The author discusses the difficulties of travel to distant stars, given the limitation of the speed of light.
| Date | Publication | Publisher | Type | Page | |
|---|---|---|---|---|---|
| November 1964 |
|
Playboy, November 1964 | HMH Publishing | MAGAZINE | 115 |
| September 1965 |
|
Voices from the Sky: Previews of the Coming Space Age | Harper & Row | Nonfiction | 38 |
| 1966 |
|
Voices from the Sky: Previews of the Coming Space Age | Gollancz | Nonfiction | 38 |
| October 1967 |
|
Voices from the Sky: Previews of the Coming Space Age | Pyramid Books | Nonfiction | 40 |
| 1969 |
|
Voices from the Sky | Mayflower | Nonfiction | 39 |
| 1969 |
|
Voices from the Sky | Mayflower | Nonfiction | 39 |
| January 1971 |
|
Voices from the Sky: Previews of the Coming Space Age | Pyramid Books | Nonfiction | 40 |
| February 1980 |
|
Voices from the Sky | Pocket Books | Nonfiction | 45 |