First, this isn’t going to happen - comets so large don’t hit the inner solar system any more. The most recent large impact crater of that size on Mars is the Hellas Basin. Nothing so large for nearly 4 billion years anywhere on Mars, Mercury, Moon, what we have of Earth or Venus.

If you could somehow add lots of water and CO2 to Mars it wouldn’t make it habitable for humans though. Even if you could somehow copy all of Earth’s atmosphere to Mars, the planet would be far too cold because it gets half the amount of sunlight that Earth does. It is on the outermost edge of our Sun’s habitable zone. It’s a mystery, how it managed to have liquid water in the past. Two main hypotheses there

• It’s orbit’s eccentricity varies a lot more than Earth’s orbit. At times it has a very eccentric orbit. Maybe it had liquid water only every two years when it was close to the Sun and at other times was a frozen ice ball.
• It may have had potent greenhouse gases, perhaps methane, in its atmosphere

Ideas for terraforming Mars involve megatechnology all the way through into the foreseeable future for as long as it stays habitable - either planet sized thin film mirrors in orbit, or else producing greenhouse gases, with mining of cubic kilometers of fluorite ore every century, and using power equivalent to 500 nuclear power plants to continuously make greenhouse gases. Otherwise it would be like Antarctica or the high Arctic all over. Too cold for trees. Seas frozen over if it had seas. That’s even with a pure carbon dioxide atmosphere.

It also has just over third of Earth’s gravity which means you need 26 tons of atmosphere per square meter instead of ten tons, to get Earth normal atmospheric pressure (that’s tons of mass, of course it is ten tons by weight in the Mars lower gravity).

Also once you have a carbon dioxide atmosphere then if you use photosynthesis, with half the amount of sunlight of Earth, well to turn carbon dioxide to oxygen, you have to take all that carbon out of the atmosphere. Build meters thick layers of organics all over Mars taken out of the atmosphere using life processes. That would take about 100,000 years on Mars according to Chris McKay, to have an atmosphere humans can breathe - taking account for both the reduced sunlight and the need to have more oxygen in the atmosphere by mass per square meter to make the air breathable for humans.

There aren’t any comets with oxygen in them ,just CO2 . They also have methane, some of it, but methane wouldn’t stay long in the atmosphere without some process to keep it there.

If you overcome those problems, then it will lose its atmosphere over geological timescales. First, it has no continental drift. This means that as carbon dioxide gets taken up by oceans, there is no way to restore it back into the atmosphere again. This is a very slow cycle but over a timescale of about 100 - 200 million years on Earth, all our CO2 would be gone without continental drift. It all gets locked up in rocks such as limestone. No CO2 means no plants. The Carbon Cycle. Mars enthusiasts have ideas of using microbes to digest the limestone and return the CO2 to the atmosphere, but that’s just pen and paper sketching, it’s not a worked out slow carbon cycle.

Then Mars has no magnetic field. This means that there is nothing to protect water vapour in the upper atmosphere from being split apart by ionizing radiation. Timescale unsure but Mars probably had a thick atmosphere in the past with lots of water and most of the water is gone, and we don’t know for sure how and where but probably a lot of it was lost from the atmosphere. So - maybe on a similar timescale of hundreds of millions of years, it loses its water.

Now - that might not seem such a big deal. Our civilization will probably be long gone. But if there are intelligent beings on Mars by then, humans or other creatures, well we are only separated by time and I think we have a responsibility for them.

Also - as the Sun warms up in the future, for a while Mars will have perfect temperatures for habitability without need for greenhouse gases or mirrors. That might be the perfect time to terraform it. If we do a terraforming now that messes up the planet, it might not be available when it is really needed in the future. I think that is kind of relevant because Elon Musk for instance talks about going to Mars to become interplanetary. Well there are no immediate threats to Earth that could make it as uninhabitable as Mars - even a terraformed Mars.

• Asteroid impacts - we are no longer hit by the huge asteroids that created the basins and largest craters on the Moon and Mars
• We could easily survive the Chicxulub impactor - the dinosaurs couldn’t make clothes, or boats, or make fire, or shelter, didn’t have primitive tools. We can do all those things and we are also omnivores able to eat just about everything. A Chicxulub impactor would not make us extinct. We just need something edible somewhere in the world that we can harvest and if necessary cultivate
• We have a breathing space anyway as we have plotted the orbits of all the asteroids of 10 km upwards that are in Earth crossing short period orbits - the Potentially Hazardous Object. With 100% coverage we know none will hit us in the next several centuries. Comets are still possible, but at present it’s a time of few comets (only 1 in 146 of flybys are by comets). By the time one is headed our way, which will happen almost certainly in the next few tens to hundreds of millions of years, well almost certainly we deflect it so long as we retain civilization and space capabilities.
• Ionizing radiation from gamma ray bursts and supernovae can’t get through our atmosphere in significant quantities. Only the UV light during the initial burst gets through.
  
  So they mainly affect us by damaging the ozone layer. Other ideas like ground level ozone and nitrous oxides created by reactions in the upper atmosphere don’t work out when they do the detailed calculations. Humans would be able to survive this easily.

So the only things that will definitely make Earth uninhabitable are far future hundreds of millions of years in the future events like the Sun getting too hot. That’s something we could deal with using huge shades or actually moving Earth. But we might decide to colonize Mars at that point -if we don’t move Earth then it becomes more habitable than Earth briefly as the Sun gets hotter.

Do it now and we may be denying the chance to do it then in the future. So if he is really concerned about future extinction events in the very remote future like that, leave Mars alone. It’s very unlikely that we know what to do now to make it more habitable half a billion years later when we need it.

While if concerned about near future events, there is just no way anything can make Earth less habitable than Mars. Here is where we make our stand. A future with a habitable Earth is what we need to aim for.

There are many issues with sending humans to Mars as quickly as possible - I think myself not least, the issue that many raise, that it would mess up our search for life on the planet, muddy the waters. Why rush with all our microbes (they are the problem, not us) to the place in the solar system where they can cause most havoc for science? Where they might mean we are unable to make the next great breakthrough in biology - it could be as significant as that on Mars. A precious exoplanet, with maybe extraterrestrial microbes in our own solar system.

Terraforming is a fun idea. Well worth studying and tells us a lot about Earth and exoplanets.

But what we can do right now is paraterraforming, covering surfaces with greenhouses until the whole surface is a giant greenhouse. And before that, make small self sustained habitats. But we don’t need Mars for that. We can do that in free space, use materials from an asteroid to build a Stanford Torus type habitat spinning around the asteroid as a hub to generate artificial gravity - or convert the probably vast lunar caves into habitats - may be 100 km long and kilometers in diameter. Or build habitats on the lunar poles close to ice and with 24/7 sunlight.

That’s the plan of the ESA and I think it’s the best starting point. Because the Moon is so close to Earth and it actually has many benefits for habitability.

One of the habitats for ESA’s planned “lunar village” at the poles - envisioned as a collaboration like the ISS - but with participants having separate habitats there grouped together much like a village. It is getting a lot of support in the international community, except for in the US. Will be interesting to see what happens there now that Trump is president (Obama was dead set on going to Mars and told NASA to ignore the Moon. Before him Bush said go to the Moon, so it flip flops between presidents quite often).

But there’s no rush. No panic. If we have 1000 people in space that would be a huge change. Let’s see how that works first. Millions in space - well that means that you could have anyone in space, can’t keep it to the “good guys” or harmless scientists and tourist. The likes of ISS and North Korea developing in space - how can you stop that happening? You could end up with a war in space and with spaceships traveling at tens of thousands of miles an hour, and the habitats as fragile as eggshells, and nowhere to survive if your habitat is destroyed, then I think a space war would be over quickly with no survivors remaining in space.

We have to find some other way of dealing with things before we can have millions in space - it’s a possibility for a post warfare society I think. And I think that future is a possible future, we could get there. Post warfare doesn’t mean no competition, it just means you solve issues in other ways. But we aren’t there yet, and until we reach a post warfare society at least in space, I just don’t see how you can have millions there.

It’s also very inhospitable. The coldest driest place on Earth is far far more hospitable. So is the summit of Mount Everest. A mountain 30 kilometers high on Earth would still be far more hospitable than Mars. And indeed in many ways it turns out the Moon is more hospitable than Mars too, and it has the benefit that it is really close to Earth and there is some prospect of commercial value too. There are many possible exports from the Moon. Whether any of them will pan out, I don’t know but they do have advocates and enthusiasts who think they will do and write books about the commercial case for the Moon. Typically these books have many chapters outlining ways in which they think entrepeneurs can earn money from the Moon.

Nobody writes similar books about the commercial case for Mars. The “Case for Mars” by Robert Zubrin has one brief and rather unconvincing chapter on commerce. Elon Musk and Zubrin both think that the main way Mars will pay for itself is by sale of intellectual property like publishing rights, patents to inventions and so on, claiming that Mars colonists will be so much more inventive than everyone else that there will be a huge net export of discoveries from Mars which will pay for everything easily. I find that unconvincing myself.

There are other ideas for Mars exports but they have to be very valuable to be worth doing. And the most valuable export - scientific understanding of the origins of life - may be destroyed by rushing to send humans there as quickly as possible.

See also my new online and kindle book:

If Humans Touch Mars - Like the Lascaux Story - Another Tale of Human Missteps?

MOON FIRST Why Humans on Mars Right Now are Bad for Science.President Obama, if you love science, Please protect Mars life from contamination from Earth (I should change this title now that Trump is about to become president)

Case For Moon First