The first reports of luminous fireballs and mysterious lights in the sky started to come in during February 1946. As the months went by the sightings became both more frequent and more descriptive.
Over the course of about a year over 2000 objects would be reported in Denmark, Sweden, Norway and Finland. Most, like the picture below, would turn out to be mis-identifications of meteors and the like, but some remain unexplained.
Some of the "rockets" were seen making "maneuvers" in the skying that would be impossible for a meteor. Others were described as slow moving cigar shaped objects which is a very common report in modern UFO reports and, of course, from the 19th century mystery airship flaps. "Crashes" were observed mainly into lakes, here's some examples taken from here:
1946 July 9. Lake Barken, Sweden
Object w/ blue & green flashing lights crashed into lake.
1946 July 10. Bjorkon, Sweden
Projectile trailing luminous smoke slammed into beach. Slaglike material. Cylinder 20-30m in dia. Military investigated.
1946 19 July, noon. Lake Kolmjarv, Sweden
Ghost rocket into lake.
1946 august 12. SW Sweden
Cigar shaped object, 5 ft. long trailing smoke landed on small uninhabited island.
1946 august 16 Malmo Sweden
Ghost rocket exploded breaking windows & dropping fragments.
1946 mid-October Southern Sweden
Small dart with wings & ball shaped tip exploded as it crashed into water.
1946 July 18. Lake Mjosa, Norway
2 - 8' long missiles with wings crashed into lake with notable turbulence.
These reports were taken very seriously by the Swedish military, especially given the emerging political order of the soon to start Cold War, and an investigation launched under the leadership of Swedish Air Force officer Karl-Gösta Bartoll.
Searches were conducted for the remains of objects at crash sites and whilst some things were found, all were later put down to meteor debris. Gradually the sightings dropped off in Scandinavia and the investigation found nothing concrete although Bartoll always insisted what people were seeing was something "real".
Given the sensitivity of the events, the USA got involved and a rather interesting document exists from that time:
Further sightings were reported in other parts of Europe, mainly Greece but also in Belgium, Portugal and Italy.
So what were they? It's hardly likely that the Soviets had, within a year of the end of the Second World War, reestablished the V2 experiments and moved them on to the point of having the V2 rockets be maneuverable. It is also highly unlikely, given the situation at the time, that they'd risk an international incident by testing these over the northern European skies. I find it unlikely any other player on the international scene would get involved. Whilst the idea that these were alien spacecraft of some form is widespread, again I just don't see it. Why would they do something so distinctive from other UFO reports for just that year in just that place? Makes no sense.
Personally, based on the lack of physical evidence or witness testimony to the contrary, I believe these to be meteors and other natural phenomena reported by people already paranoid because of the world events unfolding before them, and no doubt some were embellished a little. The evidence for anything else is just so weak and scarce I can't see any other sensible explanation.
Sadly I think the ghost rockets were not as exciting as the name would suggest.
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