40 years of Voyaging - Part 3

This is my last look at the Voyager Program, probably the most ambitious of its kind. Other probes have been sent to explore the solar system, but none has had the vast scope of the Voyagers in its wide-ranging travels.

The Voyager Program was similar to the Planetary Grand Tour planned during the late 1960s and early 70s. The Grand Tour would take advantage of an alignment of the outer planets discovered by Gary Flandro, an aerospace engineer at the Jet Propulsion Laboratory. This alignment, which occurs once every 175 years, would occur in the late 1970s and make it possible to use gravitational assists to explore Jupiter, Saturn, Uranus, Neptune, and Pluto. Limited funding ended the Grand Tour program, but elements were incorporated into the Voyager Program, which fulfilled many of the flyby objectives of the Grand Tour except a visit to Pluto.

Pale Blue Dot is a photograph of planet Earth taken on February 14, 1990, by the Voyager 1 space probe from a record distance of about 6 billion kilometers

Carl Sagan insisted that Voyager 1 turn its camera toward Earth registering the smallness of our world against the vastness of space, the "pale blue dot" picture. Sagan's eloquent lines, as he reflected upon our planet -"everyone you love, everyone you know, everyone you ever heard of, every human being who ever was, lived out their lives ... on a mote of dust suspended in a sunbeam."

As for the future, it is expected that in the year 40,272 AD, Voyager 1 will come within 1.7 light years of an obscure star in the constellation Ursa Minor (the Little Bear or Little Dipper). 

And in about 40,000 years, Voyager 2 will come within about 1.7 light years of a star called Ross 248, a small star in the constellation of Andromeda.

And finally, Ed Stone, Voyager project scientist, California Institute of Technology, holds a model of NASA’s Voyager spacecraft during a news conference to discuss the Voyager 1 spacecraft officially venturing into interstellar space. Now 81, in 1972,  he became project scientist for the planet-hopping Voyager mission.

References
https://en.wikipedia.org/wiki/Voyager_2
http://www.planetary.org/explore/space-topics/earth/pale-blue-dot.html
http://edition.cnn.com/2017/08/16/us/nasa-voyager-40-year-anniversary/index.html
https://en.wikipedia.org/wiki/Ross_248
https://www.rocketstem.org/2013/10/03/voyager-1-interstellar-adventure-begins/

40 years of Voyaging - Part 2

The Voyagers both contained a record of sounds and voices from Earth. No other probe sent out did. This was because it would eventually leave the solar system, and travel onwards, barring accidents, until the end of time. Here is the story of that record.

In the upper left-hand corner is an easily recognized drawing of the phonograph record and the stylus carried with it. The stylus is in the correct position to play the record from the beginning. Written around it in binary arithmetic is the correct time of one rotation of the record, 3.6 seconds, expressed in time units of 0,70 billionths of a second, the time period associated with a fundamental transition of the hydrogen atom. The drawing indicates that the record should be played from the outside in. Below this drawing is a side view of the record and stylus, with a binary number giving the time to play one side of the record - about an hour.

Electroplated onto the record's cover is an ultra-pure source of uranium-238 with a radioactivity of about 0.00026 microcuries. The steady decay of the uranium source into its daughter isotopes makes it a kind of radioactive clock. Half of the uranium-238 will decay in 4.51 billion years. Thus, by examining this two-centimeter diameter area on the record plate and measuring the amount of daughter elements to the remaining uranium-238, an extraterrestrial recipient of the Voyager spacecraft could calculate the time elapsed since a spot of uranium was placed aboard the spacecraft. This should be a check on the epoch of launch, which is also described by the pulsar map on the record cover.

Each record is encased in a protective aluminum jacket, together with a cartridge and a needle. Instructions, in symbolic language, explain the origin of the spacecraft and indicate how the record is to be played. The 115 images are encoded in analog form.

The remainder of the record is in audio, designed to be played at 16-2/3 revolutions per minute. It contains the spoken greetings, beginning with Akkadian, which was spoken in Sumer about six thousand years ago, and ending with Wu, a modern Chinese dialect. Following the section on the sounds of Earth, there is an eclectic 90-minute selection of music, including both Eastern and Western classics and a variety of ethnic music.

As Carl Sagan has noted, "The spacecraft will be encountered and the record played only if there are advanced spacefaring civilizations in interstellar space. But the launching of this bottle into the cosmic ocean says something very hopeful about life on this planet.“

The records also had the inscription "To the makers of music – all worlds, all times" hand-etched on its surface.

Many people were instrumental in the design, development and manufacturing of the golden record. Blank records were provided by the Pyral S.A. of Creteil, France. CBS Records contracted the JVC Cutting Center in Boulder, Colorado to cut the lacquer masters which were then sent to the James G. Lee Record Processing center in Gardena, California to cut and gold plate eight Voyager records. Gold plating took place on August 23, 1977; afterward, the records were mounted in aluminum containers and delivered to JPL.

References:
https://voyager.jpl.nasa.gov/golden-record/golden-record-cover/
https://en.wikipedia.org/wiki/Voyager_Golden_Record
https://voyager.jpl.nasa.gov/golden-record/

40 years of Voyaging - Part 1

I've always been interested in astronomy from an early age, and remember the glossy Sunday supplements with the first fantastic pictures of the gas giants in the solar system, transmitted back from the Voyager space craft. And they built these craft solidly: they last.

Humanity’s farthest and longest-lived spacecraft, Voyager 1 and 2, achieved 40 years of operation and exploration this August and September. Despite their vast distance, they continue to communicate with NASA daily, still probing the final frontier.

Voyager 2 launched on Aug. 20, 1977, about two weeks before the Sept. 5 launch of Voyager 1.

To get a scale of the technology, 1977 saw the Commodore PET (Personal Electronic Transactor) – a new line of home/personal computers produced in 1977 by Commodore International.

It also saw the first Apple II Home Computers. 

The Atari 2600 games console is also released. It needs connecting to a TV set.

Voyager 2, which was actually launched before Voyager 1 (I know, this gets a bit confusing) is currently at a distance of 10.713 billion miles from the Sun. It’s traveling at an estimated speed of 34,390 miles per hour. It hasn’t yet reached interstellar space.

Voyager 1, on the other hand, has. That’s due to the fact that Voyager 1 is traveling at a slightly greater speed of roughly 38,026 miles per hour. It is currently located some 12.974 billion miles from the Sun, and has successfully broken free from our star’s “bubble” called the heliosphere. Having passed that, Voyager 1 is now in interstellar space, where its fate will be determined solely by whatever it manages to run into.

The heliosphere is the bubble-like region of space dominated by the Sun, which extends far beyond the orbit of Pluto. Plasma "blown" out from the Sun, known as the solar wind, creates and maintains this bubble against the outside pressure of the interstellar medium, the hydrogen and helium gas that permeates the Milky Way Galaxy. The solar wind flows outward from the Sun until encountering the termination shock, where motion slows abruptly.

The Voyager spacecraft have explored the outer reaches of the heliosphere, passing through the shock and entering the heliosheath, a transitional region which is in turn bounded by the outermost edge of the heliosphere, called the heliopause. 

The termination shock is the point in the heliosphere where the solar wind slows down to subsonic speed (relative to the Sun) because of interactions with the local interstellar medium. This causes compression, heating, and a change in the magnetic field.

The shock arises because solar wind particles are emitted from the Sun at about 400 km/s, while the speed of sound (in the interstellar medium) is about 100 km/s.

As one moves far enough away from the Sun, the pressure of the solar wind drops to where it can no longer maintain supersonic flow against the pressure of the interstellar medium, at which point the solar wind slows to below its speed of sound, causing a shock wave.

So how can you have a speed of sound in space?

And what was space physicist Don Gurnett talking about when he stated at a NASA press conference in Sept. 2013 that he had heard "the sounds of interstellar space?"

Sound travels in waves like light or heat does, but unlike them, sound travels by making molecules vibrate. So, in order for sound to travel, there has to be something with molecules for it to travel through.

But space is not empty. The solar wind is a stream of particles and plasma (gas), and the interstellar medium, is a mixture of electron–proton plasma and hydrogen atoms. So linear acoustic waves can propagating through it.

Shocks occur when matter moves into a medium at a velocity that exceeds the local sound speed - a condition that is easily met in many different astrophysical contexts, just like breaking the sound barrier in air.

Strictly speaking, the plasma wave instrument does not detect sound. Instead it senses waves of electrons in the ionized gas or "plasma" that Voyager travels through. No human ear could hear these plasma waves. Nevertheless, because they occur at audio frequencies, between a few hundred and a few thousand hertz, "we can play the data through a loudspeaker and listen," says Gurnett. "The pitch and frequency tell us about the density of gas surrounding the spacecraft

References
http://bgr.com/2017/08/29/voyager-mission-space-distance/
https://en.wikipedia.org/wiki/Heliosphere
https://www.jpl.nasa.gov/news/news.php?feature=6907
https://science.nasa.gov/science-news/science-at-nasa/2013/01nov_ismsounds