Sometimes familiar experience clouds our ability to understand theunfamiliar. I’m sure that at an intellectual level people understandthat you need a rocket to get around the solar system, but at a moreprimitive level I think people equate flying to Mars with theirexperience of driving in a car.
So be it! If that is the common view of interplanetary travel, let mehumour the misconception by investigating what it would be like todrive a car to Mars.
I own a RAV4 that costs me approximately 25 cents a kilometre. Marsis 50 million kilometres away at its nearest approach, so the journey(one way) would cost me 12.5 million dollars. The cost of petrol isroughly two dollars a litre, so that means I would have used about6.25 million litres.
A litre is one-thousandth of a cubic metre, so that means I wouldhave burned through 6,250 cubic metres of petrol. As you know, thereare no petrol stations on the way to Mars, so I would need to load upthe tank before leaving, which means I’d need a cubic tank 18 metresalong each side, about six times the length of the car itself.
That’s just the start of the analysis. Petrol weighs about three-quartersof a tonne per cubic metre volume, which means the weight of fuelmy car would have to carry is 4700 tonnes. A Saturn-5 rocket – therocket that sent Apollo to the Moon – weighed 3000 tonnes, so this islike having one and a half Saturn-5s strapped to the roof of the car.
The tank could be made out of steel 1 mm thick and that in itselfwould add 15 tonnes to the weight of the car, which itself weighs just abit more than a tonne.
How long would the drive take? At a cruising speed of 100 kph, thejourney would take 57 years; longer if I stop for food and sleep and ...er... that other thing that people need to do daily. In that time, Marswould have orbited the Sun more than thirty times, so we’re notactually driving towards Mars but driving to a place where it will be,57 years from now.
You get the idea that Mars is a very long way away and that it needs alot of fuel to get there and that the journey will be a long one.You know of course that I can’t drive my car to Mars, but I wonderwhat would be your reason for saying so. You might say that I can’tdrive there because I can’t get my car off the ground, and you wouldbe right. But that’s just one of many reasons I can’t drive my car toMars.
A more important reason – I think – is that my car would get notraction out in space. The wheels would spin uselessly. There wouldbe no road surface to drive against. And it’s because of that thatrockets don’t behave like cars. You don’t drive a rocket to Mars, it’s more the case that you catapault a car to Mars with the aid of a very big rocket that does most of its job in the first ten minutes.
After that you use very small thrusters to fine-tune the direction andspeed once in a while, and by ‘once-in-a-while’ I mean maybe onceevery few months. Most of the journey consists of silent drifting, as ifthe rocket is sliding over some icy surface and firing only when itneeds to.
No-one ‘drives’ a rocket. When asked by his daughter who would bedriving the spaceship when Apollo-8 headed off to the Moon in 1968,one of the astronauts on board said Isaac Newton would be driving. Newton’s second law of motion – the one about action and reaction – gets the spaceship into space with the aid of a rocket. Newton’s first law – the one about inertia - keeps it going once the rocket is switched off.Newton’s law of gravitation – the one about the apple falling out of the tree –makes the spaceship drift in a curve around the Sun and makes the spaceshipfall towards Mars once it’s close enough. The reference to Mr Newton was agood one.
So let’s let go of the notion of‘driving’ to Mars. Take the carout of the picture and put arocket in its place, and seehow a journey to Mars mightreally happen.
First you need to get the rocketinto space, and currentpractise shows that a rocketloses about 99 percent of itselfgetting away from the Earth.Translated, if you want to send30 tonnes out into deep space,you need a rocket weighing3000 tonnes. That’s equivalentto a spaceship as big as theApollo orbital module andlander put together, and arocket as big as a Saturn-5.
(For those that don’tremember Saturn-5s, theystood approximately 100mhigh and took about a year tobuild. They cost billions ofdollars each and required thecombined services ofthousands of people.)
To land on Mars, you need to fire a rocket in reverse, and givenMars’s lower gravitational pull (lower by two-thirds), you’d need arocket about thirty times as big as the object being deposited on thesurface. Okay, so if the 30 tonnes you sent to Mars is mostly fuel to get you down to the surface, then the thing which actually arrives on the surface weighs about one tonne, or something about as heavy as my RAV4.
You get the idea that travelling to Mars (or anywhere else in the solarsystem for that matter) is a very, very, very, very big project requiringlots of fuel and lots of preparation.
We don’t have rockets as big as Saturn-5s anymore because they werejust so outrageously expensive. Only 15 were ever built and the lasttwo were turned into garden ornaments.But that’s a long story I don’t need to go into.
The point is that whenever anything issent to Mars, it is sent using the smallestand cheapest rockets we can get awaywith, and every trick in the book is usedto minimise the amount of fuel after that.New tricks are being added to the book aswe speak.
The tricks that have been dreamt up in the last forty years areabsolutely astounding and rank amongst the greatest intellectualachievements of our time. Movie makers in the 1950s frequently sent good-looking actors to Mars and the other planets in the universe, but never once considered that the characters in the story could be robots, and that their daily adventures would be directed by people on Earth who communicate with them 24/7, and that dozens of pictures would be sent to Earth everyday as a stream of radio waves, ...
... and that the arrival of theserobots on Mars would featuresuch things as parachutes,balloons and sky-cranes. Noteven science-fiction writersforesaw this. The pinnacle ofhuman ingenuity in getting toMars, therefore, lies strictly inthe minds of engineers.
The first bit of ingenuity in reaching Mars is to use the atmosphere toslow the vehicle down so that it doesn’t need to carry a heavy supplyof fuel. Yep, this is like driving my car at 100kph down the motorwayand opening the car doors to slow it down.
It’s a lot trickier in the case of Mars, however, because the planet is so faraway and the atmosphere is so tightly concentrated around it, and we don’treally know how dense the atmosphere is until we get there. Misjudge it by adozen kilometres and you miss it. Misjudge it by a few grams per cubic metreand you might whizz right through it, or be flattened into it like a bug on awindscreen.
Something like this actually happened a decade or so ago. A spaceshipapproaching Mars was supposed to dip into the top layer of theMartian atmosphere so that it could slow down at a suitable rate.Someone misjudged the height of the Martian atmosphere bythinking the number was measured in miles when in fact it wasmeasured in kilometres. The spaceship came into the atmosphere too steeply and plunged down to the thicker layers too quickly for the spaceship to handle. Air resistance tore it to pieces.
The only way to know how dense the atmosphere is and how high itextends into space is to go and measure it, and that means sending aspaceship there using conventional rocket systems, which are heavyand reduce the useful part of the spaceship to the size of a Christmasdinner.
It’s taken a generation of early Martian probes to get us to the pointwhere we can pull off stunts like using air-resistance in place of rocketfuel, and we should not forget this. All progress is made one step at atime, on the shoulders of people and machines that advanced thecause of science fairly anonymously and without always knowingwhat these advances would lead to.
Let me put a Martian journey into perspective for you. Say you are inan aeroplane at 10,000m – which is as high as most commercialaeroplanes ever go – and say you want to throw a stone at a circle onthe ground, representing Mars. On this scale, Mars would be abouttwo metres across. If you think that’s a difficult target to hit then Ihave bad news for you, because the circle is not the target, the edge ofthe circle is the target. You want to hit the edge of Mars, where theatmosphere is accessible without hitting the planet itself. On thisscale, the target is a line around the circle 2 cm thick.
But let’s make the task even more realistic. Remember that theMartian atmosphere is thicker near the bottom and that thespaceship is travelling at terrific speed and will tear itself to pieces if itenters the lower atmosphere too soon. So you aren’t just aiming at theatmosphere, you’re aiming at the top ten percent of it. That meansthat the part of the circular line on the ground that you want to hit isthe outermost 2 mm.
Now let’s get even more tricky. Remember that Mars is at least 50million kilometres away, and for reasons that I don’t want to go into,the true distance is likely to be much greater than that. Light travelsat 300,000 kilometres per second, which means radio signalstravelling to and from the spaceship take 3 minutes or more to coverthe distance. This is kind of like steering a car down the motorwayusing what was seen through the windscreen three minutes earlier....
...Granted, there aren’t many things to crash into in empty space. Butwhen the spaceship actually hits the atmosphere, the spaceship has todecide for itself if it’s coming in too steeply or not steeply enough. If iterrs too much to one side, it gets torn to pieces. If it errs too much tothe other side, it shoots straight through the atmosphere and popsout the other side. Pressure sensors on board tell the spaceship if it’son the right track or not, and computer code tells the spaceship whatto do if the spaceship goes off course. The nominal path is like atunnel through the sky, thousands of kilometres long and maybe tenor twenty kilometres in diameter; like shooting a bullet down agarden hose without touching the sides.
Once inside the atmosphere and falling at low speed to the surface, aspaceship can slow itself further with the aid of a parachute. Butthere’s a problem with this. Mars’s atmosphere is not as thick as theEarth’s so parachutes don’t work as well there as they do here. Airpressure at the Martian surface is like air pressure at the top of theHimalayas on Earth, so parachutes are only going to slow you downslightly, not enough to prevent damage when you reach the surface.Something else has to be used in the final few metres of the descentto soften the blow.
Early spacecraft carried descent rockets that would fire just a fewmetres above the ground to remove most of the remaining descentspeed. But here’s the thing: no matter how much descent speed youremove, you have to have a bit left over so that you can settle onto theground, and the heavier your spaceship is, the more inertia it carriesto the ground and the harder the thump. A light spaceship can’t carrymuch fuel so it can’t slow itself down by much. A heavy spaceship cancarry loads of fuel so that it can slow down but it still hits the groundhard because it’s heavy! That’s a Catch-22 situation. Is there a betterway?
About five years ago, a couple of Mars probes used giant balloons tosoften the final impact with the ground. Balloons compress andabsorb the shock of landing, and in the process bounce the spaceshipback up into the sky a few times before all that energy is dissipated.The first bounce was a real doozy, springing the spaceship back up toa height of a kilometre.That was a pretty inventive way of lowering a spaceship onto Mars,but the Curiosity probe went one better. Descent rockets brought thespaceship to a complete standstill a hundred metres off the ground,and cables lowered it from there so that the weight hitting the groundwas just a fraction of what it would have been if the whole thing hadgone to the surface.
I hope it impresses you that people can come up with such weird andclever ideas and can dare to even mention them to people whofinance these projects. I think if I was to try it I would be laughed outof the building and told never to come back.The great thing about this peculiar idea is that it actually worked! Youcan hear the reaction of the control team when their baby actuallymade it to the surface. They sound as surprised as anyone thatCuriosity made it to the surface, despite their better understanding ofthe technology that made it possible.
Now we have a roving laboratory on the surface of Mars which is justabout as big and heavy as all the previous Mars probes put together;something about as big as the car I was prepared to drive to Mars atthe start of this article. It didn’t take 57 years to get there, nor did itrequire one and a half Saturn-5s. It got there in less than a year, usinga quantity of fuel half as great as what my RAV4 would have required,and conducted the landing entirely without human intervention.What could be a greater technological achievement than that? Andwhat does that say about the grander schemes to come when wecontemplate the exploration of Mars and other places?