| MISSION_DESCRIPTION |
Mission Overview
================
The Pioneer mission set the stage for U.S. space exploration.
Pioneer 1 was the first manmade object to escape the Earth's
gravitational field. Later Pioneer 4 was the first spacecraft
to fly to the moon, Pioneer 10 was the first to Jupiter,
Pioneer 11 was the first to Saturn and Pioneer 12 was the first
U.S. spacecraft to orbit another planet, Venus. The following
table summarizes the Pioneer spacecraft and scientific
objectives of the Pioneer mission.
Name Launch Mission Status (as of 1998)
-----------------------------------------------------------------
Pioneer 1 1958-10-11 Moon Reached altitude of 72765 miles
Pioneer 2 1958-11-08 Moon Reached altitude of 963 miles
Pioneer 3 1958-12-02 Moon Reached altitude of 63580 miles
Pioneer 4 1959-03-03 Moon Passed by moon into solar orbit
Pioneer 5 1960-03-11 Solar Orbit Entered solar orbit
Pioneer 6 1965-12-16 Solar Orbit Still operating
Pioneer 7 1966-08-17 Solar Orbit Still operating
Pioneer 8 1967-12-13 Solar Orbit Still operating
Pioneer 9 1967-11-08 Solar Orbit Signal lost in 1983
Pioneer E 1969-08-07 Solar Orbit Launch failure
Pioneer10 1972-03-02 Jupiter Communication terminated 1998
Pioneer11 1972-03-02 Jupiter/Saturn Communication terminated 1997
Pioneer12 1978-05-20 Venus Entered Venus atmos. 1992-10-08
The focus of this document is on Pioneer Venus (12), the last
spacecraft in a mission of firsts in space exploration.
Pioneer Venus separated into two spacecraft on Aug 8, 1978: an
Orbiter (PVO) and a Multiprobe. The latter separated into five
vehicles near Venus. These were the probe transporter (called
the Bus), a large atmospheric entry probe (dubbed Sounder) and
three identical smaller probes (called North, Day, and Night in
accordance with their entry locations). At Venus all six
spacecraft communicated directly back to the Earth-based Deep
Space Network (DSN) and, in the case of the Multiprobe mission,
to two special receiving sites near Guam and Santiago (Chile).
The Orbiter encountered Venus on December 4, 1978, and was
inserted into orbit on that same day after a Type II
interplanetary cruise trajectory lasting 198 days and covering
more than 500 x 10^6 km. Twelve scientific experiments were
included in the instrumentation payload and a few radio science
investigations were planned using the S-band telemetry signal
carrier and a special X-band beacon included as part of the
Orbiter hardware. Scientific observations were made both
in-cruise and in-orbit. The nominal in-orbit mission was
designed to extend for one Venus year (243 days) with an
orbital period of approximately 24 hours. At the end of the
nominal mission (Aug 14, 1979), the spacecraft periapsis was
raised from its 150-200 km altitude to more then 2000 km to
conserve fuel for an extended mission. As fuel began to run
out and periapsis began to drop back down into the Venusian
atmosphere the end of the mission could no longer be avoided.
On Oct 8, 1992 the Orbiter made its final encounter with the
Venus atmosphere. More then 13 years had passed since the end
of the nominal mission.
During the nominal Orbiter mission all but two experiments
operated 100% successfully. One, the Radar Mapper, produced
unusable data for a 32-day period from December 18, 1978 to
January 19, 1979. The radar instrument was successfully used
during the extended mission until the periapsis altitude rose
too high. The instrument was not used during the final
re-entry phase of the mission at low periapsis due to the
success of the Magellan radar mapping mission. The Infrared
Radiometer was the other instrument to fail during the nominal
mission. It stopped operating after February 14, 1979, but had
collected an enormous quantity of valuable information prior to
that date. Later in the mission (orbit 3601) the magnetometer
data pickup failed such that data from only one of the three
sensors (P sensor) was returned to Earth in the telemetry
stream.
The Pioneer Venus Multiprobe consisted of a bus which carried
one large and three small atmospheric probes. The large probe
was released on November 16, 1978 and the three small probes on
November 20. All four probes entered the Venus atmosphere on
December 9, followed by the bus.
The Pioneer Venus large probe was equipped with 7 science
experiments, contained within a sealed spherical pressure
vessel. This pressure vessel was encased in a nose cone and
aft protective cover. After deceleration from initial
atmospheric entry at about 11.5 km/s near the equator on the
Venus night side, a parachute was deployed at 47 km altitude.
The large probe was about 1.5 m in diameter and the pressure
vessel itself was 73.2 cm in diameter. The science experiments
were:
* a neutral mass spectrometer to measure the atmospheric
composition
* a gas chromatograph to measure the atmospheric composition
* a solar flux radiometer to measure solar flux penetration in
the atmosphere
* an infrared radiometer to measure distribution of infrared
radiation
* a cloud particle size spectrometer to measure particle size
and shape
* a nephelometer to search for cloud particles
* temperature, pressure, and acceleration sensors
The three small probes were identical to each other, 0.8 m in
diameter. These probes also consisted of spherical pressure
vessels surrounded by an aeroshell, but unlike the large probe,
they had no parachutes and the aeroshells did not separate from
the probe. Each small probe carried a nephelometer and
temperature, pressure, and acceleration sensors, as well as a
net flux radiometer experiment to map the distribution of
sources and sinks of radiative energy in the atmosphere. The
radio signals from all four probes were also used to
characterize the winds, turbulence, and propagation in the
atmosphere. The small probes were each targeted at different
parts of the planet and were named accordingly. The North
probe entered the atmosphere at about 60 degrees north latitude
on the day side. The night probe entered on the night side.
The day probe entered well into the day side, and was the only
one of the four probes which continued to send radio signals
back after impact, for over an hour. More detailed information
on the probes is available.
The Pioneer Venus bus also carried two experiments, a neutral
mass spectrometer and an ion mass spectrometer to study the
composition of the atmosphere. With no heat shield or
parachute, the bus survived and made measurements only to about
110 km altitude before burning up. The bus was a 2.5 m
diameter cylinder weighing 290 kg, and afforded us our only
direct view of the upper Venus atmosphere, as the probes did
not begin making direct measurements until they had decelerated
lower in the atmosphere.
Probe Entry Timing Events: (Table 5-2, p141, Fimmel et al,
1995)
Time at Spacecraft, hr:min:sec* - PST - Dec 9, 1978
_______________________________________________________________
Parameter Large North Day Night
_______________________________________________________________
End of coast 10:24:26 10:27:57 10:30:27 10:34:08
Initiate telemetry 10:29:27 10:32:55 10:35:27 10:39:08
200 km entry 10:45:32 10:49:40 10:52:18 10:56:13
Radio blackout begin 10:45:53 10:49:58 10:52:40 10:56:27
Signal locked on 10:46:55 10:50:55 10:53:46 10:57:48
Jettison parachute 11:03:28 N/A N/A N/A
Impact with surface 11:39:53 11:42:40 11:47:59 11:52:05
Signal ended 11:39:53 11:42:40 12:55:34 11:52:07
Bus Entry 12:12:52
Bus signal ended 12:22:55
_______________________________________________________________
Event Duration
_______________________________________________________________
Descent time 54:21 53:00 55:41 55:52
Blackout time 01:02 00:57 01:06 01:21
Time on parachute ~17:07 N/A N/A N/A
Surface operations none none 01:07:37 00:02
* Earth receive times were approximately 3 minutes later
Probe Entry Impacts: (Table 5-3, p144, Fimmel et al, 1995)
_______________________________________________________________
Probe Latitude Longitude Solar-Zenith-Angle Venus Local
deg E. deg (SZA) deg Time hr:mm
_______________________________________________________________
Large 4.4 N 304.0 65.7 07:38
North 59.3 N 4.8 108.0 03:35
Day 31.3 S 317.0 79.9 06:46
Night 28.7 S 56.7 150.7 00:07
_______________________________________________________________
Bus Entry and Location of Sun and Earth:
(Table 5-4, p144, Fimmel et al, 1995)
_______________________________________________________________
Probe Lat E.Lon SZA Local Time
deg deg deg hr:mm
_______________________________________________________________
Bus entry at 200 km 37.9 S 290.9 60.7 08:30
Subsolar 0.5 S 328.5 0 12:00
Sub-Earth 1.6 S 1.7 123.1 03:47
______________________________________________________________
Probe Anomalies: (Table 5-5, p146 Fimmel et al, 1995)
____________________________________________________________________
Anomaly Large North Day Night
____________________________________________________________________
Apparent failure of temperature sensors X X X X
Apparent failure of net flux radiometer
fluxplate temperature sensors X X X
Abrupt changes and spikes in the data from
the net flux radiometer X X X
Change in the indicated deployment status
of the atmospheric structure temperature
sensor and net flux radiometer booms X X X
Erratic data from two thermocouples embedded
in the heat-shield X X X
Erratic data from a thermistor measuring
junction temperature of the heat-shield
thermocouples. X X X
Slight variation in the current and voltage
levels in the power bus X X X
Abrupt changes in the cloud particle size
laser alignment monitor X N/A N/A N/A
Decrease in the intensity of the beam
returned to the cloud particle size
spectrometer X N/A N/A N/A
Noise spikes in the infrared radiometer X N/A N/A N/A
Spikes in the data monitoring the ion pump
current of the mass spectrometer analyzer X N/A N/A N/A
Spurious reading from the thermocouples
when the heat shield was dropped from the
probe X N/A N/A N/A
____________________________________________________________________
Probe Anomalies: (Table 5-5, p146 Fimmel et al, 1995)
____________________________________________________________________
Anomaly Large North Day Night
____________________________________________________________________
Apparent failure of temperature sensors X X X X
Apparent failure of net flux radiometer
fluxplate temperature sensors X X X
Abrupt changes and spikes in the data from
the net flux radiometer X X X
Change in the indicated deployment status
of the atmospheric structure temperature
sensor and net flux radiometer booms X X X
Erratic data from two thermocouples embedded
in the heat-shield X X X
Erratic data from a thermistor measuring
junction temperature of the heat-shield
thermocouples. X X X
Slight variation in the current and voltage
levels in the power bus X X X
Abrupt changes in the cloud particle size
laser alignment monitor X N/A N/A N/A
Decrease in the intensity of the beam
returned to the cloud particle size
spectrometer X N/A N/A N/A
Noise spikes in the infrared radiometer X N/A N/A N/A
Spikes in the data monitoring the ion pump
current of the mass spectrometer analyzer X N/A N/A N/A
Spurious reading from the thermocouples
when the heat shield was dropped from the
probe X N/A N/A N/A
The long extended mission allowed the spacecraft controllers to
make several comet observations that were never part of the
original mission objectives. The tilt of the spacecraft was
altered during these comet observations so that the Ultraviolet
Spectrometer (OUVS) could view the comets rather than Venus.
Comets Encke (April 13-16, 1984), Giacobini-Zinner (Sept 8-15,
1985), Halley (Dec 27, 1985 - Mar 9, 1986), Wilson (Mar 13 -
May 2, 1987), NTT (April 8, 1987), and McNaught (Nov 19-24,
1987) were all observed in this way.
Mission Phases
==============
The Pioneer Venus Orbiter (PVO) phases are presented followed
by the Pioneer Venus Multiprobe (PVMP) phases.
PIONEER VENUS ORBITER PRELAUNCH
-------------------------------
The spacecraft was delivered to the launch site at Kennedy
Space Center on March 14, 1978.
Spacecraft Id : PVO
Target Name : VENUS
Mission Phase Start Time : 1968-06-01
Mission Phase Stop Time : 1978-05-20T13:13
Spacecraft Operations Type : ORBITER
PIONEER VENUS ORBITER LAUNCH
----------------------------
The spacecraft was launched atop an Atlas-Centaur from
Kennedy Space Center.
Spacecraft Id : PVO
Target Name : VENUS
Mission Phase Start Time : 1978-05-20T13:13:0
Mission Phase Stop Time : 1978-05-21
Spacecraft Operations Type : ORBITER
PIONEER VENUS ORBITER CRUISE
----------------------------
Two small trajectory correction maneuvers were made on June
1 and November 2, 1978. Interplanetary cruise scientific
data were collected. The Pioneer Venus Orbiter completed
its 300-million-mile voyage in 6+ months and was placed
into Venusian orbit on December 4, 1978.
Spacecraft Id : PVO
Target Name : VENUS
Mission Phase Start Time : 1978-05-21
Mission Phase Stop Time : 1978-12-04
Spacecraft Operations Type : ORBITER
VENUS ORBITAL OPERATIONS
-----------------------------
The orbiter was placed into Venusian orbit on December 4,
1978, less than a week before the arrival of the Pioneer
Venus Multiprobe. The planned mission ended August 4, 1979.
The extended mission began August 5, 1979 and ended when
the Orbiter entered the Venusian atmosphere on Oct. 8, 1992.
Spacecraft Id : PVO
Target Name : VENUS
Mission Phase Start Time : 1978-12-04
Mission Phase Stop Time : 1992-10-08
Spacecraft Operations Type : ORBITER
PIONEER VENUS ORBITER PHASE 1
-----------------------------
Orbiter Phase 1 was the primary mission which began on
Dec 4, 1979 at orbital insertion and ended after 1 Venusian
year (243 days) on Aug. 4, 1979. During Phase 1, the orbiter
periapsis was maintained between 150-200 km altitude using
the main thrusters.
Spacecraft Id : PVO
Target Name : VENUS
Mission Phase Start Time : 1978-12-04
Mission Phase Stop Time : 1979-08-04
Spacecraft Operations Type : ORBITER
PIONEER VENUS ORBITER PHASE 2
-----------------------------
Orbiter Phase 2 was the extended mission at high periapsis
which began on Aug. 5, 1979, immediately following Phase
1. Phase 2 was the period where the project flight
controllers did not control periapsis altitude. Periapsis
altitude was allowed to rise and fall under the influence
of solar perturbations to conserve fuel. Phase 2 ended when
the periapsis altitude fell below the 1000 km level in
early 1991.
Spacecraft Id : PVO
Target Name : VENUS
Mission Phase Start Time : 1979-08-05
Mission Phase Stop Time : 1991-04-01
Spacecraft Operations Type : ORBITER
PIONEER VENUS ORBITER COMETARY OBSERVING PHASE
----------------------------------------------
The long extended mission allowed the spacecraft
controllers to make several comet observations that were
never part of the original mission objectives. The tilt of
the spacecraft was altered during these comet observations
so that the Ultraviolet Spectrometer (OUVS) could view the
comets rather than Venus. Comets Encke (April 13-16, 1984),
Giacobini-Zinner (Sept 8-15, 1985), Halley (Dec 27, 1985 -
Mar 9, 1986), Wilson (Mar 13 - May 2, 1987), NTT (April 8,
1987), and McNaught (Nov 19-24, 1987) were all observed in
this way.
Spacecraft Id : PVO
Target Name : VENUS
Mission Phase Start Time : 1984-04-13
Mission Phase Stop Time : 1987-11-24
Spacecraft Operations Type : ORBITER
PIONEER VENUS ORBITER PHASE 3 (RE-ENTRY)
----------------------------------------
Orbiter Phase 3 describes the extended mission. Late in
1991, the periapsis began to penetrate the lower
thermosphere and ionosphere. When it had fallen to about
1000 km, Phase 3 of the mission began. As the periapsis
continued to fall, controllers again used the thrusters to
maintain periapsis. This time they kept it within an
altitude ranging from 140 km to 160 km. Also, the latitude
of the periapsis continued moving southward to about 10
degrees below the planet's equator. During Phase 3, the
spacecraft sampled the atmosphere to deeper levels than
were prudent in Phase 1.
Spacecraft Id : PVO
Target Name : VENUS
Mission Phase Start Time : 1991-04-01
Mission Phase Stop Time : 1992-10-08
Spacecraft Operations Type : ORBITER
PIONEER VENUS MULTIPROBE LAUNCH
-------------------------------
The spacecraft was launched atop an Atlas-Centaur from
Kennedy Space Center.
Spacecraft Id : PVMP
Target Name : VENUS
Mission Phase Start Time : 1978-08-08
Mission Phase Stop Time : 1978-08-08
Spacecraft Operations Type : ATMOSPHERIC PROBE
PIONEER VENUS MULTIPROBE ENCOUNTER
----------------------------------
The Multiprobe encountered Venus on December 9, 1978 (just
five days following the Orbiter insertion) after a Type I
interplanetary cruise trajectory lasting 123 days and
covering 330 x 106 km. The Sounder was released from the Bus
on November 15, 1978, and the three small probes were
released simultaneously on November 19, 1978. All probes
entered (200-km altitude) the Venus upper atmosphere within a
time span of about 11 min and descended to the surface in a
period from 53 to 56 min, all the time performing scientific
observations. The Bus made a delayed (~90 min) entry
relative to the probes into Venus' upper atmosphere and
burned up at about 110-km altitude since it was not
protected, as were the probes, with entry heat shields.
Scientific observations were made during the one-minute
interval from 700 to 110 km. Although not designed for
`survival' after impact, the Day probe managed to transmit
for over 67 min on the surface (it in fact continued to
transmit after the Bus transmission ceased). Seven
scientific experiments were included in the Sounder
instrumentation payload, three identical experiments in each
small probe, and two in the Bus. Again, Radio Science
experiments were performed using, separately or together, the
S-band telemetry signal carriers emanating from the
spacecraft and received at the Earth-based tracking stations.
In general, all instruments performed nominally, although
certain instruments behaved anomalously on all four probes
near the surface.
Spacecraft Id : PVMP
Target Name : VENUS
Mission Phase Start Time : 1978-12-07
Mission Phase Stop Time : 1992-12-07
Spacecraft Operations Type : ATMOSPHERIC PROBE
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| MISSION_OBJECTIVES_SUMMARY |
Mission Objectives Summary
==========================
NASA established the Pioneer Venus Science Steering Group in
Jan. 1972 in order to enlist widespread science community
participation in designing the mission science requirements.
This group concluded that a successful mission to Venus would
answer the following list of scientific questions (Fimmel et
al, 1995, Table 2-2, p25):
1) Cloud layers: What is their number and where are they
located? Do they vary over the planet?
2) Cloud forms: Are they layered, turbulent, or merely hazes?
3) Cloud physics: Are the clouds opaque? What are the sizes
of the cloud particles? What is the density of the cloud
particles?
4) Cloud composition: What is the chemical composition of the
clouds? Is it different in the different layers?
5) Solar heating: Where is the solar radiation deposited in
the atmosphere?
6) Deep circulation: What is the nature of the wind in the
lower regions of the atmosphere? Is there any measurable
wind near the surface?
7) Deep driving forces: What are the horizontal differences
in temperature in the deep atmosphere?
8) Driving force for the 4-day circulation: What are the
horizontal temperature differences at the top of the cloud
layer that could cause the high winds there?
9) Loss of water: Has water been lost from Venus? If so, how?
10) Carbon dioxide stability: Why is molecular CO2 stable in
the upper atmosphere?
11) Surface composition: What is the composition of the
crustal rocks?
12) Seismic activity: What is its level?
13) Earth tides: Do tidal effects from Earth exist at Venus,
and if so, how strong are they?
14) Gravitational moments: What is the figure of the planet?
What are the higher order gravitational moments?
15) Extent of the 4-day circulation: How does this circulation
vary with latitude and depth in the Venusian atmosphere?
16) Vertical temperature structure: Is there an isothermal
region? Are there other departures from adiabaticity? What
is the structure near the cloud tops?
17) Ionospheric motions: Are these motions sufficient to
transport ionization from the day to night hemisphere?
18) Turbulence: How much turbulence is there deep in the
atmosphere?
19) Ion chemistry: What is the chemistry of the ionosphere?
20) Exospheric temperature: What is the temperature and does
it vary over the planet?
21) Topography: What features exist on the surface of the
planet? How do they relate to the thermal maps?
22) Magnetic moments: Does the planet have any internal
magnetism?
23) Bulk atmospheric composition: What are the major gases in
the Venus atmosphere? How do they vary with altitude?
24) Anemopause: How does the solar wind interact with the
planet?
The wide range of science questions to be answered by the
Pioneer Venus mission could not be answered by an orbiter
mission alone. An orbiter could address the questions
regarding the upper atmosphere, ionosphere, solar wind, and
surface topography, but a probe would be needed to make in
situ measurements in the lower atmosphere. Likewise, a single
probe would be inadequate to address the all of the questions
posed about the lower atmosphere. Multiple probes targeting
different parts of the planet simultaneously were required to
meet the science objectives. Thus the Pioneer Venus mission
developed into an Orbiter and Multiprobe spacecraft.
A large number of scientific instruments were required on
both the orbiter and the various probes in order to meet the
mission science objectives. The orbiter carried 4 remote
sensing instruments and 8 in situ experiments. In addition,
several radio science experiments were performed using the
S-band telemetry signal plus a special X-band beacon that was
included on the Orbiter. The first letter of the instrument
acronym designates which component of the spacecraft carried
the instrument: O for Orbiter, L for Large Probe, S for the
Small Probes, and B for the Bus. The remote sensing
instruments were: 1) Cloud Photopolarimeter (OCPP), 2) Radar
mapper (ORAD), 3) Ultraviolet Spectrometer (OUVS), and 4)
Infrared Radiometer (OIR). The in situ instruments included 4
plasma and gamma ray detectors: 1) Ion Mass Spectrometer
(OIMS), 2) Electron Temperature Probe (OETP), 3) Retarding
Potential Analyzer (ORPA), 4) Plasma Analyzer (OPA), and 5)
Gamma Burst Detector (OGBD). In addition, the neutral
atmosphere composition was sampled by the Neutral Mass
Spectrometer (ONMS), the magnetic field was measured by MAG
(OMAG) and the wave electric fields were measured by the
Electric Field Detector (OEFD).
The large probe, bus, and small probes each carried slightly
different instrumentation. The probe instruments were:
Neutral Mass Spectrometers (LNMS, SNMS, BNMS), Gas
Chromatograph (LGC), Atmospheric Structure experiment (LAS,
SAS), Nephelometers (LN, SN), Cloud Particle Size (LCPS),
Solar Flux Radiometer (LSFR), Infrared Radiometer (LIR), Net
Flux Radiometer (SNFR),
Finally, the spacecraft and probe telemetry signal
distortions were used to probe the Venus atmosphere and
ionosphere. The radio science experiments included the
Atmospheric Propagation Experiment (OGPE), Atmospheric Drag
(OAD), Differential Long Baseline Interferometry (DBLI),
Doppler tracking of the probes (MWIN), Atmospheric Turbulence
(MTUR/OTUR), Dual Frequency Occultation (ORO), Internal
Density (OIDD), and finally, Celestial Mechanics experiments
(OCM).
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