The problem with your reasoning is that you neglect the fact, as do almost all humans in technological societies, that easily 90% of the energy we consume is wasted. You are not to blame; when you have no firsthand experience of a thing, it is normal for one to be oblivious to its existence.
I have lived for many years within the energy budget provided by photovoltaics and the wind. It is possible to live well under such circumstances, and to be as technological as a city dweller, but first you have to eliminate the waste.
Take, for example, your houses and apartment buildings. They are so poorly constructed that the planet would be considerably better off, despite the waste, if we tore down every single last one of them and rebuilt them for longevity and energy efficiency.
A few buildings (maybe three in a city of 5 million people) are constructed well. They use (at the moment) ICF techniques and they are so well insulated that, even in the coldest days of January, the heating system can fail completely for over three days BEFORE ANYONE NOTICES that it is broken. They can use geothermal heating exclusively both winter and summer and provide complete comfort for 10% of the normal cost.
But you do not build your stuctures like this because energy efficiency is not a priority; money is the priority. The scumbag builders build shit buildings to maximize their profits; the scumbag realtors sell units to the ignorant masses for a fortune, and nothing changes.
What is wrong with humanity is that we are just too fucking stupid and far too much in love with ourselves to do the reasonable thing. We are crazy for money and it has become the arbiter of all success. Instead of killing off 90% of the species, we could begin by simply eliminating the 90% of our activity that is wilfully wasted and continue to live well, so long as the reduction in resource usage was not simply an excuse to increase our already ridiculous numbers.
So that is my alternative: stop being monsters. Otherwise I cast my vote for us all to die of a virus incubated in the backstreets of Shenzhen, and good riddance.
December 2013, North Atlantic Ocean between Florida, Bermuda, and the Eastern Caribbean: nearly devoid of sea birds and fish; plastic garbage common.
January 2014, Coastal Everglades of Florida: silent. No birdsong. None.
February 2014, Florida Bay and the Keys: enough lobster pots that you could walk from Cape Sable to Key West without getting your shoes wet. This is absolutely not a sustainable fishery--this is an unmitigated rape of the planet for the almighty dollar, even inside the supposedly protected waters of the marine sanctuary.
If you want to build a functioning starship along the lines of Icarus by the year 2100, there is a lot of basic science that has to be done. It is very difficult to predict the course of basic science, because you don't know what you don't know, and it is therefore hard to anticipate how far away the frontiers might be.
The main problem with Icarus is that it aims to create a ship that can do the trip on the scale of a human lifetime. With space travel, the amount of energy required, and therefore the difficulty of the project, is related almost exclusively to how fast you want to get there, the delta-v, if you will.
Therefore, it is unreasonable for Icarus to set a timetable.
If you really want to set a timetable, there is an easier way: stop worrying about how long it takes. Concentrate all of your effort into curing death, and then build a starship with a solar-sail, or some other sort of environmentally-friendly, but slow, technology.
The advantages to this approach are many: easier to get funding, direct economic payoff, no difficulty convincing people that your effort is worthwhile, and, oh yes, you get to cure death.
Then, while you are aboard your starship, you can spend the hundreds of years (say) that it takes you to get to Alpha Centauri further advancing science. If you are adequately equipped, as a nuclear-pulse propulsion vessel would likely be, you can invent better propulsion technology as you go along.
In a sense, the problem of human senescence and its attendant illnesses and the problem of interstellar travel are one and the same. If someone asked me to solve the latter as quickly as possible, I would begin by solving the former.
Q: Why did you choose fusion based propulsion for Project Icarus?
A: Well, we thought about it and we realized that the only appropriate power source for an imaginary starship was a likewise imaginary one. And while there are certainly many options in the field of imaginary power generation, we rejected some of the main contenders for a variety of reasons.
"Maxwell's Demons" would have alienated both the new-agey spiritual sci-fi crowd and also the fundamentalists because it sounds medieval and just evil at the same time.
"Dilithium crystals" was taken. The zero-point people are bunch of wack jobs and we certainly didn't want to be tarred with that brush. "Fairy wings" just sounds too wussy for our tastes.
So, you see, there aren't all that many choices of power left for an imaginary starship. Fusion sounds just about right, especially if you say is slowly, with gravitas, like "fuuuuuuusion".
Tangent: fusion will require carrying...how much hydrogen? I once calculated that trying to gather it along the way (interstellar hydrogen) is pointless, as a 1m^2 swath from here to Alpha Centauri would net 0.01g of the stuff.
ETA: Yup, I was considering the Ramjet design. Scale of the scoop overwhelmed itself trying to acquire enough to function. I was just startled at how little interstellar hydrogen there is; I knew it wouldn't be much, but 0.01g in ten million cubic kilometers was even less than I'd assumed.
But this isn't meant to dispute your calculations. The linked article comes to the same conclusion: it's not feasible, too little interstellar hydrogen.
Most notional implementations of the Bussard Ramjet depend upon some sort of heretofore undiscovered force field rather than a physical structure to collect hydrogen, because size matters.
Naturally, a very large force field ramjet will deplete interstellar hydrogen along the main routes between stars, leading to a situation known as "peak hydrogen", and possibly prompting the invasion of developing hydrogen-rich planets on some sort of flimsy pretext.
Most scientists working on imaginary spacecraft projects therefore eschew the Bussard, correctly anticipating the moment when, shortly after the leader gives a speech on the deck of a starship in front of a huge "Hydrogen Accomplished" banner, the price of hydrogen rises to over $5 a gallon and starships are all left to rust in the front yard.
"Major Tracy Bunko — then posted at the Pentagon's Air Force press desk"
Now who can we get to really blow some smoke on these classified projects? Well, there's Major Bunko, sir. They say he can really sling some major... uh... bunko.
I have lived for many years within the energy budget provided by photovoltaics and the wind. It is possible to live well under such circumstances, and to be as technological as a city dweller, but first you have to eliminate the waste.
Take, for example, your houses and apartment buildings. They are so poorly constructed that the planet would be considerably better off, despite the waste, if we tore down every single last one of them and rebuilt them for longevity and energy efficiency.
A few buildings (maybe three in a city of 5 million people) are constructed well. They use (at the moment) ICF techniques and they are so well insulated that, even in the coldest days of January, the heating system can fail completely for over three days BEFORE ANYONE NOTICES that it is broken. They can use geothermal heating exclusively both winter and summer and provide complete comfort for 10% of the normal cost.
But you do not build your stuctures like this because energy efficiency is not a priority; money is the priority. The scumbag builders build shit buildings to maximize their profits; the scumbag realtors sell units to the ignorant masses for a fortune, and nothing changes.
What is wrong with humanity is that we are just too fucking stupid and far too much in love with ourselves to do the reasonable thing. We are crazy for money and it has become the arbiter of all success. Instead of killing off 90% of the species, we could begin by simply eliminating the 90% of our activity that is wilfully wasted and continue to live well, so long as the reduction in resource usage was not simply an excuse to increase our already ridiculous numbers.
So that is my alternative: stop being monsters. Otherwise I cast my vote for us all to die of a virus incubated in the backstreets of Shenzhen, and good riddance.