Pioneer Venus: Mission Characterization Dr. Andrew Ketsdever Lesson 2 AME 5595
Background Pioneer Venus evolved from recommendations from the Space Science Board of the National Academy of Sciences Need for relatively low-cost orbiters and landers to explore the planet Venus Earth s closest neighbor, yet relatively little was known (especially about the lower atmosphere) What was known raised many scientific questions
Background What was known Planet is covered with clouds Atmosphere primarily CO 2 (Traces of sulphuric acid) Surface pressure is 95 Earth atmospheres Surface temperature is 493 ºC Questions Why do two planets with about the same mass, probably formed out of similar materials and situated at comparable distances from the sun, have atmospheres that evolved so differently? Why is the surface of Venus baked by a searing heat, while Earth is not? Answers to these questions should enhance knowledge of Earth s atmosphere and weather Venus represents a relatively simple weather machine absent of the influence of oceans
Subject: Venus Diameter: 12,100 km ( 12,745km) Mass: 0.81 M Density: 5.26 g/cm 3 ( 5.5 g/cm 3 ) Mean Orbital Radius: 108.2 Million km Orbital Period: 224.7 days Rotation: Once per 243.1 days Clouds rotate in about 4 days (at the top) Rotation is retrograde Opposite to direction that planet travels around sun One day is 117 Earth days Only 6º tilt of axis with respect to its orbital plane Minimum energy launch opportunities come every 584 days
Subject: Venus Surface Pressure 95 Atm 9,616 kpa Surface Temperature 750 K 480 ºC Albedo 1.82 Nearly 1.98 solar intensity Similar atmospheric absorption of solar energy
Mission Objectives Subject: Characterization of the atmosphere, ionosphere, and surface of Venus Objectives Determine the composition of the clouds Determine the composition and structure of the atmosphere from the surface to high altitude Determine the composition and structure of the ionosphere Determine the characteristics of the surface on a planetary scale Investigate the interactions of the magnetic field and the solar wind Investigate the planet s gravitational field harmonics
Mission Concept Two spacecraft Orbiter Spacecraft subsystems 12 scientific instruments (payloads) Multi-probe Spacecraft bus 3 small probes 1 large probes DRIVER: Determine the composition and structure of the atmosphere from the surface to high altitude
Mission Details Pioneer Venus 1 (Orbiter) Launched 20 May 1978 Atlas SLV-3D/Centaur Planet Arrival 4 Dec 1978 Pioneer Venus 2 (Multi- Probe) Launched 8 Aug 1978 Atlas SLV-3D/Centaur Planet Arrival 9 Dec 1978 (Probes)
PV1: Orbit Details Type II Interplanetary Trajectory Travels more than 180º around the sun Used to reduce the spacecraft s velocity upon arrival at Venus Less propellant required (180kg propellant used 545 kg total spacecraft mass) Travel of 480 million km, in 7 months Venus Orbit 28 sec burn of solid propellant motor Elliptical (300 km periapsis / 66,000 km apoapsis) 24 hr period 75º inclination Later: Periapsis commanded to 150 km
PV1: Interplanetary Orbit
PV1: Venus Orbit
PV1: Venus Orbit
PV1: Orbit Details
PV2: Orbit Details Type I Interplanetary Trajectory Travels less than 180º around the sun Launched a few days after PV1 crossed back inside Earth s orbit 4 month trip time Arrival speed at Venus 19,500 km/hr (5.4 km/s) 24 days to Venus: Large probe released 20 days to Venus: Small probes released Bus re-entry in Venus atmosphere
PV2: Interplanetary Orbit
PV2: Orbit Details
PV2: Orbit Details
The Spacecraft Built by Hughes Aircraft Company Project was managed by NASA Ames RC Expertise High velocity (hypersonic) flight dynamics Re-entry Planetary atmosphere sensing (Earth demonstrations) We will look at some of the design specifics Orbiter (PV1) Accomplish retro-fire at Venus to achieve specified orbit Accommodate 47.6 kg of scientific payloads Support dual frequency (S and X bands) for occultation exp Multiprobe (PV2) Mechanically and electrically support probes (1 large / 3 small) Target and release probes at Venus Accommodate 57.6 kg of scientific payloads
The Orbiter (PV1) Requirements Return data for 243 days in Venus orbit Spin stabilization Disk shape Mass concentrated at perimeter (I z /I p = 1.2) Solar thermal environment of up to 2 Instrument pointing requirements ~0.2º accuracy Atlas / Centaur upper stage Limited mass (587.4 kg completion / 523 kg inception) 15g max vibration load Fundamental frequency >4Hz Shroud constrained dimensions Clamping arrangement (LV interface)
The Orbiter (PV1) Spacecraft element Spacecraft bus (subsystems) Six basic assemblies Despun antenna assembly Bearing and power transfer assembly (slip ring) and support structure Equipment shelf Solar panel Orbit insertion motor Thrust tube 12 scientific instruments Mounted on periphery of equipment shelf to provide adequate viewing angles Many instruments required diamond and sapphire windows Isolated from spacecraft contaminants For example, magnetometer is mounted at the end of a 15.7 ft. boom to isolate it from the spacecraft
Orbiter (PV1)
Orbiter
Orbiter Spin-stabilized platform Flat Cylinder 2.5 m diameter 1.2 m high Despun antenna 1.09 m diameter High gain, parabolic S and X band operation
Payloads 12 scientific instruments Cloud Photopolarimeter (OCPP) Measure vertical distribution of cloud and haze particles 5 kg, 5.4W 3.7 cm aperture telescope with filter wheel Surface Radar Mapper (ORAD) Produce first maps of large areas of Venus not observable from Earth 9.7 kg, 18W 150 m resolution Infrared Radiometer (OIR) Measure infrared radiation emitted by the atmosphere at various altitudes 5.9 kg, 5.2W Determine where maximum deposition from solar energy is located
Payloads Airglow Ultraviolet Spectrometer (OUVS) Measure UV light scattered or emitted by clouds 3.1 kg, 1.7W Airglow is the absorption of UV light by gases in the upper atmosphere Neutral Mass Spectrometer (ONMS) Measure neutral atom and molecular densities 3.8 kg, 12W Vertical and horizontal distribution of neutral gases Solar Wind Plasma Analyzer (OPA) Measure properties of the solar wind at Venus (density, velocity, flow direction, temperature) 3.9 kg, 5W Electrostatic energy analyzer Magnetometer Investigate weak magnetic field of Venus 2 kg, 2.2W Weak magnetic field may play an important role in solar wind interactions
Payloads Electric Field Detector (OEFD) Measure electric fields of plasma waves and radio emissions from 50-50,000Hz 0.8 kg, 0.7W Answer questions about how the solar wind is deflected around Venus Electron Temperature Probe (OETP) Measure thermal characteristics of ionosphere 2.2 kg, 4.8W Electron temperature, density and spacecraft potential Ion Mass Spectrometer (OIMS) Measure distribution of charged particles in the atmosphere 3 kg, 1.5W Positive charge distribution and concentrations Charged-Particle Retarding Potential Analyzer (ORPA) Measures the energy of ions in the ionosphere 2.8 kg, 2.4W Velocity, temperature and concentration of most abundant ion species Gamma Ray Burst Detector (OGBD) Measure gamma ray bursts from outside the solar system 2.8 kg, 1.3W Gamma ray energies from 0.2 to 2 MeV Radio Science Experiments Occultation of X and S bands (atmosphere) Doppler shifts (spacecraft accelerations)
Spacecraft Configuration
Attitude Determination and Control Shape and weight distribution conform to basic mechanical requirements for a spin stabilized vehicle Roll-to-Pitch Ratio greater than one Attitude determination Dual slit sun sensor (x3) Star sensor Propulsion provided control Spin rate Attitude control Orbit insertion
Attitude Control Thrusters
Propulsion Attitude control 7 total thrusters Liquid monopropellant hydrazine (N 2 H 4 ) 23.78 kg of propellant required (spin, despin, orientation) Catalytic decomposition 4.45 N of thrust (each) Blow-down mode System Tanks and Pressurant (He) Resevoir (60 sec of propellant 5 sec of spin up thrust required) Filters Feedlines Heaters (to prevent freezing) Valves Thrust chamber Nozzle
Propulsion
Propulsion Orbit insertion 1 thruster Solid propellant 18000 N of thrust provided V ~ 1.05 km/sec System Tank Insulation Safe and arm unit (igniter) Nozzle
Power Requirements 305.1W (EOL) at Venus 28V bus (30V max) 432 W-hr stored batter energy Two, 24 cell 7.5 A-hr NiCd batteries Launch Eclipse Periapsis (high temperature) Solar Arrays provided power in sunlit portion of orbit (7.4 m 2 ) More efficient cell identified then baselined in CDR Mass savings of over 10 kg $12K/kg cost for weight savings Better resistance to high temperature degradation Better resistance to radiation degradation High reliability with either cell Assembly time is the same New cells are more easily replaced Interconnects had to be shielded from ambient plasma environment
Power
Thermal Up to 1.98 Earth solar intensity Passive and Active thermal control techniques utilized Insulation Blankets Thermal finishes (white paints) Power Subsystem heat dissipation to equipment shelf Heaters (propellant) Louvers and Radiators
Thermal
Comm and Data Handling Spacecraft to Earth S-Band Transmitter (2.295 GHz) Receivers on ground: Deep Space Network US, Australia, Spain, Guam, Chile Ground Stations Antenna dish on board Parabolic, 109 cm diameter Two additional omni-directional antennas on board (redundancy) X-band used only for occultation experiments Twelve telemetry data rates between 8 and 2048 bits/sec available 1024 bits/sec used during interplanetary travel Data storage of 524Kb 1024 minor frames of telemetry Mostly for occultation (behind Venus) 26 min occultation implies 1Mb at 672 bits/sec
Operations Commands for orbit insertion sent to spacecraft from the ground hours before maneuver No communication with the orbiter during or immediately after the 30 sec burn Occultation event Each subsequent occultation event Stored commands to operate instruments Store data in memory Send telemetry Power budgets addressed by operational and scientific need Periapsis vs. apoapsis
The Multiprobe (PV2) Requirements Same basic structure as PV1 (for bus) Bus spin stabilization Disk shape House 1 large and 3 small probes near cg plane Probes to inner atmosphere and surface Not required to survive impact with surface Solar thermal environment of up to 2 Bus pointing accuracy for probes and instruments Atlas / Centaur upper stage Limited mass (905.4 kg completion / 848.2 kg inception) 15g max vibration load Fundamental frequency >4Hz Shroud constrained dimensions Clamping arrangement (LV interface)
The Multiprobe (PV2) Spacecraft element Spacecraft bus Similar in design to Orbiter 2 scientific instruments Five basic assemblies Large probe support Small probe support Equipment shelf Solar Panel Thrust tube Large probe Released from bus Seven scientific instruments Three small probes Released from bus after large probe Three scientific instruments
Multiprobe (PV2)
Large Probe (Payload) The Pioneer Venus large probe was equipped with 7 science experiments, contained within a sealed spherical pressure vessel. 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. Pressure vessel was Titanium filled with 102 kpa of Nitrogen. Weight: 315 kg 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
Large Probe 1 Radio Transparent Window 2 Aft Cover 3 Antenna 4 Pressure Vessel 4.1 Parachute Tower 5 Cloud Particle Spectrometer 5.1 Neutral Mass Spectrometer 5.2 Solar Flux Radiometer 6 Deceleration Module 4.1 5.2 5.1
Large Probe
Small Probes The three small probes were identical 0.8 m in diameter, 90 kg Spherical pressure vessels filled with 102 kpa of Xenon Unlike the large probe, they had no parachutes Aeroshells did not separate from the probe Instruments 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.
Small Probe 1 Antenna Housing 2 Temperature and Pressure Sensors 3 Deceleration Module (carbon phenolic heat shield) 4 Hermetically Sealed Container 5 Nephelometer 6 Net Flux Radiometer
Probes
Attitude Determination and Control For the spacecraft bus, the ADCS is similar to the Orbiter Bus spacecraft had to position itself for Large Probe Release Reposition itself for release of Small Probes (all three simultaneous) Final reposition for Bus re-entry Bus center of mass shifts significantly during these maneuvers
Propulsion Attitude control Same as for Orbiter with one less thruster 6 total thrusters Liquid monopropellant hydrazine (N 2 H 4 ) 15.34 kg of propellant required ( V, spin, despin, orientation) Catalytic decomposition 4.45 N of thrust (each) Blow-down mode No orbit insertion motor required
Comm and Data Handling Data Handling for Multiprobe was identical to the Orbiter except it had no data storage Data systems for probes was handled by the individual probes after separation Two omni-directional and one medium-gain horn antenna were used During separation and entry into the atmosphere, the DSN communicated with 6 separate spacecraft (Orbiter, Bus, Large Probe, 3 Small Probes) All spacecraft communicated directly to Earth
Scientific Results
REFERENCES Fimmel, R., Colin, L., Burgess, E., Pioneer Venus, NASA SP-461, 1983. Brodsky, R., Pioneer Venus: Case Study in Spacecraft Design, AIAA, 1980. Pioneer Venus, Press Kit, Release 78-68, NASA, 1978. Venus and Mars: Atmospheres, Ionospheres and Solar Wind Interactions. (edited by J.G. Luhmann, M. Tatrallyay and R.O. Pepin) 225-236, American Geophysical Union, 1992.