This story is Part 2 of Quirks & Quarks' Pathway to Mars series. Each instalment will look at one part of the huge challenge of the most ambitious journey of exploration we've ever attempted — a human mission to Mars.

It's easy to dream of putting humans on Mars, but designing the spacecraft to actually get there will be no easy feat.

One of the open questions about how we'll do it is whether it will be with established rocket technology or something entirely new.

Whatever spacecraft makes it to the Red Planet one day will need to be extremely large, able to carry lots of heavy equipment and capable of sustaining a crew for many months on end.

It may have to be assembled in orbit, from components brought up in multiple launches by a new generation of heavy-lift rockets, which themselves will be a major engineering challenge.

There are a number of players already developing rockets to try to achieve that goal, by harnessing both old and new technology.

Here's a look at some of the possibilities.

NASA's Space Launch System

It's been a while since the last human travelled to the moon — since 1972 , to be exact. But NASA is trying to change that.

The space agency is constructing a giant rocket called the Space Launch System (SLS) that NASA describes as the most powerful ever developed. It's expected to have the capacity to send astronauts and heavy cargo to the moon in one go. It'll also have the capacity to do the heavy lifting to orbit of the pieces that could be assembled into a Mars vehicle.

NASA's goal is to send the spacecraft to the moon by 2024 and to have a mission to the Red Planet ready by the 2030s.

Critics have argued that the SLS is costing too much money and taking too long to build, says Space.com writer Michael Wall. (NASA)

The SLS Is largely based on existing technology developed in the space shuttle program, according to Michael Wall, a senior space writer at Space.com. It's also been been controversial. Wall said critics have argued that the SLS is costing too much money and taking too long to build, especially compared to projects in the private sector.

The complexity is the cause for the expense and delay.

"There are millions of individual parts that go into this," Wall explained. "When you're designing a rocket that's going to carry humans, there is a lot of bureaucratic red tape that you have to go through, and you have to demonstrate everything being safe to the nth degree."

Unfortunately, it also involves re-learning lessons from the Apollo missions which used enormous Saturn V rockets to send astronauts to the moon. But those missions were so long ago that experts have lost access to important institutional knowledge, Wall said.

"It's not necessarily a measure of them having to come up with a whole new way to actually do things, but to do things on a scale that they haven't done in like half a century," he said. "You've got to kind of build up those same capabilities again."

SpaceX Starship

A lot of smart money is betting tech entrepreneur Elon Musk's company, SpaceX, will be the first to put people on Mars.

"That's what everything that company has been doing is actually working toward," Wall said.

SpaceX Starship is designed to be a reusable rocket. (SpaceX)

SpaceX has designed a system dubbed the Starship that the company claims will be the world's most powerful launch vehicle. Unlike the SLS, it will be designed to be a reusable rocket, which is a technology SpaceX has become famous for with its Falcon 9 rockets.

"Basically it's a spaceship that sits on top of a giant rocket and they have huge numbers of engines," Wall said.

According to SpaceX's website , it will be capable of carrying more than 100 tonnes to Earth orbit, and could carry a crew and cargo to the moon and Mars. Musk has in the past said Starship will be able to carry 100 passengers , although some people are skeptical of that claim, said Wall.

But it's certainly ambitious.

"They want to build a whole fleet of starships," Wall said, adding that the rockets will be powerful enough to launch themselves off Mars to return to Earth again.

So far, the company has built two prototypes, one of which was recently damaged in a test. It hopes to have a fully functional Starship available to launch satellites, by 2021, Wall said.

Compared to NASA's SLS, Wall thinks, "Starship will be a little more powerful. And then it comes down to price."

If Starship does turn out to be reusable, unlike the SLS, "that could open up everything in a way that we haven't seen before," said Wall.

A more efficient solution?

Both the NASA and SpaceX plans in their current form depend on using huge chemical rockets for propulsion for a vehicle that will travel to Mars. This is a proven technology, but it's slow. It would take a chemical rocket six months or more to travel from Earth to Mars.

Retired Canadian astronaut Chris Hadfield thinks that won't be good enough. Earlier this year on a Quirks & Quarks debate about human exploration of space he said, "I don't think we're going to Mars until [...] we go from propellers to jets, or whatever that equivalent will be, from rockets to something else."

Another former astronaut is working on that something else. Something he hopes might bring about a "paradigm shift in transportation in space."

Franklin Chang Diaz, president and CEO of Ad Astra, has been developing a plasma rocket engine called VASIMR. (Ad Astra)

Franklin Chang Diaz flew on the space shuttle seven times and holds a PhD in plasma physics from MIT, so he knows a few things about rockets. Through his firm Ad Astra Rocket Company he's been developing a plasma rocket engine called VASIMR. It could be the key to a Mars vehicle that will be faster and more efficient than any old-style chemical rocket.

Chemical rockets work using a very old technology — fire. They burn gases, like hydrogen and oxygen, which then reach high temperatures and produce thrust as that hot gas escapes through a rocket nozzle.

"This is a very sort of primitive way of transporting anything in space," Chang Diaz said.

The VASIMR rocket engine works a little differently. There's no combustion. Instead, high-intensity electromagnetic fields are used to heat up a gas to temperatures much hotter than ordinary combustion can reach — millions of degrees hotter.

When this gas is allowed to escape through a magnetic rocket nozzle, the high temperature translates into far greater thrust than chemical rockets can provide.

What's more, the power for the rockets comes from electricity, which can come from solar power or from a nuclear power source on the rocket. So, large amounts of bulky, heavy chemical fuel isn't needed and the rocket is lighter as a result.

The scenario Chang Diaz projects for a Mars mission would be a spaceship about the size of the International Space Station, with a crew of perhaps six people and with a VASIMR engine powered by a nuclear reactor about on the scale of those used in nuclear submarines.

"It will deliver about 60 metric tons of cargo to Mars in about 90 days," he said.

An equivalent mission powered by chemical rockets would take two or three times as long, and would likely require either a bigger spacecraft to carry the extra supplies, or a smaller crew.

Diaz cautions that these plasma engines are only for the space-based component of the voyage to Mars. Chemical rockets are still the technology of choice for lifting loads into orbit or for the last stage of landing on Mars.