Humans aren't going to Mars anytime soon.

For a very long time I wanted to go to Mars. Yes, I wanted to take that three year long lonesome trip through space with a small crew and a communication delay. It's the adventure of a lifetime! The adventure of a generation. It's hard to really conceptualize how far away Mars is, and while that is part of the appeal, I now realize just how significant the challenges are. The last 1.5 years I've been working on a lunar lander, which I viewed as a great step into seeing what it takes to send a lander to another body in the solar system. I've learned a tremendous amount about travel beyond Earth in the last 1.5 years. None of this is really proprietary, in fact, most of the learnings are from the Apollo days in the 1960s and 1970s, with only a few from the recent Mars rovers.

When you read the news articles, one of the recurring themes of a human trip to Mars is that we don't have the technology to do it. I would see that statement and not truly understand what that means. So let me give a few examples of the technology that we don't have ready yet:

• We don't have a spacesuit to be used on Mars. This is modestly significant. Mars has a lot of advantages, it has a thin atmosphere, the temperature is more temperate than the Moon or even the International Space Station, so the suit will likely be slimmer than the bulky in space suits that are currently used for spacewalks, and were used on Apollo on the Moon. So while it should be easier to design and manufacturer the suit, as far as I know there has been no serious work on a Mars suit yet, and it's the kind of thing that really will need a few years of dedicated work before it's ready to be used by humans on Mars.
• It's not clear how to launch a vehicle from the surface of Mars back into Mars orbit. There are a couple sub points here. 
• First, the baseline architecture for the Mars Sample Return mission costs around $11 billion, and that's to get back about 15 kilograms (34 lbs.) of samples from the surface of Mars. One astronaut weights a lot more than 15 kilograms. Now, Rocketlab proposes they can do it in house for only $2 billion... but remember this is for low tens of pounds of payload from the surface of Mars back to Earth. This first point being it's very expensive, and we've never done any launch from the surface of Mars. 
• Second, a human lander will likely be significantly different than the small sample return rocket. SpaceX is proposing using Starships, which definitely work on paper, but there is a question about how exactly to fully fuel those Starships. Mars has a lot of CO2, so just bring along a big garden and turn that into a lot of oxygen, which you can cool and condense into liquid oxygen. However, to get the hydrogen for the sabatier process, I'm not clear on exactly how to get that. The baseline is to harvest water, and then do hydrolysis to get hydrogen and oxygen, but we've never harvested water on Mars. However, you're going to need likely at least a hundred metric tons of water, maybe 1000 metric tons of water, and that's a lot. We haven't even harvested 1 kilogram of water, let along the possible 1,000,000 kilograms that could be needed to launch a large rocket off the surface of Mars. Now it's possible to take all of the return trip fuel with you, say bring five ships, one with the people, and four with fuel, and just refuel the human ship on the surface and/or in Mars orbit, and leave the four tankers behind. But the point of this is that exactly how we are going to launch a human crew sized vehicle off the surface of Mars is not clear and will require a few years of engineering to make sure we can actually do it.
• My assumption is that almost everything on a short Hohmann transfer orbit to and from Mars with an astronaut crew of say four people will essentially use expendable consumables. What I mean by consumables:
• Oxygen for breathing and water for drinking, they will essentially take a whole lot of each and then CO2 and urine and excrement will essentially be dumped overboard. Some amount may be recycled, but in the event the recycler fails, you'll need to have a lot of spare.
• Clothing will maybe be washed by hand on Mars, but for a fifteen month stay it has to be expected that clothing, boots, gloves, and likely whole spacesuits will wear out. So does each astronaut need to bring one Mars suit and some spare parts like o-rings, or four whole spacesuits? Again, we don't have a good baseline for how we will wear out clothing on Mars. 
• You're going to have to take a lot of food for the three year trip. We probably can't grow potatoes fast enough like in the book and movie The Martian. It may sound super minor, but the 33 month trip will need roughly half of the time in microgravity, and half the trip at 1/3 Earth gravity on Mars, and the systems for food and living will need to work in both gravity environments. Again, sounds trivial but I'm not sure we really know it will be that easy.
• We're going to need a lot of solar panels, a whole lot. I don't even know how many. Let's say that one human for habitation and some driving around the lunar surface needs 10 kilowatt hours per day, and four people land on Mars, that's 40 kilowatt hours per day. Given that solar panels are less efficient on Mars, you're going to need a lot of solar panels. And honestly, depending on the amount of driving that is baselined, as well as electricity needed for food production experiments or rocket fuel production, 10 kilowatt hours per person per day could be far too little. Again, this is an area where presumably we can modify already existing solar panels, and batteries for use on Mars, and we don't really have to worry about clouds on Mars, just the changing of the seasons, but it's going to take some work to make a mass efficient system.
• All of the pressure vessels, engines, heat shields, and human habitations, assuming that they are the only one of their kind, will need to be repairable. The risks of a crack or accident leading to higher than expected pressure loss will necessitate the ability to repair these pressure vessels. Repairs have been done on the International Space Station, and Apollo 13 is a famous case study in making do with what you have.  The items for for heat shields, engines, pressure vessels will need to be repairable, because a tiny micrometeoroid could render most pieces of equipment inoperable. Will people need to weld on Mars? Will people need to repair wire harnesses on Mars when a pin gets bent? Will people need to patch holes on their habitation walls? How many o-rings and seals will we need to replace because Martian dust cuts the seals? This is all pretty basic stuff, but a single cut o-ring could mean mission failure. 
• Rover wheels for a human sized rover on Mars is a big problem. The Curiosity rover shredded it's wheels faster than than expected, and traveled not that far of a distance. A human rover will probably go something like 1000+ kilometers in 15 months, compared to the 33.5 kilometers for Curiosity rover in 12 years. In other words, we're going to need better Mars rover wheels, and we're going to need spare tires. Again, this is not trivial and will take years of development here on Earth. While there are full size prototypes that NASA has developed, the whole logistics of a rover and spare wheels is a really big deal. The rover itself would be much more useful if it's a pressurized rover and could make multiday trips farther away from the landing site. This adds a fair amount of complexity and mass to the rover, something that we really should be seeing qualification prototypes of say two years before launch so that we can make sure to work out the details.
• People will get sick and hurt. I've thought about it a lot over the years and I am pretty confident that the single most important person on a mission to Mars, would be a doctor. There are so many different ways that people could get sick or hurt, and it's very possible that we discover ways we hadn't even thought of, such as what Mars dust does to lungs when it gets into the habitation. I'm not actually that worried about radiation, it's a relatively known phenomena and while clearly dangerous, would again be rather known for a three year trip. While this bullet point isn't stopping us from going to Mars, we're going to have to take a small emergency room worth of medical supplies and hope that we brought the right stuff. If a person gets cancer on the outbound trip to Mars, orbital mechanics dictates that you have to stick out the whole mission, because the fuel required to do a direct abort and get back to Earth is not part of the plan. Similarly, there is a psychological component to leaving Earth, and barely being able to even see it in the sky that is a whole new challenge we don't really know how to handle. Presumably the people on the trip will be highly qualified and highly trained and have very strong emotional fortitude, but when you can't see Earth, and there is a five minute communication delay, and your parent died, and the X-band antenna broke so you only have S-band text messages, that's a lot of stress
• Last point, it's going to take a lot of launches from Earth in 2028 in order to land humans and the equipment on Mars, with the capability to launch off Mars and come back to Earth in 2031. In round numbers, if we say one Mars landing ship will carry people, and four ships will carry equipment and fuel, and each of those five ships will require 10 refueling flights, we're talking 55 rocket launches in the second half of 2028, all for this one mission. That's simply a lot of coordination for launches, rendezvous and docking maneuvers, even if 50 of those are short 24 hour long refueling flights. This is certainly not impossible, in fact it's probably even easy. However, rendezvous and docking is actually still not a trivial activity despite the fact we have been doing it for nearly 60 years. So 55 events mean that we need pretty high reliability of all the sensors and mechanisms to make this happen.

I do think that humans can in fact launch to Mars in 2028, all of these technology issues can be solved in the next four years, but in order to do that, we need to work on it as soon as possible. More likely is a 2030s human trip to Mars. 

A final note on the rover, when studying the Apollo missions, the first three landing missions, Apollo 11, 12, and 14 did not have a rover and only covered a few miles walking on foot. Apollo 15, 16, and 17 covered 10s of miles of ground and did excellent science while on the surface of the Moon. Going to Mars without a rover is foolish. And going with only an open buggy style rover is slightly better, but still short sighted. A human can only do a space walk for in the neighborhood of 8 hours, and then there are issues like eating, getting tired, having a bowel movement, and even sleeping. The oxygen and carbon dioxide part is the easy part to solve. If we assume that a Mars space suit is very flexible and an astronaut will be able to do 2 miles per hour, that's a maximum radius of 8 miles that a person could walk. That's wildly optimistic, on Apollo 14 the astronauts struggled to walk about one mile away from the LEM before struggling with navigation and not being able to see the LEM. Similarly on an open buggy with a top speed of 20 kilometers per hour (12 miles per hour) would be able to cover more ground, but that has to be traded with the possibility that the buggy breaks down and the astronauts have to walk back to the habitat. On Apollo 17 the furthest that the two astronauts drove the lunar buggy was about 5 miles away from the LEM, so that worst case they could hike back to the lander. Two buggies would provide redundancy, but also add quite a bit of mass. Also, for people that have not done much off roading, there isn't much point in going a lot faster than those speeds, you're going to be bouncing all over the place even at those slow speeds. The point of all this is to say, without a pressurized rover, you would essentially be stuck on a deserted island without the capability to explore much more than maybe 10 miles in any direction, which over 15 months is somewhat limiting.