Hydrogen, Voyager and Pioneer

On the case of Voyager and Pioneer spacecrafts there an important subject about Hydrogen. As you may knew the mentiond Plaque or disc mounted to the exterior of the exploratory spacecraft and etched with information identifying our civilization to alien cultures.

Do you know that The drawing containing two circles in the lower right-hand corner of this is a drawing of the hydrogen atom in its two lowest states? Ther is a connecting line and digit 1 to indicate that the time interval associated with the transition from one state to the other is to be used as the fundamental time scale, both for the time given on the cover and in the decoded pictures.

The Voyager Golden Record
The Voyager Golden Record is a phonograph record included in the two Voyager spacecraft launched in 1977. It contains sounds and images selected to portray the diversity of life and culture on Earth. It is intended for any intelligent extraterrestrial life form, or far future humans, that may find it. The Voyager spacecraft will take about 40,000 years to reach the distance of the star nearest to our Sun called Alpha Centauri, though neither craft is travelling in the correct direction. 'Near' meaning in this case about 4.35 light-years' distance; hence, if other beings do not come in the direction of the spacecraft to meet them, it will take at least that long for the Golden Record to be found.
As the probes are extremely small compared to the vastness of interstellar space, it is extraordinarily unlikely that they will ever be intercepted. If they are ever found by an alien species, it will be far in the future, and thus the record is best seen as a time capsule or a symbolic statement rather than a serious attempt to communicate with aliens.


Pioneer Plaque
This is the diagram that was placed on the pioneer spacecraft in 1973 to communicate our existence to alien life forms
At the top left of the plate is a schematic representation of the hyperfine transition of hydrogen, which is thought to be the most abundant element in the universe. Below this symbol is a small vertical line to represent the binary digit 1. This spin-flip transition of a hydrogen atom from electron state spin up to electron state spin down can specify a unit of length (wavelength, 21 cm) as well as a unit of time (frequency, 1420 MHz). Both units are used as measurements in the other symbols. Note that since the plaque is 229 mm wide, the actual unit of length could have been depicted, although it wasn't.

This is a present from a small, distant world, a token of our sounds, our science, our images, our music, our thoughts and our feelings. We are attempting to survive our time so we may live into yours.
U.S. President Jimmy Carter