I don't understand the difference between these two examples, because in both cases, the goal is scientifically impossible!
What does "if the means used to achieve the purpose are impossible" mean?
4.5 Impossibility and Common Law Offences
According to the House of Lords in DPP v Nock126 impossibility is a defence to a charge of common law conspiracy. However, this is only where the conspiracy could never be achieved: impossibility is not a defence if the means used to achieve the purpose are impossible [emphasis mine]. So if A and B conspire to project indecent images into the night-time sky, the fact that such a display was not scientifically possible would not prevent the possibility of a conviction for conspiracy to corrupt public morals. An example of where impossibility would be a defence to a common law conspiracy is where A and B intend to defraud V, while V had already died. It should be stressed that impossibility is assessed at the time of the agreement. The fact that the plan subsequently becomes impossible (e.g. the company it was planned to defraud has ceased to exist) does not prevent the original conspiracy being committed.
126 AC 979 (HL).
Herring, Criminal Law: Text, Cases, and Materials (2020 9 ed). p 824.