The brainchild of astronomer Percival Lowell, and backed financially by William Randolph Hearst, the space cannon is both the cause of the incidents in Martian Dreams, as well as what made the 1895 rescue effort -- and a return trip -- possible.
Lowell had discovered a new substance called phlogistonite, the explosive properties of which were so powerful, that it could launch a train-sized "bullet" into space. Hearst financed the construction of the first space cannon for Lowell, in exchange for exclusive rights to the story. Unfortunately, the cannon was sabotaged and launched a day early (by Rasputin, though this was still unknown at the time) in 1893, with many prominent world figures on board (as seen in the Martian Dreams introduction cinematic), stranding them on Mars.
When signals were spotted originating from the red planet in Morse code, a rescue expedition was mounted. Completed in 1895, the second space bullet launched successfully, with the time-traveling Avatar and Dr. Spector on board, as well as a number of other well-known names of the Victorian era.
Soon after landing on Mars, the rescue expedition discovered that construction of another space cannon made for a return trip to Earth was already underway, under the supervision of Andrew Carnegie. Following a complex series of events involving gathering various resources to get the second cannon fully-operational, it was eventually finished and sent the stranded humans -- as well as some hitchiking Martians -- back to Earth.
The members of the stranded expedition were no less fascinating than their would-be rescuers. Those aboard Lowell's craft when it blasted off from the World's Columbian Exposition represented a veritable Who's Who of the Victorian Era.
The year was 1893 and all the world had turned its attention to Chicago, the site of the Exposition. There, ornate edifices housed displays of mankind's greatest achievements. A Ferris wheel 264 feet in diameter and capable of carrying over 2,000 people at a time towered over the fair. The world's nations sent emissaries to show off the lifestyles and accomplishments of their peoples.
Perhaps the most remarkable attraction stood near the Manufacturers and Liberal Arts Building. there, spectators marvelled at Percical Lowell's space cannon, designed to take mankind to Mars. A crew was gathered, a launch date set. A day before the scheduled launch, Lowell gave a tour of his cannon to a select group of fair attendees and workers. Suddenly the cannon discharged. Some at the scene claimed the explosion had destroyed the bullet within, killing all aboard. Some thought the whole event was a colossal hoax. In fact, the bullet rocketed toward Mars with many of the Victorian era's leading lights aboard.
I can't imagine the world without the crucial contributions these people made to the advancement of science, the arts, and government. You will, no doubt, understand what I mean when you read the list of the missing.
- Space Gun on Wikipedia.