Artistic reproduction of a "solar sailing boat".
Paris Observatory / UFE
By the middle of the twenty first century, we should have a multi-color mapping of the exoplanets, including planets with a size comparable to the diameter of the Earth. Thus we will start to see details, at the scale of continents, on the surfaces of the exoplanets. As the base of the interferometers increases, these details will become thinner and thinner.
However this approach has some limitations : we can not increase ad infinitum the bases of the interferometers. Therefore, with this method, our knowledge about the surfaces of the exoplanets is limited.
That's why we should plan an in situ exploration of the exoplanets, similar to the exploration of the Solar System today. However this prospect also poses some limitations.
First let us make a remark on quantum teleportation. This hypothetical method of transportation makes it possible to establish some instantaneous statistical correlations, with distance. But for a teleportation between two points, A and B, two observers must already be at points A and B!
For a classic trip, there are two constraints to overcome :
If we want to go in a moderate time (at human scale) to a planet, possibly in the neighbourhood of the closest stars (4 light-years), let's say 40 years, we must travel with a velocity of about 10 percent of the speed of the light. This supposes considerable propulsion resources, probably nuclear, with serious security problems.
The hostility of the interstellar medium
The interstellar medium is not empty! It contains some matter with a mean density of about 1 hydrogen atom per cm3. This matter is essentially a mixture of gas and small "sand" grains (called "interstellar dust"). We do not know exactly the mean size of these grains. But a grain with a diameter of 0.1 mm, that hits a spaceship travelling at a velocity of about 10 percent of the speed of the light, has as much kinetic energy as
a car launched at 100 km/hour. For a 1mm grain, the equivalent speed of the car is 3600 km/hour. Therefore,
either we must use considerable resources to cope with such impacts, which increases the mass of the spaceship and makes the propulsion problem worse, or we must decrease the speed of the spaceship, which increases the length of the mission.