by Shane L. Larson
Each morning, I roll out of bed, dutifully feed the three cats that own me, help my fourth-grader get her backpack put together for the day and put my daily secret note in her lunch, enjoy a few brief moments over morning coffee with my spouse, and then it is off to work.
For my day job, I’m a scientist. My friends and I work in a completely new branch of astronomy called gravitational wave astronomy. Our express goal is to detect a phenomenon that was predicted almost a century ago by Einstein: the undulations and propagating ripples in the fabric of spacetime that signify the dynamic motion of matter in the Cosmos.
Gravitational waves are expected to be a phenomenal probe of the Cosmos because they are readily generated by objects that are otherwise hard to detect by other means. This includes objects of intense interest to astronomers, like neutron stars, stellar mass black holes, white dwarfs, cosmic strings, and supermassive black holes at the hearts of galaxies. Despite their apparent utility in astronomy, the are exceedingly hard to detect. When Einstein first deduced their existence, he famously showed that the waves were so weak he thought we might never be able to measure them. But as is often the case, the future is full of wonders, and with the advent of the Space Age, people began to question that judgement. Maybe, with some cleverness and awesome technology, we could gaze at the Universe with gravity rather than light.
As with many scientific endeavours, gravitational wave detection is a difficult task because we’ve never built machines to do this before. We are learning how to do everything for the first time. You try things out, making your best guess as to how it is all going to work, but when you finally flick the switch to “on” you can debug your experiment because it is right in front of you. That’s all well and good when your lab is here on planet Earth, but when you shift your experiments to space, it becomes a bit more difficult.Someday we want to build a space observatory for measuring gravitational waves, called LISA — the Laser Interferometer Space Antenna. LISA consists of three spacecraft, each about 2 meters in diameter and 50cm deep. They fly in space, 5 million kilometers apart, and shine lasers back and forth between themselves. We time the flight of those lasers (nominally just over 16.6 seconds from one spacecraft to another) and if a gravitational wave blasts through LISA, we see the laser times change.
So how do we go about building new spacecraft for the first time? We take things in stages, just like you and I do when we try to learn something new. When I want to learn to play guitar, I don’t take the stage on Day One with Dr. Brian May; instead I get an old beater guitar out of the basement and I plunk out riffs of “Old Sussanah” until my fingers bleed. Then I work on the guitar solo in “Brighton Rock.” Building spacecraft is kind of the same thing.No space observatory like LISA has ever been built before, so we have to figure out how to do it. How do you build the laser timing system? How do you set up the spacecraft thrusters to respond to external influences like the solar wind? How do you get the whole thing into orbit in one piece, then set it up so it works? How do you control spacecraft temperature to the precision we need? The best way to answer all of these questions, and to discover all the pitfalls we haven’t imagined, is to build one. This is one of the primary reasons we built a spacecraft called LISA Pathfinder.
LISA Pathfinder is an “almost LISA”. The spacecraft itself is roughly the size and shape of a LISA spacecraft, but it’s guts are slightly different. Deep down inside, it has a linked laser system that is easiest to think of as if it is just an entire LISA arm, shrunk down to fit on a single spacecraft. This is not ideal for doing astrophysical work, but it is perfect for understanding how the spacecraft are going to work in space.Throwing a robot into space is hard. You have to get it to outer space, and get it there in one piece! The usual way you get things into space (so far) is with rockets. Putting aside the fact that they sometimes explode, a rocket ride to space is not the gentlest experience in the world. It’s loud — noise levels in proximity to a typical rocket engine are a million-billion times louder than sound you encounter at home every day. It shakes a LOT — rocket vibrations back and forth across the body of a rocket can be so strong they have led to catastrophic destruction of the rocket itself. The launch forces are enormous — human spaceflight engineers keep launch forces low for crew comfort (the maximum on space shuttle flights was about 3 times Earth gravity), but rockets without human crews regularly reach 5 to 10 times Earth gravity during launch. Add that all together, and the ride to space can be pretty rough. So how do you get a sensitive gravitational wave experiment into space, all in one piece and undamaged, on a rough and tumble rocket ride?
Hucking robots into space is hard, to be sure, but using a robot you threw into space to do science can be even harder. First, everything has to work. When your robot is tens of thousands of kilometers away from the closest space engineer, you can’t tinker with it — there’s no tightening up bolts, no replacing faulty lasers, no kicking stuck gear boxes, nor swapping out new battery packs. Second, the environment of space is harsh — there’s no air, the Sun is constantly blasting and heating one side of your spacecraft while the other side is turned toward the frigid chill darkness of deep space. And all the while, your dedicated space robot is bathing in a constant wash of hard cosmic radiation. Every ultra-sensitive space experiment has to weather through those hardships, while collecting data that would be hard to collect even under controlled laboratory conditions on Earth.
So you take a baby step, and you test everything first on Earth, then in space. This is the purpose of LISA Pathfinder. To teach us how to build a spaceborne gravitational wave detector, then to show we know how to get the thing safely to space, then once we’re in space, we turn it all on to show that we can do the actual experiment we want to do.On December 2, after many years of design and laboratory work, LISA Pathfinder was launched atop a Vega rocket from Kourou Space Center in French Guiana. It has gone through a series of orbital burns that are sending it to a neutral “Lagrange point” between the Earth and Sun, where it will enter a “halo orbit” to test its lasers, thrusters, and spacecraft guidance systems in the very same way that LISA will have to work. So far, the flight has been flawless. What constantly amazes me about the people who build these machines is their diligence and tenacious attention to detail. A robot that we huck into space is not just a dumb hunk of metal. It is an amazing complex machine that is capable of thinking and taking care of itself. It conducts experiments that we tell it to do, stores the results of those experiments and faithfully beams the information back to Earth. At the same time, it is surviving one of the most hostile environments known: the vacuum of space. The influence of the Sun produces drastic temperature shifts across your spacecraft. Cosmic radiation is constantly bathing the spacecraft in a wash of seething, energetic particles. And all the while it has to gather and store energy, and all the zillions of parts and components have to work together, flawlessly and seamlessly.
Your car is also an amazingly complex machine. But if some piece of it stops working and leaves you on the side of US Route 50 in Nevada (the Loneliest Highway in America), a passing motorist will still happen along to help you, or you can make a quick call to the motor club to come tow you. There are no such luxuries in the game of space exploration.
The scientists and engineers who contemplate these things every day are ingenious and clever. The delivery of LISA Pathfinder was the culmination of a decade long effort by an enormous team of scientists and engineers. And all the while they were designing and building LISA Pathfinder, they were teaching classes, and training new students and young scientists who will go on to do new and awesome things in the future. These are the people who make our modern world go ’round. I have nothing but admiration for my colleagues who have built and flown this marvelous machine.
So, at long last, the beginning has arrived. We are all simultaneously exhilarated, relieved, joyous, and eager for the next bit of news and the latest results to get here. Because this is only the beginning, the culmination of decades of hard work, difficult hardships, and anticipation. The BEST stuff — the detection of gravitational waves from the Cosmos — is yet to come.