Tiny SpaceChips to take us to stars
Something remarkable happened at Sriharikota on 23 June. I realize that name itself gives away half the secret: that’s where India launches rockets from. That day, the Indian Space Research Organisation (Isro) launched its PSLV (Polar Satellite Launch Vehicle) rocket into space there, which was the 40th satellite launch of the PSLV. While each such launch is a remarkable feat of science and engineering, I’m not trying to persuade you that the 40th one was particularly noteworthy by itself.
But actually, it was. It carried 31 satellites into space. Quite something, but even that number is not the remarkable feat I mean. After all, the 39th launch, last February, carried 104 satellites, so what’s a mere 31? What I do mean is that of those 31, six were actually not satellites, but spacecraft. Interstellar spacecraft. Or more accurately, prototypes of interstellar spacecraft. These six are now successfully orbiting Earth, demonstrating that they are viable space travellers. That’s important, because the whole idea is that later versions of these prototypes will indeed travel, and rather far. They will head for the nearest star to us, Proxima Centauri. (Why that one? Because it is the nearest. But also because last year, we discovered a planet, Proxima b, orbiting Proxima Centauri, and planets hold at least the possibility of life.)
And here’s the really remarkable thing about these doodads: each one is the size of a passport photo—yes, 3.5cm by 3.5cm. Each weighs just four grams, about what that one-rupee coin in your pocket does. That is, these are the smallest spacecraft we have ever put in space. They are really just small printed circuit boards, those green things that sit inside laptops or other electronic devices. They are called Sprites.
Question: How on earth—or off it—will something like this get all the way to Proxima Centauri?
To answer that, it’s worth remembering first of all that Proxima Centauri is about 4 light years away. That is, light takes 4 years to travel from there to us, a distance of 40 trillion km. Mankind’s fastest spacecraft, Voyager I, is somewhere beyond our solar system as you read this, scudding along at 17km per second. At that speed, it will take 70,000 years to reach Proxima Centauri. The plan with these micro spacecraft, in contrast, is to get them to Proxima Centauri in 20 years. In other words, they will be scudding along at about 60,000 km per second, or a fifth the speed of light.
Stop just a moment to think about that. Voyager’s speed is itself hard for Earthlings to comprehend: it will zoom from Churchgate to Borivli in just over a second. Nothing we know here on Earth travels that fast. But that’s worse than sloth-like compared to what’s planned for the little voyagers that will aim for Proxima Centauri. At one-fifth the speed of light, they will circumnavigate the Earth in less than a second. They will have to move like that, or it’s meaningless to try to reach even that nearest star.
Thus the motivation for the size of Sprites. The heavier an object, the harder we have to work—the more energy we need—to get it moving at any speed. Think of pushing a car stalled on the road, as opposed to pushing a toy car around on the floor. So the lighter we can make our spacecraft, the easier it will be to accelerate it, even to unimaginably high speeds. And these Sprites are stripped-down bare-bones space machines indeed: they have a panel of solar cells, a radio and antennae for communication, a gyroscope to stay stable and not much else.
Still, there’s the question of how we accelerate even this tiny craft to that kind of speed. This is the still more radical part of this scheme. Typically, a spacecraft must carry fuel of some kind to propel it forward—and fuel is heavy, which in turn makes the craft heavy. But what if you had a Sprite—both lightweight and free of fuel?
In fact, the final Proxima-bound craft (“SpaceChips”) will be even smaller than Sprites. Once in space, they will unfurl gosammer-like “light sails”. Back on Earth, a gigantic array will beam lasers through the atmosphere at these sails. Now here on Earth, we don’t typically come across objects that move when a beam of light, or a laser, hits them. (Try it, even with a one-rupee coin). But in the emptiness of space, things are different. Over 400 years ago, the great astronomer Johannes Kepler observed that the tails of comets pointed away from the sun, and realized that a “solar wind” was responsible. Essentially, light from the sun pushes the dust and debris that surrounds the comet into the shape of a streamlined tail. SpaceChips will harness the same power that light possesses. Laser beams from Earth will strike the sail of a SpaceChip—like you might blow on a feather, like a gust of wind acts on a sailboat—and push it in a given direction. With enough time, the laser beam can get our little SpaceChip moving at a pretty fair clip indeed.
Even a fifth the speed of light.
This is the reasoning behind “Breakthrough Starshot”, the programme that envisioned these spacecraft and built the prototype Sprites that rode into space aboard Isro’s PSLV in June. It is backed by Yuri Milner, a Russian billionaire entrepreneur; the famous physicist Stephen Hawking is also involved. For now, with their Sprites orbiting the Earth and able to communicate with us, they have shown that space probes like these can work. But they are still some years away from deploying SpaceChips and laser-blowing them towards Proxima Centauri. There are problems to resolve.
First, it’s one thing to accelerate a SpaceChip to 60,000 km per second—even if that process seems akin to magic—and follow its progress across the galaxy. But what happens when, twenty years from now, it nears its destination? How do we slow it down? It has no rocket engines to fire in reverse, nor any brakes to apply. Yet slow down it must, because how else will we get any useful images or data about Proxima b? If there’s no way to slow down, it will have to start shooting images from pretty far away—tens of millions of kilometers at least, to allow for its speed—and trust that the resolution is good enough to tell us something interesting about the planet. Then it will keep gathering and dispatching to us what information it can until it plunges past Proxima b into the void beyond.
Second, while the universe is indeed largely made up of empty space, there is such a thing as space dust: tiny or not-so-tiny pieces of stone or ice or debris that simply float about. At the best of times these can be a terrible nuisance. But Breakthrough Starshot scientists have calculated that if a SpaceChip travelling at 60,000 km/s collides with a dust fragment comparable in size to the width of a human hair, it will be totally shattered. Fragments that large are rare, but even smaller ones can cause serious damage to the craft and its precision equipment.
Third, communicating with a speeding SpaceChip presents its own headaches. Think of what’s involved. Let’s say that 10 years into the trip, we learn that it is slightly off course and needs a corrective laser-beam pulse to point it back at Proxima Centauri. That knowledge will already be at least two years old. The pulse will take over two years more to reach the craft, by which time it will have strayed even further off course. Besides, over such long distances, any communication between us and the SpaceChip may happen at rates we haven’t seen on Earth for decades, rivalling our most ancient modems.
How do we run a mission with communication lines like these?
There are plenty more problems, too. They may not be insurmountable. But Breakthrough Starshot will have to address them before launching SpaceChips into space.
The idea of reaching across the universe to our nearest star is a delicious, seductive one. But let’s be realistic: with our current knowledge and technology, it’s impossible to conceive of sending humans there. Still, we can certainly conceive of sending a microlight spacecraft there, even many of them to travel and gather data in concert. Circa 2017 and Isro’s latest PSLV launch, this idea has moved beyond conception to early execution, if only to show that it can actually become reality a few years from now.
Maybe at some still later stage we can plan for what might happen if we find another civilization on Proxima b. What will we say to them, via our intrepid little SpaceChips?
Once a computer scientist, Dilip D’Souza now lives in Mumbai and writes for his dinners.