Voyager 2 is a space probe weighing 722 kg that was launched in August 1977 from the Space Launch Complex-41 at Cape Canaveral. The purpose of this mission was to study the outer solar system and the interstellar space.
Though it was launched two weeks earlier than its twin spacecraft Voyager 1, the latter overtook Voyager 2. This was because Voyager 2 was launched with a trajectory that was longer and more circular in nature. It has been operational since then for about 35 years now. Along with Voyager 1 and Pioneer 10 and 11, it is one of the most distant manmade objects in space. It is currently at a distance of 100.675 AU.
The spacecraft is on an extended mission with the task of studying the outer end of the solar system, the heliosphere, the Kuiper belt and the interstellar space. Voyager 2 is credited as the only spacecraft that has visited two of the giant planets Neptune and Uranus.
The whole program was started off to exploit a planetary alignment and make a journey to the outer planets in the solar system: Jupiter, Neptune, Saturn, and Uranus. The spacecraft was designed to make use of the gravity assist technique in its journey so that it would use only low amounts of the propellant and take shorter transit time between planets. However, due to budget constraints, this trip was downscaled to make a flyby of Jupiter and Saturn.
Though initially based on the Mariner missions, the name was changed to Voyager because of the vast differences from the former.
The intended mission of Voyager 2 ended on 31 December 1989 after a flyby of Jupiter in 1979, an encounter with Saturn in 1980, and visits to the Uranian and Neptunian systems in 1986 and 1989, respectively. After a flyby of Titan, Saturn’s moon, by Voyager 1, Voyager 2′s mission was extended to Uranus and Neptune.
The Voyager 2 spacecraft, designed by the Jet Propulsion Laboratory, uses 16 thrusters with hydrazine as propellant, gyroscopes and three-axis stabilization instruments, all of which help to direct the high-gain antenna towards the earth. The hydrazine tanks are spherically shaped. Most instruments have identical substitutes and 8 backup thrusters are also provided. The instrumentation forms part of a subsystem for Altitude and Articulation Control. The spacecraft contains 11 instruments in addition to all of these to study the objects in space and make celestial observations as the probe passed by them.
The high-gain antenna is part of the communications system of Voyager 2. The antenna is large (3.7 m parabolic) and receives and sends data via the Deep Space Network. Communications take place in the S-band (for uplink communications, wavelength of 13 cm) and X-band (downlink communications, wavelength of 3.6 cm) frequencies. The data transfer rate was as high as 115.2 kbps from Jupiter.
Following the inverse square law in radio communications, the digital data rates have been consistently decreasing at increased distances. At times when the space probe is unable to communicate through the antenna, Voyager 2 has a Digital Tape Recorder that records up to 62,500 kb which is transmitted at a later point in time on resumption of communication capability.
The spacecraft is powered with the help of three radioisotope thermoelectric generators. The three generators are mounted end to end. Each generator has 24 plutonium-238 spheres and generated 157 watts of power at launch. The three generators provided a cumulative power output of 470 watts at launch for the spacecraft. The half life of plutonium-238 is 87.74 years. It is therefore estimated that after 34 years after its launch the generator would provide close to 76 percent of the initial power. However, in 2011 the power generated in Voyager 2 had dropped down to a value of 269.2 watts. As the power decreases, some load in the spacecraft has to be turned off, thus limiting its capabilities.
The thermoelectric generators are expected to power the spacecraft and it has sufficient propellant to continue its operations up to the year 2025. After this, there may not be sufficient electrical power available to keep the scientific instruments active, when its operations will stop and there will be no transmission of scientific data.
Of the 722 kg, the weight of the instruments is approximately 105 kg. Some of the scientific instruments include ultraviolet spectrometer (active), infrared interferometer spectrometer (disabled), triaxial fluxgate magnetometer (active), plasma spectrometer (defective) and plasma wave system (partially disabled), among others. The onboard digital computers control the cameras.
Voyager 2, like Voyager 1, carries with it a Golden Record that contains recorded sounds and pictures (117 images and greeting sounds in 54 languages) of the earth. It has directions for playing the record and contains details of the location of our planet. It is intended to provide information to any extra form of terrestrial intelligence that may intercept this spacecraft.
Voyager 2 is the only manmade spacecraft to study four of the giant planets of the solar system. Voyager 2 discovered the presence of a few rings around Jupiter. It also discovered some volcanic activity on the moon called Io. Voyager was instrumental in finding out that the Great Red Spot was actually a storm which moved in the anti-clockwise direction. Analysis of the banded clouds also revealed numerous other storms. However, the high resolution pictures of Europa taken by Voyager 2 did not help much to dispel the doubts about the topographic structure of the moon that arose with the low resolution pictures taken by Voyager 1.
Voyager 2 flew closest to Saturn in August 1981 and gathered information about the density and atmospheric pressure details of the planet from its upper atmosphere.
Though the cameras went into temporary dysfunction after the flyby of Saturn, it was serviced back into action by the engineers heading the mission and Voyager 2 was given the green signal to explore Uranus and Neptune. Voyager 2 made observations of the twisted magnetic field of the planet whose axis is parallel to the elliptical plane.
The rings around Uranus were starkly different from that of Jupiter and Saturn. Voyager 2 made a study of Neptune and its moon Triton during the flyby in 1989. Voyager 2 also discovered the presence of the Great Dark Spot neighboring Neptune. However, this spot has since disappeared.
Currently, Voyager 2 is in the outermost reaches of the solar system where the sun’s magnetic field and the solar wind dominate the space. In the year of 2007, Voyager 2 crossed a point where the speed of solar wind is less than that of sound. This was at a distance of about 13 billion km (84 AU) from the sun. Presently, Voyager 2 is in the region of the heliosheath, a region that is about 3.7 billion kilometers thick.