@wnderer raises an important point - that eventually our earth will cease to exist and that humanity needs an exit plan if it is to survive. While the timeframe is much greater than the total time than humans have lived on earth to date, it is a topic that eventually will need to be addressed.
Within this century, I'd think that the advances in robotics and technology make remote exploration of other solar systems the only practical approach. Not only can they send back data at the speed of light but also the spacecraft is spared the need to carry life support and food and provide sufficient space for human beings. Exploration of the moon is viable because the trips are relatively short; expecting someone to remain healthy for a 20 year trip is not.
This is really something as Moon and Mars are the newest edition to the boring destinations to go to. This is when NASA is planning a manned Mars mission and China is on the same lines planning Moon and Mars voyage.
One more problem (aside from mass increas and enormous energy required) is, when you travel in near lightspeed, every free-floating space particles become high-energy beam against the space ship. It could be protected by thick foward-facing shiled, but of course it increases space ship mass and will require even more energy to accelarate to near lightspeed.
Then, when getting to your destination, we must slow down. We need same amount of energy to decelarate to "normal" speed. So the space ship must acceralate to near lightspeed from full-tank, use only (about) half of fuel/propelant on board.
Einstein's space is not friendry to space travelers.
My goal for the space program would be to double the number of people living in space every 50 years. 12 people by 2050, 24 by 2100, 1 trillion by the year 4000. Then start sending the generation ships to the stars. Humanities (perhaps non-human) descendents cover the entire galaxy in 65 million years. That's only 4% of the years of habitability left for the Earth. The same amount of time since the dinosaurs went extinct. It also calls into question the existence of extra-terrestrial life. If intelligent life has been in the galaxy for 100 million years, they should not be hard to find.
@Wnderer: I see your point. I don?t understand why NASA exec thinks Moon is a boring place to be. NASA should build an observatory there, maintaining the Hubble is costlier than maintaining an observatory on the Moon.
If NASA has such a big budget to fund concepts like solar sail and faster than light travel, then why not? As far as we know, the planet we live on will cease to support this life in 1500 years or so, if the current rate of deterioration continues. For this we must be developing time travelling portals and near light speed for continuation of our life on other planets. NASA is really embarking on a tight budgeted mission.
@Prabakhar, thanks for reminding me how old I am!! I remember, at 13, looking up at the moon in 1969 and wondering if I would ever be able to go there, and thinking it was probably quite likely. Alas, the human race seems more focussed on pulling each other apart than pulling together to accomplish something like that.
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