To Mars and Beyond

Note: This article was first published on 31 July 2019.

Beyond Earth

Last June, NASA’s Curiosity rover detected surprisingly high amounts of methane in the Martian atmosphere. At 21 parts per billion of methane, it was nowhere close to the levels seen on Earth, but it was enough to have people ask the tantalizing question again. Is there life on Mars? On our planet, methane is produced by microbes called methanogens or ruminants like cows. However, it can also stem from geothermal reactions, which have nothing to do with biology or living organisms. 

Adding to the mystery is the fact that this isn’t the first time methane has been detected on Mars. In 2013, the Curiosity rover detected a similar spike in methane levels, a finding corroborated by a new analysis of old readings from Mars Express, an orbiting spacecraft built by the European Space Agency. What’s interesting is that these surges in Martian methane appear to be seasonal, following the rhythm of the red planet’s seasons. Furthermore, it only takes a few centuries – a blink of an eye in geologic time – for methane to be broken down by sunlight or chemical reactions, which means that this methane is relatively new. 

But as scientists scramble to analyze existing data and collect more precise measurements in order to determine the source of this methane, some of us on Earth are instead looking to take life to Mars itself. SpaceX CEO Elon Musk has gone on record as saying that an off-planet colony is humanity’s best hope of coming back from what he thinks is the high possibility of a nuclear war or some other cataclysmic event.

With NASA having fallen so far from the heady rush of the Apollo days and the Moon landing, it often seems like progress in space today is in the hands of billionaires and their private space companies like SpaceX and Blue Origin. Musk in particular is really impatient and eager for progress. He wants to be there on Mars in 2024, an entire decade ahead of NASA’s planned 2034 arrival. But is NASA just slow, or is Musk simply foolhardy? 

A really big stainless steel rocket

Of course, there’s a giant rocket at the center of Musk’s plan. In September 2017, Musk unveiled the Big Falcon Rocket (BFR) at the 68th International Astronautical Congress in Adelaide, Australia. This was a two-stage design, featuring a huge booster powered by 31 Raptor engines and the spaceship itself. In fact, Musk thinks that we can build a sustainable colony on Mars by 2050. 

Fast-forward to November 2018, and Musk was ready to christen the BFR with a new name, even though it didn’t – and still doesn’t – exist in any fully operational capacity. The part of the rocket that will actually carry people is now called Starship, while the rocket booster is referred to as Super Heavy. Starship may not be going to any stars in the near future, but ambitious as always, Musk has said that later versions will. 

The current prototype of Starship is now built out of 301 stainless steel, a departure from the aluminum and carbon fiber that was originally proposed. Stainless steel doesn’t sound as advanced or even as light as carbon fiber, but it is also a lot cheaper and may even perform better. It also suits the purposes of the rocket quite well, since the type that SpaceX is using, which has a high chrome-nickel content, actually gets stronger in cryogenic conditions. It essentially has a high fracture toughness, so it can withstand small structural imperfections like cracks and prevent them from propagating. 

But while you want your rocket to be able to withstand the cold in space, you also want it to be able to tolerate heat when it reenters the atmosphere. Stainless steel has a really high melting point, and it can go up to around 1,500°F. 

The metal also allows SpaceX to work toward Musk’s goal of a regenerative heat shield. The rocket will need a heat shield anyway for reentry, but by replacing the tiles used on the current ship with something akin to a stainless steel sandwich, SpaceX can inject water – or some other suitable liquid – into the space between the two layers. The water will absorb heat, and the shield will bleed water through micro-perforations to cool the windward side (the side facing the atmosphere) of the rocket, a process known as transpiration cooling.

This type of heat shield is referred to as "regenerative" because you technically only have to refill the heat shield's reservoir, instead of replacing all the insulating material that burned away in more conventional designs. That said, there are still significant hurdles to overcome. For instance, this design is particularly vulnerable to clogged pores, which can happen when water vapor suddenly freezes.

Truth be told, the Starship prototype looks glorious, and its stainless steel hull looks like a vision straight out of 60s science fiction. If all goes according to plan, Starship will stand roughly 118m tall atop the Super Heavy booster. That’s taller than even the Statue of Liberty and would make for a really imposing sight. There's also a reason it's so shiny – the metal surface will get too hot for paint, so SpaceX is opting for a stainless steel mirror finish instead. 

In the meantime, a second prototype is being built in Florida, an example of a bit of competition going on within the company itself. These engineering "contests" are not uncommon, and SpaceX is likely to incorporate the best elements of both designs into its final craft.

Journey to Mars

Getting to Mars is a complex affair though. The Super Heavy booster will first need to power Starship and help it get to orbit, before returning to the launch site itself. Next, Starship needs to be refueled in orbit by multiple flights of a similarly sized tanker before it can head to Mars. 

This tanker is basically an alternate version of Starship, and is also powered by a Super Heavy booster. There are actually three possible versions of Starship, comprising Starship, the tanker, and a craft for satellite delivery. After topping up the ship, the tanker returns to Earth. 

But before Starship can even think about lifting off, tests have to be conducted. SpaceX is using a brand new Raptor engine, and the top priority is to ensure that it works and, well, doesn't undergo a "rapid unscheduled disassembly". To do that, the Starship prototype, also colloquially dubbed Starhopper, needs to make several “hop” tests, comprising both low- and high-altitude flights up to heights of 16,400 feet. Each hop is also performed on a tether, which keeps Starhopper close to the ground for safety purposes. 

These tests are supposed to demonstrate that Starhopper can hover and land safely. In addition, SpaceX is using them to explore various fuel mixture ratios, among other things. The current version has just one Raptor engine, but later versions will have up to three, while the fully operational Starship is expected to have six. Three of these will be specially optimized to work in the vacuum of space, while the other three will be optimized for sea-level operation. 

Eventually, SpaceX hopes for Starship to replace the Falcon 9 and Falcon Heavy, and even the crewed and cargo versions of the Dragon capsule. Starship will handle all of SpaceX’s loads, including, but not limited to, the settling of the Moon and Mars. In fact, that's a big part of Musk's plan to pay for Starship. By cannibalizing its existing portfolio, SpaceX thinks that it can keep Starship going simply by diverting all its existing resources to Starship.

A bit of rocket science

Starship’s ambitions also call for a completely new engine design. The Raptor engine is far more powerful than the existing Merlin units used in the Falcon 9 and Falcon Heavy. More importantly, while Merlin uses a mix of RP-1 propellant, a type of kerosene, and liquid oxygen, Raptor runs off liquid methane, which would allow it to be refueled on Mars, where the components of methane – carbon and hydrogen – are easily obtained. Similarly, Blue Origin’s new BE-4 engine will also use methane. 

The Raptor is built around a full-flow staged combustion cycle, which has only been used by two rocket engines so far, including the Soviet RD-270 and Aerojet and Rocketdyne’s integrated powerhead demonstrator (IPD) project. However, none of these have actually flown, so if all goes according to plan, the Raptor would be the first such engine to send a rocket into space. 

Instead of having just a single preburner, the Raptor features one oxidizer-rich and one fuel-rich preburner. In modern rocket engines, small volumes of fuel and oxidizer are piped to the preburner, where the resulting reaction powers a turbine that in turn drives the pumps that send more fuel and oxidizer into the combustion chamber. 

The reaction in the combustion chamber is what produces thrust, and it’s what enables the rocket to lift off. 

The full-flow staged combustion engine is generally considered to be the pinnacle of rocket design, where it allows for the most efficient use of its liquid propellants. To understand why, you first have to look at the history of rocket engine designs. 

The open-cycle engine

Some of the best known rockets, such as the Soyuz, Saturn V, and Delta IV, have used an open-cycle engine. The technology has been immensely successful, and it works. However, it had one major flaw. While the preburner is required to turn the turbine, you can’t use the same ratio of fuel and oxidizer as the engine, because the resulting exhaust would be too hot for the turbine. 

Instead, you have to use a fuel-rich mixture, which brings its own set of problems like incomplete combustion, if you’re using a carbon-based fuel. This sooty exhaust cannot be recirculated through the engine, so it ends up being dumped overboard. That’s plenty of waste right there, and you may have even noticed this in pictures as a distinctive black plume next to the fiery exhaust.

Closing the cycle

That said, the last thing you want on a rocket is wastage, and Soviet and American scientists ended up solving two halves of the same problem. To get around the problem of incomplete combustion, the Soviets decided they would opt for an oxidizer-rich mixture instead, where all the oxidizer, but only some of the fuel, was shot at the preburner. Unfortunately, this created its own problem. The oxygen-rich gas produced by the preburner was now so hot that no metal turbine could survive it. 

Fortunately, they managed to solve this by creating a special alloy that could actually stand up against the heat. The cleaner exhaust produced by the oxidizer-rich mixture also allowed them to close the cycle and pipe the preburner exhaust – comprising hot gaseous oxygen – into the combustion chamber, where it reacts with the liquid fuel. A version of this design is used on the RD-180 engine, which powers the Atlas V rocket today. 

On the other hand, the Americans stuck with a fuel-rich mixture, but they swapped out carbon-based kerosene for hydrogen instead. However, the engine had to be adapted for hydrogen, which is significantly less dense than RP-1 or even liquid oxygen, requiring a larger pump to get the right amount to the combustion chamber.

The differing densities and pump sizes also meant that the engine now needed two preburners and shafts, one each for hydrogen and oxygen. The oxidizer still goes directly to the combustion chamber, where it meets the hydrogen fuel that has passed through the preburner.

Either way, wastage is reduced as the exhaust is recaptured and not simply thrown out.

Putting them together

But what if you could combine the best elements of both designs? Put them together, and you get the full-flow staged combustion engine. You have the dual-preburner design that the Americans eventually put into the Space Shuttle’s RS-25 engine, except that there’s now independent oxidizer-rich and fuel-rich preburners. The oxidizer-rich preburner also means that a very strong metal alloy is needed to withstand the heat, and SpaceX developed something Musk calls the “SX500 superalloy”.

On the fuel-rich side, you can’t use RP-1 because of all that soot, which is where the liquid methane comes in. While RP-1 can create carbon deposits that clog up engines, methane burns cleaner, producing carbon dioxide and water as byproducts.

You also have the freedom to pipe all the fuel and oxidizer you need through the preburners without worrying about wastage as in the open-cycle design. In effect, the fuel is burned twice, once in the preburners and again at greater efficiency in the combustion chamber. Wastage is completely minimized and you end up making the most of the available fuel and oxidizer. Both of them also arrive in the combustion chamber as hot gas, which further improves combustion. 

Finally, you have to worry less about the seals between the turbines and the pump, which was an area of concern on the RS-25. The last thing you wanted was for liquid oxygen to leak into hot fuel-rich gas, but there is little risk of liquid fuel leaking into already fuel-rich gas or liquid oxygen coming into contact with oxidizer-rich gas, as is the case with the respective fuel-rich and oxidizer-rich preburners in a full-flow staged combustion engine. This improves the reliability of the Raptor design, which is crucial given that SpaceX expects to reuse the rocket. 

What does space do to the human body?

But even if we pull off amazing feats of engineering and devise a flawless system to get humans to Mars, there’s still one very pesky problem. The human body just wasn’t built to exist for long periods in a zero-gravity environment bombarded by cosmic radiation. 

“The NASA Twins Study: A multidimensional analysis of a year-long human spaceflight” is a cross-disciplinary study of the long-term effects of space on the human body. The study was just published in the April 2019 issue of Science, and it is a one-of-a-kind study, conducted on identical twin astronauts Scott and Mark Kelly. Scott Kelly spent an entire year aboard the International Space Station between 2015 and 2016, while his twin Mark remained on Earth as a control. 

Over the course of 25 months, the brothers submitted to the collection of a multitude of biological samples, in addition to a battery of cognitive and physical assessments. The idea behind the study was to determine what exactly happens to the human body after prolonged periods in space, and by extension, assess the dangers of long-term space habitation.

The answer is pretty simple. The human body does not like space. As adaptable as we are, cosmic radiation wrecks havoc on our DNA, leading to scary changes that scientists are only beginning to understand. Scott Kelly lived in space for a year, and it’s difficult to extrapolate what would happen to Mars colonists who would be exposed to far more radiation. What’s more, the ISS orbits low enough to still be protected by Earth’s magnetic field, even if the astronauts are exposed to more radiation than someone on Earth because they’re above the planet’s atmosphere. 

A crew journeying to Mars would have to contend with eight times more radiation than Scott Kelly. While the study ultimately concluded that it was still possible to sustain human health for a year-long spaceflight, longer space missions are an entirely different ball game, and round trips to Mars could take up to three years

What is Galactic Cosmic Radiation (GCR)?

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GCR is a type of ionizing radiation that mostly comes from outside our solar system, but still within the Milky Way galaxy. Ionizing radiation can blast through substances and alter them. It has been described as an "atomic-scale cannonball", so you can imagine the damage it brings about.

Specifically, GCR comprises heavy, high-energy ions of elements that have had their electrons stripped away. They travel at nearly the speed of light, and can cause ionization of atoms as they pass through a spacecraft or an astronaut's skin. Often, these high-speed particles are shot out into space by the magnetic fields of supernova remnants. Astronauts on the ISS have even reported seeing flashes of light light up their field of vision when their eyes are closed, the result of radiation impacting on their retinas.

Twisted chromosomes

Space sure did a number on Scott’s DNA. Sections of his chromosomes were inverted or translocated, which possibly ended up affecting gene expression as well. The study found that over 10,000 genes in Scott’s genome were activated in space, no doubt a response to stressors like lift-off and zero gravity.

And even though 91.3 percent of these genes whose expression changed in space returned to normal within six months of Scott returning to Earth, a full 811 genes never did. More worryingly, these genes existed in different cell types and were almost all involved in DNA repair and immune function. If humans are to survive for long in space, these genes will be crucial to protecting us from radiation. Cells that cannot repair DNA damage are prime fodder for mutations to accumulate and give rise to cancer and heritable diseases.

Shortened telomeres

Telomeres are the caps that protect the ends of our chromosomes from deterioration. They shorten as our cells divide, and when they get short enough, the cell simply stops dividing and dies, which is what eventually leads to aging and death.

Oddly enough, while Scott’s telomeres lengthened onboard the ISS – likely because of the recommended exercise regime – they rapidly shortened within 48 hours of returning to Earth, perhaps a result of the stresses of reentering the atmosphere and landing. Most of his telomeres regained their baseline levels eventually, but things were never quite the same. Scott now has significantly fewer telomeres overall and higher numbers of critically short telomeres, and telomere loss may eventually make astronauts more susceptible to cancer and other diseases associated with old age.

Visual deficits

Up to 40 percent of the astronauts who have lived on the ISS have suffered some sort of damage to their eyes. NASA isn’t completely sure how this happens, terming it “spaceflight-associated neuro-ocular syndrome”, or SANS. The condition is characterized by optic disc edema, globe flattening, choroidal folds, and other structural changes. Some astronauts also develop cotton wool spots, which appear as fluffy white patches on the retina.

The most common problem is probably globe flattening, where what we think of as the eyeball essentially becomes less round at the rear-end. It’s thought to be a result of fluid build up and increased intracranial pressure, which end up squishing the eyeballs and flattening them.

Poorer cognitive performance

Space also affected Scott Kelly’s brain. He did worse on cognition tests even six months after returning home, responding more slowly and making more mistakes. The findings were slightly odd, since Scott’s performance actually increased in space before declining sharply upon return. Obviously, the limitations of the study – the sample size of 1, for instance – make it difficult to attribute reasons to this decline. 

It’s entirely possible that Scott was simply less motivated once he was back on Earth, but there could also be more insidious mechanisms at work. Either way, it’s concerning, since any crew landing on Mars will need to be ready to tackle more challenges. 

Should we even try?

Mankind isn't a complete stranger to Mars. NASA put the Viking lander on the red planet in 1976, followed by the Sojourner rover in 1997 and twin rovers Spirit and Opportunity in 2003. The most recent rover, Curiosity, landed in 2012 and is still there today. 

But living there is a whole different ball game, and there are myriad challenges to overcome, from rocket science to the equally important question of how humans will stay healthy in space in the long term. Assuming we figure out how to survive the perilous journey through space, we'd still have to make Mars home. The red planet is a barren landscape. It barely even has an atmosphere, which means that if you stood unprotected on the surface, you'd either suffocate or freeze to death. 

Some fantastic ideas have been floated, such as the use of silica aerogel to create an artificial atmosphere of sorts that could recreate the greenhouse effect and warm up the Martian surface. 

Stephen Hawking said that he believed humanity only had 100 years left on Earth.

Ultimately, setting up a human colony on Mars would require astronomical amounts of resources and the smartest minds on the planet. That in turn only raises the question as to whether this is the best way to spend our time and resources. 

Other than ethical issues like the possibility of contaminating Martian life, if it exists, with microbes from Earth, the fact remains that there's a lot wrong with Earth that seems to require fixing before we even think about going to another planet. Climate change, overpopulation, and the prospect of nuclear war has created the impression of an ailing planet on the brink of destruction. In fact, a huge asteroid just flew past Earth last week, and scientists didn't even see it coming until it was mere hours from Earth. Asteroid 2019 OK was so big that it could have obliterated a city, hitting with a force 30 times that of the blast at Hiroshima. How close did it come? Just 73,000km, or one-fifth of the distance to the Moon. 

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Before he died, Stephen Hawking said that he believed humanity only had 100 years left on Earth. Elon Musk also views Mars as something of a backup plan. But not everyone agrees, and a recent Pew survey found that the majority of US adults believe that NASA's top priority should be solving the problems on Earth. Why spend billions on Mars when you could be investing in renewable energy? 

There's also the issue of ensuring that Mars doesn't just become a haven for the rich. And even if Earth does collapse under the weight of all its problems, there's no guarantee that Mars would be the solution. Are we all going to pile onto giant ships and flee for a planet that practically has no atmosphere and 100 times more radiation than Earth?

Having said that, I'd argue that the important thing is to take things slow. We shouldn't shoot for grandiose plans like a second home for all mankind. Instead, we should focus on fixing the problems at home, and in the spirit of science and exploration, look toward other planets. 

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