On 2020-06-23, Luigi Caselli wrote:

Something newer?
The document was written in 1999 and after 20 years we haven't had no
progress at all about propulsion systems.

There are newer documents on the same concept. Some improvements have
been made when it comes to materials (since the sail would need to be
very light, have good reflective properties, and be incredibly thin).
You can search some of them he https://ntrs.nasa.gov/
For example, this one from this year discusses options for interstellar
propulsion in general and mentions workshops on the topic:
https://ntrs.nasa.gov/archive/nasa/c...0200000759.pdf
This one is a more technical paper tackling part of the topic:
https://ntrs.nasa.gov/archive/nasa/c...0100036571.pdf

There is 'Breakthrough Starshot' which is a very limited
(proof-of-concept) private initiative for nanoprobes capable of reaching
Alpha Centauri using laser sails. I doubt they will go too far, but they
got some funding and media exposure (googling it brings up plenty of
results).

They are all _conceptual studies_, of course, because at present the
energy requirements are too burdensome for us (ie, even aside from
engineering challenges concerning nanomaterials or scaling up lasers,
building one or more space lasers with the required power, ability to
focus the beam at large distances, etc., would be impossibly expensive).
But for a civilization with a developed presence in its own system,
these requirements would be trivial.

I never implied that one of us would go on vacation to exoplanets any
time soon anyway: it was only meant to show that conceptually
interstellar travel should be feasible (within centuries, perhaps) and
isn't 'unscientific'. No violation of physics is required, and the
engineering issues are solvable given enough time. Travel between the
stars *can* happen, even if not for us right now.

We use the same rockets, only smaller, of mission Apollo.

I wouldn't be so pessimistic. The main reason why human spaceflight
programs have stagnated after Apollo has been that they were approached
in ways that were economically unsustainable (especially if you look at
some 1980s projects for Mars landings), along with some efficiency
issues within the aerospace industry that have kept launch costs high.
With high launch costs and expensive projects you're hardly going to get
anything beyond Earth orbit outside of one-off missions ("flags and
footprints").

Since it was mentioned, I think that SpaceX has made good progress
towards a sustainable space presence: both the Raptor engine and the
ability to return boosters to launch site are a step ahead compared to
what had been accomplished until now (and is influencing other
companies' choices too), and hopefully other stuff like in-orbit
refueling and the whole 'Starship' project will pave the way for more.