Pioneer Venus: Mission Characterization

Dr. Andrew KetsdeverLesson 2AME 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 CO2 (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/cm3 (⊕ 5.5 g/cm3)• 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 (Iz/Ip = 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 (N2H4) • 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 m2)– 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 Probe1 – Radio Transparent

Window2 – Aft Cover3 – Antenna4 – Pressure Vessel4.1 – Parachute Tower5 – Cloud Particle

Spectrometer5.1 – Neutral Mass

Spectrometer5.2 – Solar Flux Radiometer6 – Deceleration Module

5.2

5.1

4.1

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 Probe1 – Antenna Housing2 – Temperature and

Pressure Sensors3 – Deceleration Module

(carbon phenolic heat shield)

4 – Hermetically Sealed Container

5 – Nephelometer6 – 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 (N2H4)• 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.