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SYFY WIRE Bad Astronomy

LISA Pathfinder got whacked by dozens of hypervelocity 'comet crumbs'

By Phil Plait
A somewhat fanciful depiction (there would be no trail in the vacuum of space) of a micrometeorite hitting the LISA Pathfinder spacecraft. Credit: NASA

I love it when a space mission designed to do one thing winds up also doing something entirely different.

LISA Pathfinder launched into space on 3 December 2015, starting a two-year mission to test the engineering necessary for the much more ambitious LISA (Laser Interferometer Space Antenna) mission. LISA (currently still being designed) will detect gravitational waves, ripples in the fabric of spacetime when massive compact objects — think neutron stars and black holes — collide. The hardware it will use is incredibly sensitive, and if it works as planned will be able to detect mergers clear across the Universe.

In a nutshell, LISA would be comprised of three spacecraft separated by millions of kilometers. Despite that, they have hardware that will determine their relative distances with an accuracy of a picometer: one trillionth of a meter. When gravitational waves pass by, the distances between the spacecraft change minutely, and LISA will be able to detect that. I describe this in more detail in an earlier article.

LISA Pathfinder has some of this hardware on board, and was put into orbit around the Sun about 1.5 million kilometers from Earth (what's called the L1 point), far from Earth's and Moon's disturbing gravity. One part of the test was to make sure it could maintain its attitude, its orientation, with extreme accuracy. If it detected any sort of disturbance, it would use extremely low-force thrusters to counteract it and keep the spacecraft oriented correctly.

That's where the fun comes in. Looking at over 180 days worth of data, a team of scientists and engineers found that LISA Pathfinder encountered at least 54 sudden jolts to the spacecraft. These were extremely small — the largest of them would be about the same force a housefly would apply sitting in your hand. The smallest was a thousandth of that!

What caused these wee pings? Cosmic dust. Literally tiny particles shed from comets as they orbit the Sun, cosmic crumbs, essentially the same sort of thing that causes meteor showers on Earth but with grains that are far far smaller. Even though they're microscopic, they're moving so rapidly, at tens of thousands of kilometers per hour relative to the spacecraft, that they pack a much larger punch than you might expect.

Because LISA Pathfinder was so sensitive, they could look at the microthruster logs and tell where the particles hit, with what force, and even what direction they came form. This allowed some interesting science to be done: They determined that most of the comets that shed these bits of debris were what are called Jupiter-family comets, ones that used to be on very long orbits around the Sun, but were pulled by Jupiter into much shorter orbits. These comets have orbits less than about 20 years long, and are aligned with the orbital planes of the planets. To a lesser extent, some of the comets responsible for the dust had longer orbits, and are called Halley-type comets (Comet Halley orbits the Sun once every 76 years).

This is pretty consistent with other studies about the type of dust out there. For example, if you happen to be at an extremely dark site on Earth you can see a faint glow extending above the horizon, following the ecliptic, the path the Sun and planets take relative to the stars during the year. This dim glow — called zodiacal light — is from dust understood to be shed from Jupiter-family comets, reflecting sunlight back toward you.

Zodiacal light, sunlight reflected off the scattered remains of thousands of long-dead comets. Credit: Rogelio Bernal Andreo

And that's what I love about this. LISA Pathfinder is designed to be the testbed for detecting gravitational waves, theorized to exist for a century but only directly detected for the first time in 2016. This tech is cutting edge (I call it Star Trek tech) and the science behind gravitational waves extremely advanced. So there's just something really lovely about it being used to look at objects much closer to home, especially teeny ones which reveal their presence by slamming into the spacecraft itself. Even better: We have pretty good data on micrometeorites really close to Earth, but not from farther out, so LISA Pathfinder extended our understanding of this population of microscopic dust at a distance of over a million kilometers from Earth.

And that's why this is so cool to me. When you put an extremely sensitive and finely calibrated machine in space, a lot of things are going to affect it. You may not know what all those things are, but the spacecraft itself can tell you. The science you get from it goes beyond what it was directly designed for. And, in this case, is laying the path for even more sophisticated and cooler science yet to come.