I'm not so sure I'd have the LXG investigating the Tunguska event after the fact, as structuring my adventure so that they are, perhaps, the cause-- or at least there to witness it.
I may not have made myself clear in my earlier post...
The initial investigation is separate from and leads to Tunguska.
One idea is that the cause of Tunguska is taken from the station to the detonation site. The party follows the clues there, perhaps engages some mooks in a chase scene, and winds up causing or witnessing the event.
There is no way in heck I'm going to include something that dramatic and
not have the party involved in it.
During the research scene the party learns that alien technology was taken from the station and who
appears to be responsible for the theft, but they don't know what was taken, or if they do, not how devastating it could be.
We next have a chase, leading to Tunguska. The first major combat of the adventure occurs, the climax of which is the explosion. It's a real wow moment, when they realize exactly what is at stake if they fail. If the party causes the explosion, so much the better, we just need to have some way of keeping them alive!
Then the party realizes the second canister is heading toward London. Now the chase is on to stop the antagonist before the city is completely destroyed.
Now, if you'll pardon my ramblings...
It's hard for me to picture what kind of action can take place at the research station until we decide who our bad guy is. But it is easy for me to imagine that the bad guy would leave behind someone to watch the station, to eliminate any teams sent by the English to investigate.
Rasputin is an interesting figure of the region during that time. He arrived in St. Petersburg from Siberia in 1911, where he quickly gained prominence with the royal family. He was born in Tobolsk, which is in Siberia.
We could easily imagine agents of Rasputin hitting the research station, retrieving the canister (if that's what we go with) and returning it to Siberia. If the station is not in Siberia though, how did he find out about it?
If the ultimate goal is the destruction of London, it could be a competing nation has hired the bad guy to topple the English government as a preemptive strike. Germany would be a good candidate. Or, it could be that a force inside England wants the government completely eliminated to set up it's own regime. Maybe industrialists run amok?
Another problem is the Tunguska region itself. Tunguska is in the Krasnoyarsk region of Siberia. Literally the *middle* of nowhere. I originally assumed we could have Nemo transport the party there, but I don't see an inlet that would get us even close to the area. I know that The Trans-Siberian RR traverses the region, we could use that. Of course, the Event is in June, so maybe the seas aren't completely frozen.
Looking at a map, though, there are some major rivers in the area. Maybe Nemo could get them close, and then they could travel on foot or sled to the location. Or we could go with a more extraordinary means of travel.
Tunguska is also in the middle of the taiga, a coniferous forest so huge you could hide the U.S. there and
lose it.
Must stop now, my brain is starting to hurt....