Voyager program
The Voyager program is a series of U.S. unmanned space missions that consists of a pair of unmanned scientific probes, Voyager 1 and Voyager 2. They were launched in 1977 to take advantage of a favorable planetary alignment of the late 1970s. Although they were officially designated to study just Jupiter and Saturn, the two probes were able to continue their mission into the outer solar system. They are currently on course to eventually exit the solar system. These probes were built at JPL and were funded by NASA. Voyager 1 is currently the farthest human made object from Earth.
Both missions have gathered large amounts of data about the gas giants of the solar system, of which little was previously known. In addition, the spacecraft trajectories have been used to place limits on the existence of a hypothetical post-Plutonian Planet X.
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History
The Voyager probes were originally conceived as part of the Mariner program, and designated Mariner 11 and Mariner 12, respectively. They were then moved into a separate program named Mariner Jupiter-Saturn, later retitled Voyager because it was felt that the probes' designs had moved sufficiently far from the Mariner family that they merited a separate name.[1] Voyager is essentially a scaled-back version of the Grand Tour program of the late 1960s and early 1970s. The Grand Tour's plan was to send a pair of probes to fly by all the outer planets; it was scaled back because of budget cuts. However, in the end, Voyager fulfilled all the Grand Tour flyby objectives except for Pluto, which at the time was considered a planet by the IAU.
Of the pair, Voyager 2 was launched first. Its trajectory was designed to take advantage of an unusually convenient alignment of the planets allowing the inclusion of Uranus and Neptune fly bys in the probe's mission. Voyager 1 was launched after its sister probe, but on a faster trajectory which enabled it to reach Jupiter and Saturn sooner at the consequence of not visiting the outer planets.[2]
In the 1990s, Voyager 1 overtook the slower traveling Pioneer 10 to become the man-made object most distant from Earth in the universe. It will keep that record until such time as mankind develops newer, radically-faster forms of space propulsion than are currently known—even the faster (at launch) New Horizons probe will not outrace it, since the final speed of New Horizons (after maneuvering within the solar system) will be less than the current speed of Voyager 1. Voyager 1 and Pioneer 10 are also the most widely-separated man-made objects in the universe, because they are traveling in roughly opposite directions from the Sun (and each other).
Periodic contact has been maintained with Voyager 1 and Voyager 2 to monitor conditions in the outer expanses of the solar system. The crafts' radioactive power sources are still producing electrical energy, fueling hopes of locating the solar system's heliopause. In late 2003, Voyager 1 began sending data that seemed to indicate it had crossed the termination shock, but interpretations of this data are in dispute. It is now believed that the termination shock was crossed in December 2004, with the heliopause an unknown distance ahead.
As of August 2009, Voyager 1 was over 16.5 terameters (16.5 × 1012 meters, or 16.5 × 109 km, 110.7 AU, or 10.2 billion miles) from the Sun, and has thus entered the heliosheath region between solar wind's termination shock and the heliopause (the limit of the solar wind). Beyond heliopause is the bow shock of the interstellar medium, beyond which is interstellar space, a vast area where the Sun's influence gives way to that of the Milky Way galaxy in general. At this distance, light from the sun takes over 15 hours to reach the probe.
As of August 2009, Voyager 2 is at a distance of around 89.7 AU (approximately 13.4 terameters) from the Sun, deep in the scattered disc, and traveling outward at roughly 3.3 AU a year. It is more than twice as far from the Sun as Pluto is. On December 10, 2007, instruments on board Voyager 2 sent data back to Earth indicating that the Solar System is asymmetrical. It has also reached the termination shock, about 10 billion miles from where Voyager 1 first crossed it.
Spacecraft design
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 3.7 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.
Scientific instruments and sensors
The two planetary radio and plasma wave antennas extend diagonally downwards left and right. 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 susbsystem (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 investigator teams are still supported, though data is collected for two additional instruments.[3] 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.[4] There is only a minor software modification for one craft that has a scientific subsystem the other lacks.
- 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, however, used in the later Galileo spacecraft. The custom built CCS systems on both craft are identical.
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.
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 3.7-meter 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,[5] 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 2006, 29 years after launch, such an RTG would produce only 470 W × 2-(29/87.74) ~= 373 W — or about 79.5% — of its initial power. Moreover, the bi-metallic thermocouples that convert heat into electricity also degrade, so the actual power will be even lower.
As of 2009-09-25 the power generated by Voyager 1 and Voyager 2 had dropped to 276.4 W and 277.6 W respectively. This is about 58% of the power at launch. This level of power output so far has been better than the pre-launch predictions based on a conservative thermocouple degradation model. As the electrical power decreases, spacecraft loads must be turned off, eliminating some spacecraft capabilities.
Voyager Interstellar Mission
The Voyager primary mission was completed in 1989, with the close flyby of Neptune by Voyager 2. The Voyager Interstellar Mission (VIM) is a mission extension, which began when the two spacecraft had already been in flight for over 12 years.[6] The Heliophysics Division of the NASA Science Mission Directorate conducted a Heliophysics Senior Review in 2008. The panel found that the VIM "is a mission that is absolutely imperative to continue" and that VIM "funding near the optimal level and increased DSN (Deep Space Network) support is warranted."[7]
As of the present date, the Voyager 2 and Voyager 1 scan platforms, including all of the platform instruments, have been powered down. The ultraviolet spectrometer (UVS)[8] on Voyager 1 was active until 2003, when it too was deactivated. Gyro operations will end in 2010 for Voyager 2 and 2011 for Voyager 1. Gyro operations are used to rotate the probe 360 degrees six times a year to measure the magnetic field of the spacecraft, which is then subtracted from the magnetometer science data.
The two Voyager spacecraft continue to operate, with some loss in subsystem redundancy, but retain the capability of returning scientific data from a full complement of Voyager Interstellar Mission (VIM) science instruments.
Both spacecraft also have adequate electrical power and attitude control propellant to continue operating until around 2020, when the available electrical power will no longer support science instrument operation. At that time, science data return and spacecraft operations will cease. It is possible that one or both craft may have enough RTG energy to last until 2025, but there is only a small probability of this.[9]
Telemetry
The Voyager craft have three different telemetry formats
High rate
- CR-5T (ISA 35395) Science [1], note that this can contain some engineering data.
- FD-12 higher accuracy (and time resolution) Engineering data, note that some science data may also be encoded.
Low rate
- EL-40 Engineering [2], note that this format can contain some science data, but not all systems represented.
- This is an abbreviated format, with data truncation for some subsystems.
It is understood that there is substantial overlap of EL-40 and CR-5T (ISA 35395) telemetry, but the simpler EL-40 data does not have the resolution of the CR-5T telemetry. At least when it comes to representing available electricity to subsystems, EL-40 only transmits in integer increments -- so similar behaviours are expected elsewhere.
As both telemetry formats are mostly documented on paper that has not been published on the web. Therefore scant information is available for the general public on the telemetry subsystems. It is assumed that minor updates to the telemetry subsystem have been made -- such that the printed documentation that exists may be potentially inaccurate.
Memory dumps are available in both engineering formats. These routine diagnostic procedures have detected and corrected intermittent memory bit flip problems, as well as detecting the permanent bit flip problem that caused a 2 week data loss event mid-2010.
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 recovers either of the Voyager craft. The contents of this record were selected by a committee chaired by Carl Sagan.
Fiction and popular culture
The Voyager program's discoveries during the primary phase of its mission, including striking never-before-seen close up color photos of the major planets, were regularly documented by both print and electronic media outlets. Among the most well known of these is an image known as Pale blue dot, taken in 1990 by Voyager 1, and popularised by Carl Sagan.
As a result, the Voyager program, especially at the high points of its mission, has seen significant public limelight. There are a number of references to the Voyager program or to the particular probes themselves within popular culture.
- The Alan Parsons Project album "Pyramid" (Arista Records) from 1977 features a track entitled "Voyager".
- Space: 1999 "Voyager's Return" (1975) A probe called Voyager One - vastly different in design to the real Voyager spacecraft - is en route back to Earth after its propulsion system has had devastating effects on an alien world, and is intercepted by the members of the Moonbase Alpha community, attempting to retrieve the valuable information in its memory banks.
- Star Trek: The Motion Picture (1979) features an alien intelligence V'ger, which turns out to be a fictional Voyager 6, highly transformed.
- Larry Niven and Jerry Pournelle's Footfall (1980) opens with the discovery of an interstellar Bussard ramjet in in the vicinity of Saturn by Voyager I.
- Stephen Baxter's novel Titan (1997) describes what will happen to the Voyager probes billions of years in the future as the metal from which they are constructed gradually disintegrates.
- In L. Ron Hubbard's novel Battlefield Earth, aliens who stumble on a Voyager probe and its golden contents proceed to find and conquer Earth.
- In the X-Files episode "Little Green Men" in Season 2, Special Agent Fox Mulder hears part of the golden record played back to him in an extraterrestrial transmission received at Arecibo Observatory.
- John Carpenter's 1984 movie Starman opens with the Voyager 1 probe being intercepted by an alien spacecraft, whose occupants play the golden record.
- The Moody Blues' 1981 album Long Distance Voyager contained images of a Voyager probe on its cover.
- NASA: Symphonies of the Planets Vol. I-V A series of recordings (5 volumes) that was created from original Voyager recordings. Although space is a virtual vacuum, the sounds were created through the conversion of electronic vibrations. The specially designed instruments on board the Voyagers performed special experiments to pick up and record these vibrations, all within the range of human hearing.
The recordings come from a variety of different sound environments and are listed as follows: 1. From the interaction of the solar wind with the planet's magnetosphere, which releases charged ionic particles within a vibration frequency in an audible range (20 - 20,000 Hz). 2. From the magnetosphere itself. 3. From the trapped radio waves bouncing between the planet and the inner surface of its atmosphere. 4. Electromagnetic field noise within space itself. 5. From charged particle interactions of the planet, its moons, and the solar wind. 6. From charged particle emissions from the rings of certain planets.[10]
See also
- Timeline of Solar System exploration
- Pioneer program
- Family Portrait (Voyager)
References
- ↑ Miller, E: From Engineering Science To Big Science, Chapter 11 "Voyager: The Grand Tour of Big Science" sec. 268. NASA, 1998 | url=http://history.nasa.gov/SP-4219/Chapter11.html
- ↑ Miller, E: From Engineering Science To Big Science, Chapter 11 "Voyager: The Grand Tour of Big Science" sec. 269. NASA, 1998 | url=http://history.nasa.gov/SP-4219/Chapter11.html
- ↑ -Nasa Website Voyager
- ↑ Tomayko, James (April 1987). "Computers in Spaceflight: The NASA Experience". NASA. http://history.nasa.gov/computers/Ch6-2.html. Retrieved February 6, 2010.
- ↑ The Actinide Research Quarterly: Summer 1997
- ↑ "Interstellar Mission". NASA. http://voyager.jpl.nasa.gov/mission/interstellar.html.
- ↑ "Senior Review 2008 of the Mission Operations and Data Analysis Program for the Heliophysics Operating Missions" (PDF). NASA. p. 7. http://www.igpp.ucla.edu/public/THEMIS/SCI/Pubs/Proposals%20and%20Reports/Senior%20Review%202008%20Report%20Final.pdf.
- ↑ "Ultraviolet Spectrometer". Voyager: The Interstellar Mission. NASA JPL. http://voyager.jpl.nasa.gov/spacecraft/instruments_uvs.html. Retrieved 2006-06-11.
- ↑ http://voyager.jpl.nasa.gov/spacecraft/spacecraftlife.html
- ↑
External links
NASA sites
- NASA Voyager website - Main source of information.
- Voyager Spacecraft Lifetime
- Space Exploration - Robotic Missions
- NASA Facts - Voyager Mission to the Outer Planets (PDF format)
- Voyager 1 and 2 atlas of six Saturnian satellites (PDF format) 1984
- Mission state (more often than not at least 3 months out of date)
NASA instrument information pages:
- "Voyager instrument overview". http://starbrite.jpl.nasa.gov/pds/viewHostProfile.jsp?INSTRUMENT_HOST_ID=VG2.
- "CRS - COSMIC RAY SUBSYSTEM". http://starbrite.jpl.nasa.gov/pds/viewInstrumentProfile.jsp?INSTRUMENT_ID=CRS&INSTRUMENT_HOST_ID=VG1.
- "ISS NA - IMAGING SCIENCE SUBSYSTEM - NARROW ANGLE". http://starbrite.jpl.nasa.gov/pds/viewInstrumentProfile.jsp?INSTRUMENT_ID=ISSN&INSTRUMENT_HOST_ID=VG2.
- "ISS WA - IMAGING SCIENCE SUBSYSTEM - WIDE ANGLE". http://starbrite.jpl.nasa.gov/pds/viewInstrumentProfile.jsp?INSTRUMENT_ID=ISSW&INSTRUMENT_HOST_ID=VG2.
- "IRIS - INFRARED INTERFEROMETER SPECTROMETER AND RADIOMETER". http://starbrite.jpl.nasa.gov/pds/viewInstrumentProfile.jsp?INSTRUMENT_ID=IRIS&INSTRUMENT_HOST_ID=VG2.
- "LECP - LOW ENERGY CHARGED PARTICLE". http://starbrite.jpl.nasa.gov/pds/viewInstrumentProfile.jsp?INSTRUMENT_ID=LECP&INSTRUMENT_HOST_ID=VG2.
- "MAG - TRIAXIAL FLUXGATE MAGNETOMETER". http://starbrite.jpl.nasa.gov/pds/viewInstrumentProfile.jsp?INSTRUMENT_ID=MAG&INSTRUMENT_HOST_ID=VG2.
- "PLS - PLASMA SCIENCE EXPERIMENT". http://starbrite.jpl.nasa.gov/pds/viewInstrumentProfile.jsp?INSTRUMENT_ID=PLS&INSTRUMENT_HOST_ID=VG2.
- "PPS - PHOTOPOLARIMETER SUBSYSTEM". http://starbrite.jpl.nasa.gov/pds/viewInstrumentProfile.jsp?INSTRUMENT_ID=PPS&INSTRUMENT_HOST_ID=VG2.
- "PRA - PLANETARY RADIO ASTRONOMY RECEIVER". http://starbrite.jpl.nasa.gov/pds/viewInstrumentProfile.jsp?INSTRUMENT_ID=PRA&INSTRUMENT_HOST_ID=VG2.
- "PWS - PLASMA WAVE RECEIVER". http://starbrite.jpl.nasa.gov/pds/viewInstrumentProfile.jsp?INSTRUMENT_ID=PWS&INSTRUMENT_HOST_ID=VG2.
- "RSS - RADIO SCIENCE SUBSYSTEM". http://starbrite.jpl.nasa.gov/pds/viewInstrumentProfile.jsp?INSTRUMENT_ID=RSS-VG1S&INSTRUMENT_HOST_ID=VG1.
- "UVS - ULTRAVIOLET SPECTROMETER". http://starbrite.jpl.nasa.gov/pds/viewInstrumentProfile.jsp?INSTRUMENT_ID=UVS&INSTRUMENT_HOST_ID=VG1.
Non-NASA sites
- Spacecraft Escaping the Solar System - current positions and diagrams
- NPR: Science Friday 8/24/07 Interviews for 30th anniversary of Voyager spacecrafts
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