Sarah Silverman was considered to join the regular cast for the fourth season, after executive producer Brannon Braga enjoyed her work in this two-part story. But he eventually chose Jeri Ryan to join the cast instead.
In-joke about the Star Trek Communicators, right after Harry Kim receives the "Greeting from Earth" message from Rain Robinson he proceeds to contact the away team on the surface. As the Captain's Communicator beeps all of the people walking past the away team immediately reach for their cellphones to answer them. This was probably a reference to the similarity of 1990's cellphone design and technology to the "futuristic" flip-top Communicators from the Star Trek: The Original Series (1966) Original Series.
47-reference: When Rain Robinson's computer crashes, it displays a Fatal System Error #0047. A further appearance of the number 47 is in 2047, the year that the Hermosa earthquake submerged Los Angeles.
In this episode, Voyager's crew discovers that they are in the past because they cannot pick up Starfleet signals, but are receiving radio transmissions. The same occurrence helps Captain Kirk and his crew determine that they are in the past in the episode TOS: "Tomorrow is Yesterday."
When Tuvok and Paris leave the observatory, walking towards the truck, Tuvok looks at Paris and says "question, what is groovy?" This is alluding to Star Trek IV The Voyage Home when after getting on a bus, and then being kicked off, Spock turns to Captain Kirk and says "question, what is exact change?"
Janeway says "For all I know, she could be my great, great, great, great grandmother." This echoes Dr. Crusher's line in TNG: "A Matter of Time": "Anyway, I could be your great, great, great, great grandmother." (Both lines say 'great' four times.)
When Paris and Tuvok enter Rain's office at the observatory Paris picks up and looks over a snow-globe. Inside you can see it contains a Gorn, an alien species shown in The Original Series and Enterprise.