It's mainly because they haven't got much bandwidth to communicate with Earth.
The lunakhods could travel up to 2 km / hour. The lunar rovers were even faster, fastest 11 mph, and with more bandwith we could drive rovers on the Moon as fast as them from Earth. We could send vehicles like that to Mars. Lunakhod 2 was 840 kg, and Curiosity, 899 kg, so they are similar size.
So this was technologically possible already back in the 1970s, though they didn't yet have our ability to land the rovers on Mars safely. Top Ten Rovers: Distance Driven Off-Earth.
So power and maximum speed are not the main issue.
The autonomous navigation can be sorted out - we are working on that and future rovers will be better at it, also the rover can be designed to be better at traversing rugged terrain.
But without more bandwidth, then those improvements may be of limited value.
Which you can see by comparing Opportunity with Lunakhod - Lunakhod with 1970s technology traveled as far in 4 months as Opportunity did in a decade. That's mainly because of the higher communications bandwidth - and that is with 1970s technology for Lunakhod. And the terrain is roughly comparable in roughness and difficulty to traverse.
Or it could also be sorted out with much better autonomy for the rovers, so they don't just drive, but do experiment decision making as well. Or a mix of the two, better autonomy combined with more bandwidth.
In detail:
Sometimes they can communicate directly from the surface but more usually through a relaying satellite.
The data rate from Curiosity direct to Earth varies from 62 bytes per second to 4 k / sec. So a 1 MB image would take over 4 minutes to download if it sent it to us directly. Data Rates/Returns - Mars Science Laboratory
Typically it communicates with the Mars Reconnaissance orbiter, up to 244 Mb per second, or to the Odyssey orbiter, up to 31 Mb per second. But it can only communicate with orbiters in low orbit for about 8 minutes per sol when they fly overhead (longer if they are in high orbits), and then, they have to send the data back to Earth, and have more bandwidth than the rovers because they have more power and have solar power 24/7 but they have their own data to send back too. Space Communications with Mars
Mars Reconnaissance Orbiter has a downlink speed of about half a megabyte per second when closest to Earth. So it doesn't matter how fast our rovers can communicate with Mars Reconnaissance rover, it still has to get to Earth. And the satellite it has to send all its own data down this narrow pipeline too. http://descanso.jpl.nasa.gov/DPS...
(Not sure if these are the most up to date figures for bandwidth, but its around the right figures, not been any dramatic improvements - any corrections do say in the comments).
And then we also have limited listening radio bandwidth back on Earth in the deep space network to pick up the transmissions from all our spacecraft in the solar system.
Home Page - Deep Space Network
If the signals are stronger, you can return more data in less time. So this radiobandwidth doesn't equate directly to a data bandwidth. But you need either more powerful radio transmitters for our spacecraft, or more listening radio receivers if we want to return radically more data from the solar system.
The end result of this is that they schedule communications with a Mars rover typically once every Mars day. So they download all the images an all other observations it took from the previous day, analyse it, and decide its movements for the next day.
It could be as far away as Pluto, or the Kuiper belt, and they would do the science at the same pace. So the light speed delay isn't a major factor here.
They also have limited power, from solar panels, with the sun power levels half that on Earth.
Curiosity's top speed, for instance, is 1.5 inches per second. Or 140 meters per hour. NASA Jet Propulsion Laboratory
But they can't sustain that speed continuously because they have only so much power per day and if traveling continuously it relies on automatic navigation. Curiosity is designed to be able to travel up to 200 meters per day. Mars Curiosity: Facts and Information
In practice, it spends a lot of time in one spot, as you can see from this video
Given how long it spends on each site, it really wouldn't speed things up so much to have it traveling faster between them.
If we had more bandwidth, those experiments also could be speeded up hugely.
You can get an idea of what would be possible with better communications, by looking at Lunakhod 2 which in a few months in the 1970s traveled as far as Opportunity traveled in a decade. It's the same weight as Curiosity pretty much and the power was supplied using solar panels with 1970s technology.
So, the power problem surely can be cracked, yes Mars has less sunlight than the Moon. But not that much less and modern solar panels are far more efficient.
NASA's Opportunity Mars rover breaks driving distance record
Lunokhod 2 - traveled about 39 km on the Moon in about 4 months.
Lunakhod 2 Typically traveled less than 1 km per day but had a top speed of around 2 km per hour
Our fastest rover to date though is the Apollo lunar rover
On the Apollo 17 mission it reached a top speed of 11 mph. It nearly beat the Lunakhod record with a distance of 35.9 km, in a total of 22 hours 3 minutes 57 seconds in three days. That's an average of 1.63 km / hour or about 1 mph.
There is no real reason why we couldn't send the likes of lunakhod to Mars.
Or even the lunar rover. Okay its batteries would run out, but until that happened, it would have the ability to travel many kilometers every day. If it traveled at the same speed as the lunar rover, it could travel the same distance as Opportunity did every decade in roughly 3-4 days, but it could travel further if needed. Basically it could travel pretty much as fast as a geologist or astrobiologist would want to travel on Mars.
The lunakhods had only 180 watts from its solar panels, not much more than Opportunity's 14o watts, and had a top speed of 2 km / hour. And Lunakhod 2 traveled as far in four months as Opportunity in ten years.
Lunokhod 1
The Lunar Roving Vehicle at 290 kg had two pre-charged silver oxide batteries on board able to power the whole thing for 92 km with two astronauts added to the weight.
It would be a bit more payload to add precharged batteries, but not a huge amount. And when they ran down they could be recharged from solar panels or RTG whenever the rover isn't using all the power for scientific experiments or traveling.
One idea for that - its an idea suggested by Mars One for humans, but could adopt it for automated rovers - if you could cover a flat area of the Mars surface with thin film solar panels, they wouldn't weigh much, and it could keep coming back to that area to charge up for its next mission and keep doing forty km missions from there, or whatever is it's maximum distance after a recharge.
When it comes back, swap batteries with one that has been charging up throughout its mission, and repeat (with extra solar panels on the rover for the science experiments and so it can recharge itself slowly in case it gets slowed down and the battery runs out).
Or use Zubrin's idea of hydrogen feedstock from Earth to generate fuel for the rovers using the CO2 in the atmosphere via the Sabatier reaction.
One way or another we can solve this and get plenty of power to the rovers if it is needed, not too difficult.
So the problem isn't power supply, for Mars.
So what do we need to do to fix this:
First, better communications. If we had a fleet of two or three dedicated communications satellites around Mars, and more radio time to receive the signals, this would speed things up a fair bit.
We also need to build the rovers so they are able to communicate to the satellites and receive from them 24/7. (Both for communications, and so we can download images and video from them quickly, so they can send data night and day back to Earth).
But as well as that we need to be able to receive all this data on Earth. That means, either building more radio receivers to receive all the data, or making the radio transmitters on our orbital satellites somehow much more powerful - or - using another method of communication. There's a lot of interest at present in using optical laser communication to solve these problems.
Laser optical communications could speed it up even more, maybe to the point where we have live HD streaming from Mars. Optical Comm
In 2014, the OPALS satellite demonstrated the power of laser communication, communicating with Earth and downloading a 22 megabyte video in 3.5 seconds.
Space, Stars, Mars, Earth, Planets and More Laser communication in space
Something like this is what they want to do for Mars.
Then we can build in better autonomy into our rovers.
Something like this, if we could transport it to Mars, would have no trouble with just about any terrain on Mars.
It uses a lot of power, there are simpler ways to deal with rough terrain. E.g.
Or
Or indeed rovers that are able to float, or fly or hop to get about.
For more ideas like this, see Soaring, Buzzing, Floating, Hopping, Crawling And Inflatable Mars Rovers - Suggestions For UAE Mars Lander
Or to build more autonomy into our normal wheeled rovers
ExoMars Rover Starts Autonomous Fields Trials in Chile
One way or another, faster traveling autonomous rovers should be possible.
This is another approach, a small robotic scout that goes ahead to check the terrain for the larger robot which follows behind
They hope it will make it possible to travel 20 km in six months. Could mini scouts be sent with rovers to Mars in future?
Our rovers already do some decision making, for instance this is how the rovers were able to take photographs of dust devils - no way we could react quickly enough on Earth to tell it to take a photograph when we don't know about it until the next day.
Curiosity also does automatic obstacle avoidance - you tell it where to go, and it decides itself how to get there to some extent.
In future they could make more decisions. For instance this idea for Texture Cam, in future for instance, you could tell a rover to travel over a dune and take a photograph of every rock it sees, then next day download close up photographs of all the rocks to look at.
'Smart' Rovers Are the Future of Planetary Exploration
Also, even if we can't control it continuously 24/7, for the experiments, and for decisions about which rocks to sample and such like - it would be a huge speed up if you can communicate with the rover multiple times every day instead of just once a day.
If you can communicate ten times a day, communicating with a semi-autonomous rover, able to make many decisions itself, and two way communication that is, download data, make decisions based on it and upload new commands - you could do many things ten times faster, basically.
But you can speed it up even more if you can use artificial real time. This builds up a virtual 3D copy of the Mars landscape as you travel over it, so you navigate your copy of the landscape as you travel over Mars, and it feels as if you are navigating it in real time yourself.
Parts of the terrain would be hidden from view until you go over a rise or round a boulder, and these would be marked as uncertain in some way so you know you need to take care and go more slowly there until it catches up.
In practice, geologists and exobiologists on Mars would typically spend only a fraction of their time actually traveling to get to another site, and most of their time traveling around to different parts of a site of special interest.
So they would soon build up detailed terrain maps of any parts of interest, and then could drive around pretty much in real time. This is of course a textured map - they can look closely at the rocks in artificial real time and see what they look like (if they drive up to a rock never examined closely before they'd get a lower resolution texture that would update to higher resolution as the new images are downloaded).
With of course, mechanisms that detect collisions or wheels getting stuck in sand etc so the rover has some override autonomy as well to stop you driving it into trouble.
With these methods we could travel far faster on both Mars and the Moon.
I expect to see this some time in the next decade or two. But the big bottleneck here is communications, sort that out and then you have motivation to sort out all the other things needed to get this working.
Until we do this, then faster rovers are of limited value because you'd have to drive them slowly anyway. And rovers would need a lot of autonomy to make up for this disadvantage that you typically only communicate with them once per Martian sol.
It might be that we start on this on the Moon first, because communications there are much easier as we already saw with Lunakhod. If we were to send rovers to the Moon with the capabilities of our Mars rovers, but with higher maximum speed per day, nothing else changed - basically send a souped up Curiosity to the Moon, it would be zooming around exploring kilometers every day - the Lunakhod was driven slowly, and could easily have traveled as far as the Lunar rover with its max of 2 km / day.
With modern technology and especially using telerobotics / telepresence, I think you'd be amazed how much we can do with robots on the Moon if we sent rovers there that are as capable as our Mars rovers - but capable of say the Lunakhod's max speed of 2 km / hour - or more, solar powered.
It's also far easier to send rovers to the Moon, can send them at any time of the year.
Then that could be more motivation for increased bandwidth for Mars if we have rovers on the Moon accomplishing in days and weeks what it takes years and decades to accomplish on Mars.
It might then be easier for those who are working on this to get their voice heard. Just a thought.
See also, stack exchange discussion: Why are Mars rovers so slow?
(note, more answers in related quora question Why does the Mars rover, Curiosity, move so slow?)
The lunakhods could travel up to 2 km / hour. The lunar rovers were even faster, fastest 11 mph, and with more bandwith we could drive rovers on the Moon as fast as them from Earth. We could send vehicles like that to Mars. Lunakhod 2 was 840 kg, and Curiosity, 899 kg, so they are similar size.
So this was technologically possible already back in the 1970s, though they didn't yet have our ability to land the rovers on Mars safely. Top Ten Rovers: Distance Driven Off-Earth.
So power and maximum speed are not the main issue.
The autonomous navigation can be sorted out - we are working on that and future rovers will be better at it, also the rover can be designed to be better at traversing rugged terrain.
But without more bandwidth, then those improvements may be of limited value.
Which you can see by comparing Opportunity with Lunakhod - Lunakhod with 1970s technology traveled as far in 4 months as Opportunity did in a decade. That's mainly because of the higher communications bandwidth - and that is with 1970s technology for Lunakhod. And the terrain is roughly comparable in roughness and difficulty to traverse.
Or it could also be sorted out with much better autonomy for the rovers, so they don't just drive, but do experiment decision making as well. Or a mix of the two, better autonomy combined with more bandwidth.
In detail:
BANDWIDTH LIMITATIONS
Sometimes they can communicate directly from the surface but more usually through a relaying satellite.
The data rate from Curiosity direct to Earth varies from 62 bytes per second to 4 k / sec. So a 1 MB image would take over 4 minutes to download if it sent it to us directly. Data Rates/Returns - Mars Science Laboratory
Typically it communicates with the Mars Reconnaissance orbiter, up to 244 Mb per second, or to the Odyssey orbiter, up to 31 Mb per second. But it can only communicate with orbiters in low orbit for about 8 minutes per sol when they fly overhead (longer if they are in high orbits), and then, they have to send the data back to Earth, and have more bandwidth than the rovers because they have more power and have solar power 24/7 but they have their own data to send back too. Space Communications with Mars
Mars Reconnaissance Orbiter has a downlink speed of about half a megabyte per second when closest to Earth. So it doesn't matter how fast our rovers can communicate with Mars Reconnaissance rover, it still has to get to Earth. And the satellite it has to send all its own data down this narrow pipeline too. http://descanso.jpl.nasa.gov/DPS...
(Not sure if these are the most up to date figures for bandwidth, but its around the right figures, not been any dramatic improvements - any corrections do say in the comments).
And then we also have limited listening radio bandwidth back on Earth in the deep space network to pick up the transmissions from all our spacecraft in the solar system.
Home Page - Deep Space Network
If the signals are stronger, you can return more data in less time. So this radiobandwidth doesn't equate directly to a data bandwidth. But you need either more powerful radio transmitters for our spacecraft, or more listening radio receivers if we want to return radically more data from the solar system.
TYPICALLY, COMMUNICATE ONCE PER MARS SOL WITH EARTH
The end result of this is that they schedule communications with a Mars rover typically once every Mars day. So they download all the images an all other observations it took from the previous day, analyse it, and decide its movements for the next day.
It could be as far away as Pluto, or the Kuiper belt, and they would do the science at the same pace. So the light speed delay isn't a major factor here.
They also have limited power, from solar panels, with the sun power levels half that on Earth.
Curiosity's top speed, for instance, is 1.5 inches per second. Or 140 meters per hour. NASA Jet Propulsion Laboratory
But they can't sustain that speed continuously because they have only so much power per day and if traveling continuously it relies on automatic navigation. Curiosity is designed to be able to travel up to 200 meters per day. Mars Curiosity: Facts and Information
In practice, it spends a lot of time in one spot, as you can see from this video
Given how long it spends on each site, it really wouldn't speed things up so much to have it traveling faster between them.
If we had more bandwidth, those experiments also could be speeded up hugely.
WHAT WOULD BE POSSIBLE WITH BETTER COMMUNICATIONS
You can get an idea of what would be possible with better communications, by looking at Lunakhod 2 which in a few months in the 1970s traveled as far as Opportunity traveled in a decade. It's the same weight as Curiosity pretty much and the power was supplied using solar panels with 1970s technology.
So, the power problem surely can be cracked, yes Mars has less sunlight than the Moon. But not that much less and modern solar panels are far more efficient.
NASA's Opportunity Mars rover breaks driving distance record
Our fastest rover to date though is the Apollo lunar rover
There is no real reason why we couldn't send the likes of lunakhod to Mars.
Or even the lunar rover. Okay its batteries would run out, but until that happened, it would have the ability to travel many kilometers every day. If it traveled at the same speed as the lunar rover, it could travel the same distance as Opportunity did every decade in roughly 3-4 days, but it could travel further if needed. Basically it could travel pretty much as fast as a geologist or astrobiologist would want to travel on Mars.
BATTERY POWER DETAILS
The lunakhods had only 180 watts from its solar panels, not much more than Opportunity's 14o watts, and had a top speed of 2 km / hour. And Lunakhod 2 traveled as far in four months as Opportunity in ten years.
Lunokhod 1
The Lunar Roving Vehicle at 290 kg had two pre-charged silver oxide batteries on board able to power the whole thing for 92 km with two astronauts added to the weight.
It would be a bit more payload to add precharged batteries, but not a huge amount. And when they ran down they could be recharged from solar panels or RTG whenever the rover isn't using all the power for scientific experiments or traveling.
IDEAS FOR RECHARGING HIGH CAPACITY BATTERIES QUICKLY
One idea for that - its an idea suggested by Mars One for humans, but could adopt it for automated rovers - if you could cover a flat area of the Mars surface with thin film solar panels, they wouldn't weigh much, and it could keep coming back to that area to charge up for its next mission and keep doing forty km missions from there, or whatever is it's maximum distance after a recharge.
When it comes back, swap batteries with one that has been charging up throughout its mission, and repeat (with extra solar panels on the rover for the science experiments and so it can recharge itself slowly in case it gets slowed down and the battery runs out).
Or use Zubrin's idea of hydrogen feedstock from Earth to generate fuel for the rovers using the CO2 in the atmosphere via the Sabatier reaction.
One way or another we can solve this and get plenty of power to the rovers if it is needed, not too difficult.
So the problem isn't power supply, for Mars.
WHAT CAN BE DONE?
So what do we need to do to fix this:
First, better communications. If we had a fleet of two or three dedicated communications satellites around Mars, and more radio time to receive the signals, this would speed things up a fair bit.
We also need to build the rovers so they are able to communicate to the satellites and receive from them 24/7. (Both for communications, and so we can download images and video from them quickly, so they can send data night and day back to Earth).
But as well as that we need to be able to receive all this data on Earth. That means, either building more radio receivers to receive all the data, or making the radio transmitters on our orbital satellites somehow much more powerful - or - using another method of communication. There's a lot of interest at present in using optical laser communication to solve these problems.
Laser optical communications could speed it up even more, maybe to the point where we have live HD streaming from Mars. Optical Comm
In 2014, the OPALS satellite demonstrated the power of laser communication, communicating with Earth and downloading a 22 megabyte video in 3.5 seconds.
Something like this is what they want to do for Mars.
Then we can build in better autonomy into our rovers.
Something like this, if we could transport it to Mars, would have no trouble with just about any terrain on Mars.
It uses a lot of power, there are simpler ways to deal with rough terrain. E.g.
Or indeed rovers that are able to float, or fly or hop to get about.
For more ideas like this, see Soaring, Buzzing, Floating, Hopping, Crawling And Inflatable Mars Rovers - Suggestions For UAE Mars Lander
Or to build more autonomy into our normal wheeled rovers
One way or another, faster traveling autonomous rovers should be possible.
ROBOTIC SCOUT
This is another approach, a small robotic scout that goes ahead to check the terrain for the larger robot which follows behind
They hope it will make it possible to travel 20 km in six months. Could mini scouts be sent with rovers to Mars in future?
EXPERIMENTS AND DECISION MAKING
Our rovers already do some decision making, for instance this is how the rovers were able to take photographs of dust devils - no way we could react quickly enough on Earth to tell it to take a photograph when we don't know about it until the next day.
Dust devil captured by Spirit Rover. These are very short lived and so the rovers needed to have some autonomous decision making to photograph them.
Curiosity also does automatic obstacle avoidance - you tell it where to go, and it decides itself how to get there to some extent.
In future they could make more decisions. For instance this idea for Texture Cam, in future for instance, you could tell a rover to travel over a dune and take a photograph of every rock it sees, then next day download close up photographs of all the rocks to look at.
'Smart' Rovers Are the Future of Planetary Exploration
Texture cam can distinguish rocks from soil, and also identify potentially interesting rock textures. You could, for instance, tell it to drive over a hill and take a photograph of every interesting rock it sees. Next day you look at all its photos and tell it which one you want it to go back to.
Also, even if we can't control it continuously 24/7, for the experiments, and for decisions about which rocks to sample and such like - it would be a huge speed up if you can communicate with the rover multiple times every day instead of just once a day.
If you can communicate ten times a day, communicating with a semi-autonomous rover, able to make many decisions itself, and two way communication that is, download data, make decisions based on it and upload new commands - you could do many things ten times faster, basically.
EVEN MORE SPEED UP - ARTIFICIAL REAL TIME
But you can speed it up even more if you can use artificial real time. This builds up a virtual 3D copy of the Mars landscape as you travel over it, so you navigate your copy of the landscape as you travel over Mars, and it feels as if you are navigating it in real time yourself.
Parts of the terrain would be hidden from view until you go over a rise or round a boulder, and these would be marked as uncertain in some way so you know you need to take care and go more slowly there until it catches up.
In practice, geologists and exobiologists on Mars would typically spend only a fraction of their time actually traveling to get to another site, and most of their time traveling around to different parts of a site of special interest.
So they would soon build up detailed terrain maps of any parts of interest, and then could drive around pretty much in real time. This is of course a textured map - they can look closely at the rocks in artificial real time and see what they look like (if they drive up to a rock never examined closely before they'd get a lower resolution texture that would update to higher resolution as the new images are downloaded).
With of course, mechanisms that detect collisions or wheels getting stuck in sand etc so the rover has some override autonomy as well to stop you driving it into trouble.
With these methods we could travel far faster on both Mars and the Moon.
I expect to see this some time in the next decade or two. But the big bottleneck here is communications, sort that out and then you have motivation to sort out all the other things needed to get this working.
Until we do this, then faster rovers are of limited value because you'd have to drive them slowly anyway. And rovers would need a lot of autonomy to make up for this disadvantage that you typically only communicate with them once per Martian sol.
It might be that we start on this on the Moon first, because communications there are much easier as we already saw with Lunakhod. If we were to send rovers to the Moon with the capabilities of our Mars rovers, but with higher maximum speed per day, nothing else changed - basically send a souped up Curiosity to the Moon, it would be zooming around exploring kilometers every day - the Lunakhod was driven slowly, and could easily have traveled as far as the Lunar rover with its max of 2 km / day.
With modern technology and especially using telerobotics / telepresence, I think you'd be amazed how much we can do with robots on the Moon if we sent rovers there that are as capable as our Mars rovers - but capable of say the Lunakhod's max speed of 2 km / hour - or more, solar powered.
It's also far easier to send rovers to the Moon, can send them at any time of the year.
Then that could be more motivation for increased bandwidth for Mars if we have rovers on the Moon accomplishing in days and weeks what it takes years and decades to accomplish on Mars.
It might then be easier for those who are working on this to get their voice heard. Just a thought.
See also, stack exchange discussion: Why are Mars rovers so slow?
(note, more answers in related quora question Why does the Mars rover, Curiosity, move so slow?)
About the Author
Robert Walker
Writer of articles on Mars and Space issues - Software Developer of Tune Smithy, Bounce Metronome etc.Studied at Wolfson College, Oxford
Lives in Isle of Mull
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