Pioneer Venus - Abstract

Pioneer Venus

Scott Patano

February 19, 1993

THE PIONEERS TO JUPITER Mission Characterization Assignment ASE 387.P by Sung H. Byun February 20, 1992
Introduction

Since the beginning of the space age, scientist had dreamed of sending probes to Jupiter. In 1962 the first true inter-planatary explorer Mariner 2, succeeded in escaping the Earth-Moon system and encountered Venus. By the mid 1960's a U.S planetary spacecraft had also flown to Mars. Based on this and other success with inner planets, NASA began to plan to meet the challenge of the other solar system. However, the distances between the outer planets are so vast, a way to bridge the space between the planets in a more efficient and economical manner was needed. In the late 60's celestial mechanicians began to solve fly by problems. If a spacecraft is aimed to fly close to a planet in just the right way, it can be accelerated by the gravity of the planet to higher speeds than could ever be obtained by direct launch from Earth. If a second more distant planet is in the correct alignment, the gravity boost given by the first encounter can speed the spacecraft onto the second. Jupiter, with its huge size and strong gravitational pull, could be used as the fulcrum for a series of missions to Saturn, Uranus, Neptune and Pluto. In addition the early 80's would offer an exceptional opportunity. At that time, all four giant planes would be in approxi-mateÊalignment, so that gravity assist maneuvers could be done sequentially. Such a multiplanet mission was named Grand Tour. The first essential step in the Grand Tour was a fly by of Jupiter.

Mission Objectives

Initially, the objectives of the Pioneer mission to the giant planets were:

To explore the interplanetary medium beyond the orbit of Mars.
To investigate the nature of the asteroid belt from a scientific stand point and to assess the belt's possible hazard to missions to the outer planets.
To explore the environment of Jupiter.

When the potential of the spacecraft to explore beyond Jupiter became clear, the objectives were extended:

If the first spacecraft to fly by Jupiter attained its scientific objectives, the second would be targeted to fly by Jupiter in such a way that the spacecraft would enter a trajectory that would enable it to reach Saturn.
The second spacecraft would then explore the Saturnian environment.

By early 1970, all scientific experiments had been selected:

Magnetic fields and plasma in inter-planatary space and planetary magnetic fields and trapped radiation in the magnetospheres of the planets were to be measured.

Polarimetric measurements and images of Jupiter, possibly Saturn, and of several satellites were to be taken.

Composition of charged particle beams in space were to be determined.

Cosmic rays were to be determined.

Planets were to be observed at ultraviolet and infrared wavelengths.

Asteroid and meteoroids were to be detected and the distribution of the meteoric dust observed.

The intensity and distribution of the zodiacal light were to be observed.

The radio communication signal was to be used to probe the planetary atmospheres during occultation.

The radio communications signal would be used to learn about the planetary masses from analysis of Doppler residuals.

Mission Constraints

The mission to Jupiter and Saturn posed many technical challenges. It would extend man's exploration of the solar system to a new scale : 780 million Km from Sun to Jupiter and another 650 million Km to Saturn. The vast distances to be covered by the spacecraft presented problems of communication. Not only the weakness of the radio signals but also because of the time delay in the information travelling to Earth from the spacecraft and radio commands transmitted from Earth to the spacecraft. This delay required that controllers on Earth become adept at flying the spacecraft 90 minutes out of step with the spacecraft itself at Jupiter and 170 minutes out of step at Saturn. Normally a spacecraft's power is supplied by converting sunlight to electricity. Because of great distance between the Sun and Jupiter, sunlight at Jupiter's orbit is only 1/27 as intense as at Earth's orbit and at the distance of Saturn sunlight is only 1/90. So a spacecraft bound for the outer solar system must carry a nuclear energy source to generate electricity. Because of the high velocities required to reach Jupiter and Saturn the spacecraft and its components and its scientific instruments had to be lightweight. Additionally, between Mars and Jupiter stretches an asteroid belt which might seriously damage a spacecraft crossing it.

Mission Sequence and Trajectory

The launch vehicle boosted each space-craft in direct ascent with no parking orbit. Several in flight maneuver were to be made during the Pioneer 10 mission to target the spacecraft so that it would arrive at Jupiter at a time and position best suited to observe the planet. For Pioneer 11 the inflight maneuver were planned to preserve the option of continuing the mission to Saturn. After each craft had been carefully tracked and precise orbits calculated, small on board rockets were commanded to fire to correct its trajectory for exactly the desired fly by at Jupiter. Pioneer 10 was targeted to fly by the planet at a minimum distance of 3 Jupiter Radii from the center or 2 RJ above the clouds. This close passage, inside the orbit of IO, allowed the craft to pass behind both IO and Jupiter as seen from Earth, so that its radio beam could probe both the planet and its inner most large satellite. Pioneer 11 was intended to fly even closer to Jupiter, but the exact targeting option were held open until after the Pioneer 10 encounter. Pioneer 10 passed the orbit of Mars in June 1972, just 97 days after launch and in mid-July it began to enter the asteroid belt. In February 1973, the spacecraft emerged unscathed from the asteroid belt, having demonstrated that the concentration of small debris in the belt did not exist. On November 26, 1973 the encounter with Jupiter began. On that date Pioneer 10 detected a sudden change in the inter-planatary medium as the spacecraft crossed the point -the bow shock- at which the magnetic presence of Jupiter first becomes evident. At noon, the next day, Pioneer 10 entered the Jovian magnetosphere at a distance of 96 RJ from the planet. By December 2, the spacecraft had crossed the orbit of Callisto, the outermost of the large Galilean satellites. December 3, 1973 Pioneer 10 reached its closest point to Jupiter, 130,000 Km above ten Jovian cloud tops. Pioneer 11 continued to follow steadily, emerging from the asteroid belt in March 1974. Based on the performance and finding of Pioneer 10, it was decided to send Pioneer 11 still closer to Jupiter, but on a more inclined trajectory. On April 19, thrusters (DV adjust) on the spacecraft fired to move the Pioneer 11 aim point just 34,000 Km above the clouds of Jupiter. Pioneer 11 entered the Jovian magnetosphere on November 26, 1974 just a year after its predecessor. Closest approach took place on December 02. following their encounters with Jupiter. Both Pioneers spacecraft returned to their normal routine of measuring the interplanetary medium. Pioneer 10 had gained speed from the gravity field of Jupiter and became the first craft to achieve the velocity needed to escape from the solar system. Pioneer 11 had used the pull of Jupiter to bend its trajectory inward, aiming it across the solar system toward the Saturn.
List of Hardware Elements

General Structure
Cone shaped medium-gain antenna
Dish shaped high-gain antenna
Main equipment compartment
Scientific equipment compartment
Radioisotope thermoelectric generator
Magnetometer sensor

Attitude control and propulsion
One starlight sensor
Two sunlight sensors
Three pairs of rocket thrusters

Communications
Two identical receivers
Omni-directional and medium-gain antenna.
High gain antenna
Two radio transmitters

Thermal control system
Multi-layered blankets of aluminized plastic
Temperature-responsive louvers
Electric heaters
Twelve 1-W radio-isotope heaters

Electric power
SNAP -19-type radioisotope thermo-electric generators.

Scientific payloads
Asteroid-meteroid detector sensor
Ultraviolet photo meter
Imaging photo polarimeter
Geiger tube telescope
Asteroid detector sensor panel
Helium vector magnetometer
Plasma analyzer
Trapped radiation detector
Cosmic ray telescope
Flux-gate magnetometer
nfrared radiometer
Charged particle instrument

Mission Design Strength and Weak Points The pioneer craft rotated continuously around the axis pointed toward the Earth. This design was very stable and required less elaborate guidance than a non-rotating craft. In addition, the spin provided an ideal base for measurement of energetic particles and magnetic fields in space, since the motion of the spacecraft itself swept the sky and allowed data to be acquired rapidly from many different directions. However, the spin does not allow a stabilized platform on which to mount cameras or other instruments that require exact pointing. Thus, the spacecraft design was optimized for measurement of particles and fields in interplanetary space and in the Jovian magnetosphere, but had limited capability for observations of the planet and its satellites.

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