Harpooning Space Trash, Comets and Asteroids

Christian Sager

A copy of one of Philae's harpoons. Photo courtesy of © SVEN HOPPE/epa/Corbis

When the Philae attempted to land on a comet last week, one detail caught my attention more than others. We have space harpoons. The idea of using a harpoon to snare a comet sounds like something out of some wildly cosmic Jack Kirby comic. Looking further into it, Art Chmielweski, the project manager for the mission, even joked about how the harpoons sounded very Moby Dick. But he also noted the practicality of using harpoons in space. Because the comet had 100,000 times less gravity than Earth, the mission required multiple tools to keep it Philae on its surface. These included shock-absorbing landing gear, drills in the lander's feet, an upward-firing rocket... and the two harpoons.

Space harpoons are mainly in development to help manage space junk. There's a lot of it (6,000 tons in orbit) floating around up there, each larger than a coffee cup and way more dangerous. For example, in March 2012 astronauts on the International Space Station had to take refuge when a hunk of debris from a Russian telecommunication satellite came flying at them. Luckily it missed. But NASA is currently tracking around 500,000 similar pieces of debris, whipping around Earth at 17,500 mph (28,164 kph). But there are lots of smaller pieces whizzing around too that we can't detect with ground-based radar. They may not seem like a big deal because of their size, but consider this... it only took a few loose paint flecks to damage the windows of a space shuttle.

We don't think about it often, but we rely on space for amenities like GPS, television signals and weather forecasts. Rogue space debris can collide with the satellites that maintain these services, taking them out of commission. So plans are in place to use unmanned spacecraft to fire harpoons into this space trash, tether it and tow it into the atmosphere for burn up. Potentially, these harpoons could capture 10 objects per mission.

Human beings have used harpoons since the Stone Age, so how do we employ the same tool in the Space Age? The European Space Agency has a harpoon plan in place for 2021. Their prototype is fired into a target with high-energy, pierces the object and then reels it back. They've already tested it on satellite material to make sure it doesn't generate additional smaller fragments.

Another plan is to use space harpoons for mining. On a mission they could grab samples from an asteroid or a moon by firing a few meters into their surface and then retracting via a miles-long tether. Currently we have to soft land on an object, collect samples and then blast off again. Using harpoons would use the spacecraft's kinetic energy while also saving energy for the mission.

To penetrate the surface, these harpoons would need to travel up to 2,240 mph (3,605 kmh). They would also need to be made of energy-absorbing materials that could withstand the force of impact. Tests on Earth are close to demonstrating harpoons that can survive 1 kilometer per second impacts and researchers hope that this will encourage investment in developing even better materials for space. In the meantime, there's theories that we could use the same tether-harpoon system to take samples from extreme environments like volcanoes or nuclear contaminated sites here on Earth.

As cool as the harpoons sounded for the Philae landing, it turned out they didn't actually fire. The mission's harpoons were designed to anchor the lander up to 2.5 meters into the comet's surface. But let's keep in mind that the mission was launched over 10 years ago... before our current plans to capture space junk and asteroid samples with harpoons. With the right materials in place we might still be able rein in comets like a bull at the cosmic rodeo.