Interest Article: The Voyager Probes – Earth’s First Starships

The Voyager Probes – Earth’s First Starships

A Salute to the Achievements of NASA & JPL

Kelvin F.Long

Background to the Voyager Mission

The Voyager spacecraft were US National Aeronautics and Space Administration (NASA) missions in co-operation with the Jet Propulsion Laboratory (JPL). The Voyager mission was officially approved in May 1972. Both probes were built by JPL but paid for by NASA. Voyager 1 was launched on September 1977, from Cape Canaveral, Florida, aboard a Titan-Centaur rocket. Voyager 2 was launched on August 20th 1977, also from Cape Canaveral, Florida, aboard a Titan-Centaur rocket.  It is estimated that a total of $865 million has been spent on the Voyager mission using a total work force of 11,000 people. So far, a total of five trillion bits of information has been sent back to Earth from the Voyager probes, the equivalent of over 7,000 CDs.

The Voyager Mission

The primary mission for the Voyager probes was the exploration of the gas giants, Jupiter and Saturn. Voyager 2 also went on to explore Uranus and Neptune and it remains the only spacecraft to ever have visited those planets.  The primary mission was completed in 1989 when Voyager 2 passed the planet Neptune. At the completion of the primary mission Voyager 1 was at a distance of around 40 AU and Voyager 2 at a distance of 31 AU.

The Voyager Mission

The primary mission for the Voyager probes was the exploration of the gas giants, Jupiter and Saturn. Voyager 2 also went on to explore Uranus and Neptune and it remains the only spacecraft to ever have visited those planets.  The primary mission was completed in 1989 when Voyager 2 passed the planet Neptune. At the completion of the primary mission Voyager 1 was at a distance of around 40 AU and Voyager 2 at a distance of 31 AU.

The following is the Voyager event timeline so far:

  • 1977 Mariner/Jupiter/Saturn is renamed Voyager
  • August 20th 1977 Voyager 2 launched from Kennedy Space Flight Center
  • September 5th 1977 Voyager 1 launched from the Kennedy Spaceflight Center, returns first spacecraft photo of Earth and Moon.
  • March 5th 1979 Voyager 1 makes its closes approach to Jupiter
  • July 9th 1979 Voyager 2 makes its closest approach to Jupiter
  • November 12th 1980 Voyager 1 flies by Saturn.
  • August 25th 1981 Voyager 2 flies by Saturn
  • 1982 Deep Space Network upgrades two 26 m antennas to 34 m.
  • January 24th 1986 Voyager 2 has the first ever encounter with Uranus.
  • 1987 Voyager 2 observes Supernova 1987A
  • 1988 Voyager 2 returns first colour image of Neptune.
  • August 25th 1989 Voyager 2 is the first spacecraft to observe Neptune.
  • January 1st 1990 Begins Voyager Interstellar mission
  • February 14th 1990 Last Voyager images – portrait of the solar system
  • February 17th 1998 Voyager 1 passes Pioneer 10 to become the most distant human made object in space.
  • December 15th 2004 Voyager 1 crosses the Termination shock.
  • September 5th 2007 Voyager 2 crosses the Termination shock.
  • Voyager enters Interstellar space.

The mission was then extended to the ‘Voyager Interstellar Mission (VIM)’ which has the objective of exploring the outermost edge of the Sun’s domain and beyond. This mission helps to characterise the outer solar system environment and look for the heliopause boundary, where the Sun’s magnetic fields and outward flow of the solar wind, come to an end. The spacecraft take measurements of the interstellar field, particles and waves, which are not affected by the solar wind.

The Voyager Extended mission has three phases (1) The penetration of the Termination shock (2) The exploration of the heliosheath which may be 10s AU thick (3) The exploration of interstellar space. The termination shock is the point where the solar wind slows from supersonic to subsonic speeds and the spacecraft begin to detect large changes in plasma flow direction and magnetic field orientation.

Voyager 1 crossed the termination shock at 94 AU in December 2004 and Voyager 2 crossed it at 84 AU in August 2007. The spacecraft then went on to explore the heliosheath with passage through the heliopause, the outer extent of the Sun’s magnetic field and solar wind. It is generally believed to exist between 90 – 120 AU from the Sun. Reaching the interstellar space exploration phase is ultimately the goal of the Voyager missions.

As of September 2012, Voyager 1 was at a distance of 121.727 AU (18,210,114,132 km) from the Sun which is the equivalent of a 33 hours 44 minutes round trip light time from the Sun. Voyager 2 was at a distance of 98.988 AU (14,808,525,771 km) from the Sun which is the equivalent of a 27 hours 26 minute round trip light time from the Sun.

Gravity Assist

The key to the success of the Voyager mission was a rare geometric arrangement of the outer planets which allowed for a four planet tour using a minimum of propellant and trip time. The planetary arrangement of Jupiter, Saturn, Uranus and Neptune, only occurs around every 175 years. It allows for a spacecraft to fly by one planet to the next without the need for on board propulsion. Each flyby bends the spacecrafts flight path and increases its velocity enough to deliver it to the next destination. This ‘gravity assist’ technique was first demonstrated with the NASA Mariner 10 Venus/Mercury mission in 1073-1974. As a result of the flyby technique, the total flight time to Neptune was reduced from 30 to 12 years. More than 10,000 different trajectories were studies to choose the right path for the Voyager missions.

Cosmic Perspective

In the year 2017 the Voyager probes would have been in space for 40 years. The probes have enough electrical power and thruster fuel to operate to at least the year 2020 and may survive until around the year 2025 – nearly half a century. The reason the mission ends is due to the diminishing power and hydrazine levels which prevent further operation and for the Voyager probes to be able to get a target lock on Earth for signal transmission. This is a remarkable achievement for Earth’s first robotic ambassadors to go beyond the solar system.  They are moving out into parts of space where no person or technology from Earth has ever flown previously. Whilst we on Earth go about our daily businesses of living, the Voyager achievements go largely unnoticed by the general public. Yet, every day they break a new record, and push the human frontier that bit further outwards. Both probes are now in the solar Heliosheath, the outermost layer of the heliosphere where the solar wind is slowed down by the pressure of interstellar gas.

Currently Voyager 1 is at a distance of nearly 122 AU and is escaping the solar system at a speed of 3.6 AU/year, 35 degrees out of the ecliptic plane to the North. Voyager 2 is at a distance of 99 AU and is escaping from the solar system at a speed of 3.3 AU/year, 48 degrees out of the ecliptic plane to the South. Eventually, the probes will pass other stars. Voyager 1 will drift within 1.5 light years (9.3 trillion miles) of the star AC+79 3888 in the Camelopardalis constellation, in around 40,000 years time. For Voyager 2, the interstellar pass by is less attractive, it will drift within 4.3 light years (25 trillion miles) of the star Sirius, in around 296,000 years time. By that time, other spacecraft from Earth would have passed these probes, perhaps carrying humans aboard. What should we do when we pass them? Retrieve them for some historical museum. Perhaps its best they are left to wonder on their journeys and forever serve as a beacon of hope for human kind of what we as a species can accomplish, despite the backdrop of world troubles. It would become a tourist attraction, “fly within 10 km of the Voyager probes and watch them on their journey, cameras at the ready – but no touching”.

The voyage of the Voyager probes is likely to be one of the most remembered missions by the generations ahead.

The Voyager Spacecraft

The Voyager spacecraft have a mass of 773 kg each, with 105 kg being for scientific instruments. The Voyager spacecraft are three-axis stabilized using celestial or gyro reference attitude control to maintain pointing of the high-gain antennas towards the Earth. The Attitude and Articulation Control Sub-system (AACS) controls the spacecraft orientation and maintains the pointing of the high-gain antenna towards the Earth. It also controls the attitude manoeuvres. The spacecraft is controlled by an on board computer, the Command Computer Sub-system (CCS) which provides sequencing and control functions, which includes fault detection, correction routines, antenna pointing information, and spacecraft sequencing information. The spacecraft are powered by three Radioisotope Thermoelectric Generators (RTGs) with a power level of 315 W. These convert heat produced from the natural decay of plutonium to power the spacecraft instruments, computers, radio and all other systems. The Voyager probes are too far from the Sun to use solar panels. The fault protection program on board the Voyager spacecraft is the most sophisticated every designed for deep space probes. There are seven top level fault protection routines, each capable of covering a multitude of possible failures. The probe is capable of placing itself in a safe mode in a matter of seconds or minutes. When the Voyager spacecraft pass near the Jupiter system they were exposed to hard doses of radiation. This required very special shielding requirements to protect the sensitive electronic components. The on board instruments can be pointed to an accuracy of one-tenth of a degree. This is enabled using the scan platform which moves about two axes of rotation using a gear assembly. The electronics and heaters aboard each Voyager probe operate with a 400 W power level. Attitude control and trajectory correction is provided by a set of small thrusters, producing thrust levels of around 3 ounces, which is the equivalent of a 0-60 mph time of 12 hours for a standard large car. The spacecraft is held steady by a set of gyroscopes which can detect angular motions as little as one ten-thousands of a degree.

Voyager Communications

Data from the Voyager probes is transmitted through the Deep Space Network (DSN). This is a global tracking system operated by the Jet Propulsion Laboratory and there are antenna complexes in the California Mojave desert, near Madrid in Spain and in Tidbinbilla, near Canberra, Australia. This is at a transmission rate of 160 bps received with a 34 m dish, although high transmission updates are also sent regularly at 152 kpbs requiring a 70 m dish. Al date received and transmitted at the spacecraft is via the 3.7 m high-gain antenna. The signal from the Voyager spacecraft is very weak, so that the power hitting the antenna is only 10 to the power of -16 Watts, approximately 20 billion times less than a standard digital watch in use today. An interesting fact about the Voyager probes is that because the Earth moves around the Sun faster than the spacecraft are travelling away from the Earth, at certain times of the year the distance between the Earth and the spacecraft actually decreases.

The Instruments

There is a variety of science under study with the interstellar phase of the mission. This includes the strength and orientation of the Sun’s magnetic field, the composition, direction and energy spectra of the solar wind particles and interstellar cosmic rays, the strength of the radio emissions originating from the heliopause, the distribution of hydrogen within the outer heliosphere. There are five science teams participating in the Voyager Interstellar mission, examining the data from six operating instruments on each Voyager probe. The Voyager spacecraft 1 and 2 are identical and they include many instruments including television cameras. The magnetometers are mounted on a spindly fibreglass boom which Is 13 m long. There is also an Ultraviolet Spectrometer Subsystem (UVS) on board the probes, although no science teams are assigned to working on this instrument. The other key instruments include: Plasma Science (PLS); this measures the properties and radial evolution of the solar wind (ions 10 eV – 6 keV, electrons 4 eV – 6 keV). Low-Energy Charged Particles (LECP); this is to measure the energy spectrum of low-energy particles (electrons 10 – 10,000 keV, ion 10 – 150,000 keV/n). Cosmic Ray Sub-System (CRS); this measures the energy spectrum of high and low energy electrons (3 – 110 MeV) and cosmic ray nuclei (1 – 500 MeV/n). Magnetometer (MAG); this measures high (50,000 – 200,000 nT) and low (8 – 50,000 nT) magnetic field intensities. Plasma Wave Sub-system (PWS); This measures the electrical field components of the plasma waves (10 – 56 Hz range

The Golden Record

Both of the Voyager probes carry a special greeting card from any intelligent life that may find it. This is called the Gold Record and is a continuation from the earlier Pioneer 10 and 11 probes which carried small metal plaques identifying their time and place of origin for the benefit of any other space farers. This message is carried by a phonograph record, a 12 inch gold-plated copper disk containing sounds and 115 images selected to portray the diversity of life and culture on Earth.  The sounds include music selections from different cultures and eras, and spoken greetings from people of Earth from over 55 languages (from the 6,000 year old Akkadian to the modern Chinese dialect Wu). It includes sounds such as sea surf, wind, thunder, birds, whales and other animals. The contents were chosen for NASA by a committee which was chaired by the Cornell University astronomer Carl Sagan.  Other participants to the creation of this “Murmurs from Earth” also included people such as Frank Drake, Ann Druyan, Timothy Ferris, Jon Lombert, Linda Salzman. Of the Golden Record Sagan has said: “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”.

Comments are closed.