Voyager Space Probes
by Peter Ackers
On December 17th, 1903, at Kitty Hawk, Wilbur Wright flew in the first powered aeroplane. The 337 Kg machine flew for 12 seconds. Less than 74 years later, NASA launched the first of two space probes, with a mass of 815 Kg each, that would eventually leave the confines of our Solar System and travel into interstellar space.
In less than 74 years man had harnessed the power of flight to such an extent that putting satellites into orbit is an everyday occurrence - hardly newsworthy in some cases. These probes however where special. Named Voyager 1 and 2, they would take advantage of a rare planetary alignment. This configuration of the planets meant that the probes could gain energy by using the planets gravitation to cause a "sling shot" effect. Therefore smaller on board propulsion could be utilised, minimising the size of the launch vehicle - a Titan (IIIE)Centaur rocket.
Voyager 2 was the first to be launched on August 20th 1977. Voyager 1 soon followed on September 5th 1977, however Voyager 1 had a shorter, faster path than it's earlier twin. They were initially designed to last 5 years, but due to the technological improvements on Earth: the probes were re-programmed during their mission. This re-programming vastly increased the working life of the probes.
Encounters.
Voyager 1 made the first encounter, this was with the planet Jupiter on March 5th 1979, Voyager 2 soon followed encountering the gas giant on July 9th 1979. Saturn came next, November 12th 1980 for Voyager 1 and August 25th 1981 for Voyager 2. After this most spectacular encounter with Saturn, Voyager 1 headed northwards out of the ecliptic plane towards the vast voids of space. Voyager 2 continued on through the Solar System, on January 24th 1986 the probe made it's encounter with the planet Uranus. Finally, on August 25th, 1989, Voyager 2 made it's final encounter of it's most unique "Grand Tour". This encounter was with the last of the gas giants, Neptune. With it's main mission successfully completed, Voyager 2 headed southwards on the long voyage into interstellar space.
Interstellar Postcard.
On board each Voyager probe was placed a 12 inch gold-plated record. This contained over 100 images and a variety of sounds from Earth. Also, samples of music from a variety of composers ranging from classical to jazz. Included on this postcard from Planet Earth was spoken greetings in 55 languages and printed messages from world leaders. Possibly, more significantly than the greetings, where binary messages informing any reader of the postcard where it originated and the biological make-up (DNA) of the sender. Will any intelligent life form ever read our postcard ? I will leave that question for your imagination to answer.
What of the Probes now?
Just over 20 years after their launch, both the probes are still going strong - receiving power from the onboard radioisotope thermoelectric generators. Both probes should have enough onboard fuel reserves to supply power and attitude control until 2020. In the intervening years, both probes will continue to work hard, collecting data and studying the environment within the outer limits of the Solar System. Recent data received from the two probes suggests the instruments are detecting signals which, if the interpretation is correct, are coming from the Heliopause - the outermost edge of our Sun's magnetic field. This means that the Voyager probes will be the first ever man made objects to leave the confines of our Solar System - the first objects from Earth, to cross the Heliopause.
"Today, even though Voyager 1 is now more than twice as far from the Sun as Neptune, their journey is only half over, and more unique opportunities for discovery await the spacecraft as they head towards interstellar space" said Dr Edward Stone, Voyager project scientist and the director of NASA's Jet Propulsion Laboratory, Pasadena.
Our Sun emits a Solar wind - a stream of electrically charged particles (these particles are what cause the Aurora Borealis phenomena on Earth). As the Solar Wind expands supersonically, it creates a magnetised bubble centred on the Sun - called the Heliosphere. This interacts with the charged particles and magnetic field in the interstellar gas causing the Solar Wind to slow from supersonic to subsonic velocity - called the termination shock. Reaching the termination shock and the Heliopause has long been a goal for it will give direct data with regards to structure and exact positions.
"Based on current data from the Voyager cosmic ray subsystem, we are predicting the termination shock to be in the range of 62 to 90 astronomical units (AU) from the Sun. Most `consensus` estimates are currently converging on about 85 AU. Voyager 1 is currently at about 67 AU and moving outwards at 3.5 AU per year, so I would expect crossing the termination shock sometime before the end of 2003," said Dr. Alan Cummings, a co-investigator on the cosmic ray subsystem at the California Institute of Technology.
There are four other scientific instruments still functioning on the space probes. The plasma subsystem which measures the protons in the Solar Wind, this has detected a slow, year long increase in the velocity of the Solar Wind which peaked in 1996. This increase was followed by a slow decrease in the velocity of the Solar Wind. It is thought that the peak in late 1996 coincided with the Solar minimum. The magnetometer instrument on board the space probes measures the magnetic fields that are carried out into interplanetary space. This instrument is presently collecting data which shows that the interplanetary magnetic fields are the weakest yet detected. The two other instruments, still collecting data, are the planetary radio astronomy subsystem and the ultraviolet spectrometer subsystem. All these instruments will continue collecting data, transmitting it to Earth to be collected via the 34 meter Deep Space Network antennas located in California, Australia and Spain. The probes are so distant, Voyager 1- 10.1 billion Km from Earth: Voyager 2 - 7.9 billion Km from Earth, that the signals from the probes take a full 9 hours to reach Earth.
Once the probes have left the confines of the Solar System, they will continue on their journey into the depths of space until all the onboard power has been spent, contact will be lost with the probes. They will continue on through space, a legacy left by the Human race from a lonely little blue and green planet orbiting an average star within an average Galaxy - one of billions. Many millennia from now, the probes may be intercepted by an advanced civilisation. If they are able to decode our postcards attached to the probes they may be impressed with our achievements in space reconnaissance - or maybe they will laugh at our little probes. Either way, they will know that we existed and know we were prepared to parley.
Thanks must go to the following for their contributions:
Tim Kallman and Maggie Mazzetti: NASA High Energy Astrophysics Dept
Mary Hardin: Jet Propulsion Laboratory, Pasadena, CA (JPL, a division of the California Institute of Technology, manages the Voyager Interstellar Mission for NASA's Office of Space Science, Washington, DC.
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