Orbital 'Bungee'

Scene: You've arrive in Mars orbit having travelled for many months.

The view is nice, but you naturally wish to make the final thousand kilometres trip to walk on the surface.

Now, you could attempt to use aerobraking, but many attempts to do that have ended in disaster and the final mach 5 bit is tricky at best. Or you could use an absolutely huge rocket to land, but that would cost too much to launch towards Mars. A space elevator would be great, but they take weeks to get you down to the surface, and they need lasers and power stations to get you back up again.

Instead, you attach the Mars patent grappling hook (tm) to the back of your suit, and check the hydrogen peroxide jets on your suit pack.

Attached to the grappling hook is 2400 km of carefully tapered kevlar with a counterweight on the far end...

Using the backpack you fly so that you end up 1200km ahead prograde, in a flat spin to keep the cable from sagging; meanwhile a robot at the counterweight does the same thing in the opposite direction.

When you are exactly 1200km ahead, you cancel the spin and add a bit of down. Meanwhile the counterweight cancels the spin and adds a bit
of up.

Now, tidal forces kick in and make you fall towards Mars, meanwhile tidal forces kick in and move the counterweight up, keeping the balance point in almost the same place- you're still in orbit.

However, as you fall you lose potential energy, so your speed, relative to the orbital motion increases (in this case retrograde), and the counterweight does the opposite- it's gaining speed, and you're losing speed in equal measure.

So picture the scene, you're swinging towards mars on a few hairlike cables with Mars rushing towards you. You're in a spacesuit, with no other protection. You're going to die, right?

Well... no.

At the bottom, it turns out that with a cable about 1200km long, you're now stationary with the ground (and air)- since the swing has cancelled out your orbital motion relative to the ground, so as you enter Mars very thin atmosphere you're actually going very slowly, so you're not burning up. In fact, from Mars' surface your motion is what is called a 'cycloid', which is very much straight down at the sharpest point.

Max g on the swing from orbit is about 1. Because you're exchanging mass with the surface you're taking almost no energy at all.

Now adjusting a 1200km long tether so that you just touch the surface is probably a bit too much to ask, so the tether is set so that you are a few km up at closest approach.

Now, to greet you a very small rocket that was earlier dropped on Mars has picked up a bunch of Mars rocks that weighs the same as you; and has flown up and grabbed the bottom of the tether below you. Once it does this you release your tether (if it misses, you ride back up- free ride, no charge!)

This is where your rocket pack comes in you use your peroxide rocket backpack to land on the surface (reasonably easy to do under 1/3 Mars gravity). (Delta-v from, say, 1km altitude under Mars gravity = 86 m/s).

Now, it's a good idea not to mess up your landing, but you're down! And you're forever cool ! ;-)

Meanwhile the tether swings back up, speeds back up to orbital speed, and the counterweight slows back down and it's made ready for the next astronaut, and it becomes spin stabilised, until the descent window lines up with you again and then it gets fired and the next astronaut follows you down and lands close by.

The tether/payload ratio with Kevlar PBO is about 450, I reckon it would cost about $1 billion to deliver to Mars orbit (OK call it 2 billion due to swimming pools), and can be completely reused for all of your equipment; if it's smaller than ~200 kg (i.e. a person in a
spacesuit).