Mars is a destination that seems inevitable for human exploration. We have seen a number of intriguing signs from our series of robotic probes that Mars was once a very different world than it is today. Still, even cold and dry though it now is, it remains a place where humans can go and exist with only some help from life support systems. However, the sheer distances involved, coupled with the combination of moderately strong gravity and a very thin atmosphere make Mars a challenging place to get to. That is why the mission architecture selected does matter.
To understand this idea better, consider this analogy. Suppose you want to go on a vacation trip to a distant place on Earth, let's say...Bora-Bora. There are a number of different ways you can go, depending on what matters most. If you want to get there really fast, you could pay a Russian fighter pilot to fly you there in a MiG, but you are going to have to pack really light. Just a toothbrush and a bathing suit and maybe one pair of sandals plus what you are wearing. If you want to have a few more wardrobe options and maybe some additional haircare items, sunscreen, and snacks from home that you just can't live without, you're out of luck on the fighter jet! In that case, you still can take a scheduled airline flight. Checked bags will cost you plenty, but you can pack enough stuff to get by reasonably well for several days to several weeks. Of course, it takes a bit longer to get there. Instead of hours it might take you a good day before you can sink your toes in the sand and relax with that Mai-Tai.
Now consider if you really had to take everything you needed with you, like your food and something to cook it on, water to drink, a scooter to get around on, etc - in addition to your clothes and toiletries - well you're gonna need a bigger boat. Probably a tramp steamer is your best bet. The problem with this is it will take several weeks before you reach your vacation paradise. Given that we live in the age of instant gratification, nobody really wants to wait that long to get there. So you are more likely to try a different solution. You'll crate up your stuff, your food, and big tanks of water and you'll send it on its way a month ahead. You can pack oodles of stuff that way and it is (relative to the airlines baggage fees) cheap to get it there on the boat. Meanwhile, you cool your heels for a couple of weeks, load MP3s and movies onto your tablet, and book a flight that gets you (and a few personal items that you can't be without) there within a day.
Now, of course, Mars is much, much further away than Bora-Bora. And we definitely need to take everything that we are going to need (including air to breathe by the way) with us. That adds up to a whole lot of stuff. Stuff like rovers and tools and extra spacesuits. Water and air and underwear. It all has one thing in common - mass. Well, mass and volume, but for this discussion we care about mass. And that's because it all starts out right here on the surface of the good old planet Earth. Earth has gravity and gravity wants to make it difficult for all that mass to leave the planet. That means we need to build big rockets. But even with those big rockets, it still takes a lot of extra push to get from the Earth to Mars. Robert Heinlein, the excellent science fiction writer, was reputed to have once said, "get your ship into low Earth orbit and you are halfway to anywhere in the solar system." It seems that lots of people believe in this truism. It turns out that lots of people are wrong.
Even for the ubiquitous GEO comsats that circle the globe today, half of the mass that gets lifted into Earth orbit is propellant that has to be used to raise the satellite to its final orbit. Make your destination the moon and you need even more propellant. The mighty Saturn V weighed 6.5 million pounds at liftoff. 99,270 pounds of this reached the moon. Even that was partly propellant because you still have to get off the moon and back to Earth. Therefore, the actual mass fraction of useful stuff to propellant you need just to get there is something like 1.5%. For Mars, it is even less because it takes more propellant to get there and back. When you consider it costs about $10,000 for every pound lifted to orbit that makes it expensive. Kinda like those airline baggage fees.
So coming back to the Bora-Bora vacation analogy, it is clear that something like the slow boat for all the supplies coupled with a (relatively speaking) speedy vehicle for the crew is the right way to go. NASA is looking carefully at this now. Technologies such as advanced solar power and electric propulsion are maturing rapidly and provide a very good way to send the majority of the cargo to destinations beyond LEO with high efficiency. By using this approach, a whole lot of propellant mass can be saved. And that propellant mass costs just as much to launch as the useful mass does.
So that ties back to the real reason that how we go to Mars matters. Careful selection of an architecture -- with the recognition that in-space propulsion has a significant impact on the mission cost -- can easily reduce the cost by a factor of two or more. And that gets down to the real reason architecture matters. We won't go to Mars until we can get the cost estimates down to where the accountants with the green eyeshades in places like the Office of Management and Budget don't faint when they see them. We need an affordable plan. An architecture that considers separate crew and cargo transportation, launch and in-space transportation, and the potential for using resources that can be found on Mars such as carbon dioxide and water not only saves money, it also sets up a long-term logistics pipeline that makes future missions easier. And that is good because if we choose to go, we need to go to stay.
Mr. Cassady is a Member of the Board of ExploreMars. His day job is Executive Director, Advanced Programs Engineering at Aerojet -- Rocketdyne. The views expressed here are his own and do not reflect the position of his employer.