There's very little that's more wonderful than landing a spacecraft on another world:
But how did we get there?
I'd like to say it's not rocket science, but, well...
Exponentials Everywhere
Launching things into orbit is hard. In a later essay I'll talk about orbital mechanics and the business of moving things around the solar system, but here I'm going to focus on boosters: the big rockets that we need to take our first small steps toward the stars.
The most obvious thing about booster rockets, from the Saturn V that launched the Apollo lunar missions to Space X's Falcon, is that they are big. They run from 50 to over 100 m tall. For comparison, a modern 747 is about 75 m long.
They aren't just tall, they're massive: the two stages of a fully fuelled Falcon Heavy come in at over half a million kilograms, and can deliver slightly over 10% of that mass--68,000 kilograms--to low Earth orbit.
A 747 has a maximum take-off mass of that's about 80% of that, but over half that mass is cargo.
Why the big difference?
The answer is the exponential nature of the rocket problem.
Any process where "a thing causes more of the same" tends to grow exponentially. In the case of rockets, we need X amount of fuel to accelerate the payload to orbital velocity. But then we need more fuel to accelerate that fuel, and then still more fuel... which results in the total mass of the rocket depending exponentially on its final orbital velocity.
It isn't just fuel, either: rockets don't breathe air, but pump both fuel and an oxidizer into a combustion chamber where they burn, expand, and are pushed out the nozzle to produce thrust. The fuel in most modern rockets is plain old kerosene, and the oxidizer is typically ultra-cold liquid oxygen.
The basic ideas of rocketry were worked out over a century ago by Russian pioneer Konstantin Tsiolkovsky, whose "rocket equation" is the foundation of astronautics. Because rockets require fuel to accelerate fuel, the equation is an exponential that relates the final velocity (when all the fuel and oxidizer are burned) to the ratio between payload mass and fuel mass. The 1:10 ratio of the Falcon Heavy is really good for a rocket, but nowhere near the 1:1 ratio of the jet-powered 747.
So why not use jet engines rather than rockets, at least for the lower stage where the air is thick enough that an oxidizer is not required?
The fundamental reason for this is that the goal of rocketry isn't so much height as speed. Things don't go into orbit because they go high, but because they go fast. They need to go high to get above the atmosphere, but on a body like the Moon, where there's no air, you could have an object in orbit a few metres off the ground, assuming you found a flight path with no hills in the way.
Conventional jet engines work pretty well up to about the speed of sound, and can be coaxed to about three times that, but orbital velocity is twenty times higher.
This is why a lot of "space tourism" is focused on sub-orbital flights: flying high is relatively easy. Flying fast is hard. And, as is frequently the case, the engineering is harder than the science.
Rocket Engineering: I love big pumps and cannot lie
The first stage rockets on the Falcon Heavy burn about 400 tonnes of fuel and oxider in just over three minutes. That's over three tonnes per second, which is a pumping rate that would drain the average backyard swimming pool in thirty seconds. Getting those liquids--one of which is at -200 C below freezing!--into the combustion chambers fast enough is one of the hardest engineering problems there is.
Cavitation is one issue: the boiling point of a liquid drops as the pressure goes down, which is why people living at high altitudes have trouble making a decent cup of tea. When a turbo-pump is sucking fluid through at a rate of tonnes per second, it's easy to get regions of extremely low pressure on the intake side. The liquid then boils, and the pump finds itself ingesting a mix of liquid and gas, after which it tends to violently disintegrate.
SpaceX's Merlin engines use high-pressure helium to keep the fuel and oxidizer tanks pressurized so this doesn't happen. If the helium feed fails, as it has done on occasion, cavitation ensues and the rocket blows up.
The Future: wings, orbital towers, and more of the same
Getting to orbit with rockets is hard. Might there be an easier way?
There is no shortage of ideas.
Winged rocketry was once thought to be the natural future. Bell Aircraft, maker of the X-1 rocket-plane that Chuck Yeager flew through the sound barrier, submitted a proposal to the early Apollo lunar program that was summarized by a NASA engineer as: "Wingless would be only a stunt."
Sixty years later we're still launching capsules of the kind that Bell refused to consider, but a lot has changed since then. Lighter, stronger materials make wings more interesting for the first stage: carrying a much smaller rocket on a high sub-orbital arc looks tempting. By lofting the second stage well out of the atmosphere you can give it a lot more than three minutes to reach orbital velocity, which makes the pumping problem, in particular, more tractable.
We have better air-breathing engines, too. Turbo-fans may be replaced by supersonic ram-jets or "aerospike" engines that have no moving parts (other than fuel pumps: space travel is a pump-heavy business!)
More exotically, cables reaching down from an artificial asteroid parked in geo-stationary orbit above the equator might haul payloads into orbit using nothing but solar power.
For now, though, we're likely to keep on with incremental improvements to the big boosters that have been the workhorse of every space program since the '50s. They have opened up the solar system to robotic exploration, and in the next decade they will take humans back to the Moon, and beyond.
Rocket Science
Wow, so I guess the old expression "it's not rocket science" should more accurately say "it's not rocket engineering"!