern that RTGs could cause contamination if the launch of a probe catastrophically fails, impact studies have shown that the risk is quite minimal.

The biggest drawback is that Plutonium 238 must be manufactured in a nuclear reactor. As the use of nuclear power and the construction of nuclear weapons has declined, the amount of available Pu-238 has dwindled. Since radioactive materials are somewhat fear-inducing to the general public, the use of RTGs is controversial despite the safety precautions.

But the reliability of RTGs ensure that many of our spacecraft will be nuclear powered for quite some time.

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The Japanese spacecraft Hayabusa 2 has launched successfully, and is on its way to the asteroid 1999 JU3. The mission is the successor to the first Hayabusa mission, which landed on the asteroid 25143 Itokawa, and returned dust particles from the asteroid to Earth. This new mission will also strive to return samples to Earth, but it is also more ambitious. The probe has a shape charge that will detonate on the asteroid’s surface to eject material, it has a lander, and three mini rovers.

Hayabusa 2 won’t reach it’s destination until 2018, and then it will be 2020 before it returns with samples, so it will be a while before we start getting data from it. If you are interested, you can check out a video on the mission (complete with epic background music).

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Yesterday Philae landed on a comet. It was the first successful soft landing on a comet, though it likely won’t be the last. This is something everyone should be excited about. With all the things to complain about in the world, here is a chance to celebrate a huge achievement of human ingenuity. We have a robotic probe on the surface of a comet.

Rosetta ground control at the moment Philae landed. Credit: ESA

There’s been a lot in the news about how this is the culmination of a 10 year mission, but that’s only the time since Rosetta’s launch. The idea for a comet lander dates at least as far back as the 1980s, when the Giotto spacecraft made a flyby of Halley’s comet. This led to plans for a mission to land on a comet and return with cometary material, but that was later scaled back to just a lander for budgetary reasons.

The Rosetta spacecraft under construction. Philae is circled. Credit: Cumbrian Sky

Design and construction of the Rosetta and Philae spacecraft date back to the 1990s, with major construction beginning around the turn of the century. It was originally scheduled to land on a comet known as 46P/Wirtanen, but delays led to the change to 67P/Churyumov–Gerasimenko.

The reason comets are such an important target is because they contain the most primal materials of the solar system. Unlike most of the bodies in the inner solar system, which have been mixed and bombarded over the ages, comets have an origin in the outer edges of the solar system known as the Oort cloud.

Philae has a planned mission of about 6 weeks, so there is going to be a lot of data gathered, and over time results will start being published. But we’ve already learned a few surprising things. One is that the comet itself sings. Not in the traditional way we think of, but rather in electromagnetic waves. The comet’s weak magnetic field interacts with the coma and solar wind to create a warbling effect. You can hear the sound in the video above, where it’s been sped up from its actual millihertz range.

So we’ve sent a spacecraft into space, put it in orbit near a comet, landed on it with a smaller probe, and listened to the comet’s song. What an amazingly human thing to do.

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