Voyager – The Farthest Human-made Object From Earth

The Voyager program is a U.S program that launched two unmanned space missions, scientific probes Voyager one and Voyager 2. They were launched in 1977 to take advantage of a favorable planetary alignment of the late 1970s.

By December 2010 Voyager 1 had reached a region of space where there was no net velocity of the solar wind. At this point, the wind from the Sun may be canceled out by the interstellar wind. It does not appear that the spacecraft has yet crossed the heliosheath into interstellar space.

On June 10, 2011, scientists studying the Voyager data noticed what may be giant magnetic bubbles located in the heliosphere, the region of our solar system that separates us from the violent solar winds of interstellar space. The bubbles, scientists theorise, form when the Sun’s magnetic field becomes warped at the edge of our solar system.

Spacecraft design

The Voyager spacecraft weighs 773 kilograms. Of this, 105 kilograms are scientific instruments. The identical Voyager spacecraft are three-axis stabilized systems that use celestial or gyro referenced attitude control to maintain pointing of the high-gain antennas toward Earth. The prime mission science payload consisted of 10 instruments (11 investigations including radio science).

The diagram at the right shows the 3.66 meter diameter high-gain antenna (HGA) attached to the hollow ten-sided polygonal electronics bus, with the spherical tank within containing hydrazine propulsion fuel.

The Voyager Golden Record is attached to one of the bus sides. The angled square panel to the right is the optical calibration target and excess heat radiator. The three radioisotope thermoelectric generators (RTGs) are mounted end-to-end on the lower boom.

File:Voyager spacecraft structure vector.svg

Instruments and sensors

Two 10-meter whip antennas, which study planetary radio astronomy and plasma waves, extend from the spacecraft’s body diagonally below the magnetometer boom. The 13 metre long Astromast tri-axial boom extends diagonally downwards left and holds the two low-field magnetometers (MAG); the high-field magnetometers remain close to the HGA.

The instrument boom extending upwards holds, from bottom to top: the cosmic ray subsystem (CRS) left, and Low-Energy Charged Particle (LECP) detector right; the Plasma Spectrometer (PLS) right; and the scan platform that rotates about a vertical axis.

The scan platform comprises: the Infrared Interferometer Spectrometer (IRIS) (largest camera at top right); the Ultraviolet Spectrometer (UVS) just above the UVS; the two Imaging Science Subsystem (ISS) vidicon cameras to the left of the UVS; and the Photopolarimeter System (PPS) under the ISS.

Only five investigation teams are still supported, though data is collected for two additional instruments. The Flight Data Subsystem (FDS) and a single eight-track digital tape recorder (DTR) provide the data handling functions.

The FDS configures each instrument and controls instrument operations. It also collects engineering and science data and formats the data for transmission. The DTR is used to record high-rate Plasma Wave Subsystem (PWS) data. The data is played back every six months.

The Imaging Science Subsystem, made up of a wide angle and a narrow angle camera, is a modified version of the slow scan vidicon camera designs that were used in the earlier Mariner flights. The Imaging Science Subsystem consists of two television-type cameras, each with 8 filters in a commandable Filter Wheel mounted in front of the vidicons. One has a low resolution 200 mm wide-angle lens with an aperture of f/3 (wide angle camera), while the other uses a higher resolution 1500 mm narrow-angle f/8.5 lens (narrow angle camera).

Computers

Unlike the other onboard instruments, operation of the cameras is not autonomous, but is controlled by an imaging parameter table residing in one of the spacecraft computers, the Flight Data Subsystem (FDS). Modern spacecraft (post 1990) typically have fully autonomous cameras.

The computer command subsystem (CCS) provides sequencing and control functions. The CCS contains fixed routines such as command decoding and fault detection and corrective routines, antenna pointing information, and spacecraft sequencing information. The computer is an improved version of that used in the Viking orbiter. The custom-built CCS systems on both craft are identical. There is only a minor software modification for one craft that has a scientific subsystem the other lacks.

The Attitude and Articulation Control Subsystem (AACS) controls the spacecraft orientation, maintains the pointing of the high-gain antenna towards Earth, controls attitude maneuvers, and positions the scan platform. The custom built AACS systems on both craft are identical.

It is widely reported on the web that the Voyager spacecraft were controlled by a version of the RCA CDP1802 “COSMAC” microprocessor, but such claims are not substantiated by primary references. The CDP1802 was used in the later Galileo spacecraft. The Voyager systems were based on RCA CD4000 radiation-hardened sapphire-on-silicon (SOS) custom chips, and some TI 54L ICs.

Uplink communications is via S band (16-bit/s command rate) while an X band transmitter provides downlink telemetry at 160 bit/s normally and 1.4 kbit/s for playback of high-rate plasma wave data. All data is transmitted from and received at the spacecraft via the 3.7m high-gain antenna.

Power

Electrical power is supplied by three radioisotope thermoelectric generators (RTGs). They are powered by plutonium-238 (distinct from the Pu-239 isotope used in nuclear weapons) and provided approximately 470 W at 30 volts DC when the spacecraft was launched. Plutonium-238 decays with a half-life of 87.74 years, so RTGs using Pu-238 will lose a factor of 1 – 0.5{1/87.74} = 0.78% of their power output per year.

In 2011, 34 years after launch, such an RTG would inherently produce 470 W × 2-(34/87.74) ≈ 359 W, about 76% of its initial power. Additionally, the thermocouples that convert heat into electricity also degrade, reducing available power below this calculated level.

File:MHW-RTGs.gif

By October 7, 2011 the power generated by Voyager 1 and Voyager 2 had dropped to 267.9 W and 269.2 W respectively, about 57% of the power at launch. The level of power output was better than pre-launch predictions based on a conservative thermocouple degradation model. As the electrical power decreases, spacecraft loads must be turned off, eliminating some capabilities.

Voyager Golden Record

Voyager 1 and 2 both carry with them a golden record that contains pictures and sounds of Earth, along with symbolic directions for playing the record and data detailing the location of Earth. The record is intended as a combination time capsule and interstellar message to any civilization, alien or far-future human, that may recover either of the Voyager craft. The contents of this record were selected by a committee chaired by Carl Sagan.

File:The Sounds of Earth Record Cover - GPN-2000-001978.jpg

File:Voyager Golden Record Cover Explanation.svg

Pale blue dot

The Voyager program’s discoveries during the primary phase of its mission, including never-before-seen close-up color photos of the major planets, were regularly documented by both print and electronic media outlets. Among the best-known of these is an image of the Earth as a pale blue dot, taken in 1990 by Voyager 1, and popularised by Carl Sagan.

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3 Responses to “Voyager – The Farthest Human-made Object From Earth”
  1. momofplenty Says...

    On April 20, 2012 at 8:57 pm

    Awes article, thanks!


  2. celeres Says...

    On April 21, 2012 at 9:45 am

    Space, the final frontier …..


  3. celeres Says...

    On April 21, 2012 at 10:00 am

    well done


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