Rover Control
4G is coming to the Moon. Can it be used for spacecraft on Mars?
“Download large files in a matter of minutes. Enjoy group video chat with your friends and family. Stream movies and more, wherever you are.” The new 4G technology is making mobile Internet faster, cheaper, and more widespread than ever before.
And now, telecom giants Vodafone and Nokia plan to take it to the Moon. By 2020, Earth’s nearest neighbour will have Internet coverage too!
The question you must be thinking now is: Why? Future lunar explorers, stepping onto the Moon’s lonely surface, would be comforted to know there’s a high-speed 4G network waiting for them — but that doesn’t quite seem to be the point.
Actually, the new network is not for humans, but for robots. PTScientists is a private space company based in Berlin. They want to send a rover to the Moon — a small robotic laboratory on wheels, which can drive around the place conducting experiments.
Rovers have been sent to the Moon several times before, though not very often. The Yutu rover of 2013 sent back many high-resolution images, as well as taking scientific observations and even discovering a new kind of rock. The rover before that was back in 1973: the Lunokhod 2, which sent back thousands of TV images. But those were just still, black-and-white pictures, about 20 seconds apart.
This time, if PTScientists’ plan works out, whatever the rover sees will be streamed back in high-definition video.
Controlling a rover is not an easy task. It has to be done by remote control. You’re not actually there, next to the rover, to see and take in the whole situation.
That’s because, if you could actually be there, you’d probably ditch the rover and do all the experiments yourself.
So you have to rely on what little information the rover can send you: a small bunch of photos, taken when the rover has finished its previous move. And when you send your instructions, it’s not just a matter of moving a joystick like for your remote-controlled car. The instructions have to be carefully calculated, programmed, and set up for the rover to follow.
One reason you can’t livestream video for long is that it would take up too much energy, and the poor rover won’t have power left to do anything else.
Energy usage isn’t such a problem on the Moon — that’s how Lunokhod 2 could send back some sort of livestream. But the further away you get, the bigger the problem becomes. That’s because sending signals further away will mean stronger transmitters, as well as longer antennas at the other end to receive them.
The maximum download speed from the Mars rover Curiosity is 32 kilobits per second. For comparison, loading the Google homepage at that speed would take almost two minutes. Loading a reasonable-quality picture would take even longer.
And don’t forget that the bandwidth isn’t just for photos. Curiosity has to send back all the other scientific observations and data as well.
The problem of battery usage is not unique to space. In fact, it originally showed up right here on Earth.
The first “mobile phones” were very big and bulky — and not just because the technology was new. The phones needed powerful antennas and large batteries to support them, so that their signals could reach mobile-phone towers tens of kilometres away. That made them so big that, at first, they were sold as “car phones”: emergency telephones that you carry with you in your car.
A space-probe has a similar problem. But instead of ten kilometres, the number is more like ten million.
Actually, things are not so bad for Curiosity. The rover’s signals don’t always have to travel all the way to Earth. Sometimes, Curiosity sends them to one of three “relay spacecraft” that go round Mars: Mars Odyssey, Mars Reconnaissance Orbiter, and Mars Express
At those times, the signals just have to reach the relay spacecraft, which uses its more powerful antenna to finally send them to Earth. That means just a few thousand kilometres of travelling for Curiosity’s signals. (The relay spacecraft, though, still have to beam them the 55 million or more kilometres forward).
A similar idea helped make mobile phones smaller. Instead of having only one tower for the whole area, there could be many smaller towers closer together. That way, a mobile phone wouldn’t have to send out such a powerful signal to reach the tower. A weaker signal would do, for a tower that’s much nearer. And those nearer towers could keep passing on the message, just like Martian relay-spacecraft.
Under the new system, each tower would service its own little area, called a “cell”. And mobile phones using this new technology were called “cell phones”.
With the new 4G network, spacecraft on the Moon will be able to send back data as easily as a smartphone here on Earth can.
Of course, there’s always the question of where the 4G towers will get their electricity from. But since cellphone towers are big things that don’t move around, that shouldn’t be too hard to organise — especially not for two large cellphone companies.
A more interesting question is: can the idea be extended further? Can we start controlling rovers with a joystick, by setting up a 4G network on Mars?
Unfortunately, that won’t work. Even the most high-bandwidth network won’t let you use a joystick. That’s because there’s one thing that bandwidth can never change: time gaps.
Every evening, before powering down for the night, the Curiosity rover sends a small set of pictures to Mission Control on Earth. These pictures are taken from the rover’s two pairs of navigations cameras, to show people where the rover is standing right now. The pictures are 3D images, but they’re all black-and-white.
Every morning, Mission Control sends back the roving plans for the day. This could be a very detailed set of instructions: “Move forward 3 metres. Turn 27 degrees to your right. Move another 1.6 metres forward. Turn 24 degrees to your left, then stop.” Curiosity will follow those instructions exactly, and then wait for further orders.
Other times, the instructions could be just a destination: “You need to reach this point. Get there somehow.” In this case, Curiosity will figure out the way on its own, avoiding simple obstacles and setting itself right with minor adjustments, just like a self-driving car. Such vague instructions are only given when the route is pretty straightforward — anything more complicated, and the step-by-step instructions will be sent instead.
Whatever the mode of instruction, Curiosity goes very slowly. Its average speed is less than a speeding snail, and it usually takes a two months to complete one kilometre.
Why is Curiosity controlled in this tedious, roundabout manner? Why can’t someone just sit with a joystick and instruct the rover as it’s moving?
That’s because instructions don’t reach immediately. Radio signals are electromagnetic waves, just like light. That makes the the fastest travellers in the Universe. Fast they may be, but Mars is so far away even radio-waves can take upto twenty-one minutes to arrive. That means 21 minutes before you know anything’s happened, and another 21 before your instructions get heard at the other end. Forty-two minutes is a bit long if the rover is, say, driving into a rock.
As you can see, sending signals across space is quite different from sending signals here on Earth. The Internet and instant remote-controls works quite well over a few thousand kilometres. But as the distance grows larger, more hidden problems start to show up.
You realise that, no matter how fast your Internet is, it always travels at the same speed.
Ready for more? ‘Rover Control’ is the first of a two-part series on sending data through space. Read the second part here.
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