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Thank you for the welcome and the kind words :)

Pinning down 1 breakthrough will be tough as space is so interdisciplinary where many different sciences and technologies needs to come together and work at the same time. I recently got to speak with Pete Worden (previous Director of Nasa Ames Research Center) about this very topic. He is the Chair of the Breakthrough Starshot Initiative (started by a 200 million dollar Yuri Milner investment, and supported by Mark Zuckerberg and Stephen Hawking). I suggest you look their work up.

I would personally say the work that SpaceX is doing in bringing down launch costs and enabling the reusability of rockets will have to be number 1. When things get cheaper, more minds are allowed to try out their experiments in space. Thus, new people with new ideas are also able to contribute to innovation.

Secondly, this Friday the UK announced its intention to open the world's first commercial Astronaut Training Centre in 2018. This is also a big deal. It is easy to see how SpaceX can succeed as a commercial company taking on contracts from satellite companies and the military. It is less obvious how the training of astronauts can be made profitable. Yet they seem to have cracked the code, having built a structure that can also profit from oil and gas companies, blue farming companies, and subsea robotics companies, being able to pay to use the same infrastructure.

The number 1 factor for me then is the ability for space enabling infrastructure to also profit from delivering earth-based value. It will still take some years before companies can sustainably profit from space resources and pace tourism. So it will be important to find the innovative ways to scale the business and technology also in the short run.

I hope that answered your question :)