Exploring the Outer Planets: Composition and Robotic Missions
Learning Objectives
Provide an overview of the composition of the giant planets.
Chronicle the robotic exploration of the outer solar system.
Summarize the missions sent to orbit the gas giants.
Composition of the Giant Planets
Mass Distribution: The giant planets hold most of the mass in our solar system, with Jupiter alone exceeding the mass of all other planets combined. (Figure 11.2)
Material Classes: Materials available to build these planets are divided into three classes based on their composition:
"Gases": Primarily hydrogen (H_{2}) and helium (He), which are the most abundant elements in the universe.
Hydrogen: 75\% by mass.
Helium: 24\% by mass.
"Ices": Refers to compounds that form from the next most abundant elements: oxygen, carbon, and nitrogen. This term denotes composition, not necessarily a solid state.
Common ices include water (H{2}O), methane (CH{4}), and ammonia (NH_{3}).
Other ices may include carbon monoxide (CO) and carbon dioxide (CO_{2}).
Water (H_{2}O): 0.6\% by mass.
Methane (CH_{4}): 0.4\% by mass.
Ammonia (NH_{3}): 0.1\% by mass.
"Rocks": Less abundant than ices, including elements such as magnesium (Mg), silicon (Si), and iron (Fe).
Magnesium, Iron, Silicon: 0.3\% by mass.
Planet Categorization:
"Gas Giants": Jupiter and Saturn are dominated by gases, primarily hydrogen and helium.
"Ice Giants": Uranus and Neptune are sometimes called "ice giants" because their interiors contain a significantly higher proportion of "ice" components compared to Jupiter and Saturn.
Atmospheric Chemistry:
The chemistry of all four giant planet atmospheres is dominated by hydrogen.
This hydrogen abundance created a reducing chemical environment in the outer solar system.
Reducing Chemistry Implication: Other elements tended to combine with hydrogen first, meaning less oxygen was available to form oxidized compounds (like CO_{2}) more common in the inner solar system.
Consequently, compounds detected in giant planet atmospheres are mostly hydrogen-based gases, such as:
Methane (CH_{4})
Ammonia (NH_{3})
More complex hydrocarbons like ethane (C{2}H{6}) and acetylene (C{2}H{2}).
Robotic Exploration of the Outer Solar System
Overall Scope: Nine spacecraft have ventured beyond the asteroid belt into the realm of the giant planets.
Challenges of Outer Solar System Exploration:
Flight Times: Missions take years to decades to reach the giant planets, contrasting with months for Venus or Mars.
Communication Delays: Messages take hours to travel between Earth and spacecraft, even at the speed of light. This necessitates high reliability and autonomy for the spacecraft.
Power Sources: The Sun is too distant to provide sufficient energy, so spacecraft must carry their own power sources (e.g., radioisotope thermoelectric generators) or very large solar arrays.
Thermal Control: Heaters are required to maintain instruments at proper operating temperatures.
Data Transmission: Spacecraft need powerful radio transmitters to send data back to distant Earth.
Pioneer Missions (Pathfinders)
Pioneer 10 & 11: Launched in 1972 and 1973 by NASA as pathfinders to Jupiter.
Primary Objectives:
Determine if a spacecraft could safely navigate the asteroid belt without collision damage from dust.
Measure radiation hazards within Jupiter's magnetosphere (zone of magnetic influence).
Findings:
Both spacecraft passed through the asteroid belt without incident.
The energetic particles in Jupiter's magnetic field nearly destroyed their electronics, providing crucial data for designing radiation-hardened spacecraft for future missions.
Pioneer 10: Flew past Jupiter in 1973 and continued towards the solar system's limits.
Pioneer 11: Utilized Jupiter's gravity for a slingshot maneuver to reach Saturn in 1979.
Voyager Missions (The Grand Tour)
Voyager 1 & 2: Launched in 1977, marking the next wave of outer planet exploration.
Instrumentation: Each carried 11 scientific instruments, including cameras, spectrometers, and devices for magnetosphere measurement.
Distance Record: These are currently the most distant spacecraft launched by humanity, continuing outward after their planetary encounters.
Voyager 1: Reached Jupiter in 1979 and used a gravity assist to proceed to Saturn in 1980.
Voyager 2: Arrived at Jupiter four months later (July 1979) and followed a different trajectory.
Grand Tour Trajectory: Visited Saturn (1981), Uranus (1986), and Neptune (1989).
This ambitious trajectory was possible due to a rare alignment of the four giant planets on the same side of the Sun, occurring approximately once every 175 years.
This alignment allowed a single spacecraft to visit all of them using successive gravity-assisted flybys.
Engineering Challenges and Innovations (Voyager 2 at Neptune, 1989):
Age-Related Issues: After 12 years, the spacecraft showed signs of wear:
Camera/instrument arm was "arthritic" (limited movement).
Communications system was "hard of hearing" (partial radio receiver failure).
Onboard computer suffered "memory loss."
Generators showed significant wear, depleting power.
Neptune-Specific Challenges:
Sunlight at Neptune is 900 times weaker than at Earth, requiring much longer camera exposures.
The spacecraft was traveling at ten times the speed of a rifle bullet.
Solution: Engineers preprogrammed the computer for complex maneuvers to swivel the camera backward at a precise rate to compensate for forward motion during long exposures.
Communication Distance: The distance from Earth to Neptune is about 4.8 billion kilometers.
The power received from Voyager 2 at Neptune was approximately 10^{-16} watts.
NASA used 38 different antennas across four continents to collect and decode the faint signals.
Missions to Orbit the Gas Giants (Orbiters and Probes)
Transition from Flybys: Pioneer and Voyager were quick flyby missions; more detailed studies required spacecraft to orbit the planets.
To date, no orbiter missions have been started for Uranus and Neptune.
Galileo Mission (Jupiter Orbiter)
Launch and Arrival: Launched toward Jupiter in 1989, arrived in 1995.
Dual Approach: Orbiter and entry probe.
Entry Probe:
Deployed into Jupiter's atmosphere for direct studies of its outer layers.
Entered at a shallow angle with a speed of 50 kilometers per second (fast enough to travel from New York to San Francisco in 100 seconds).
This was the highest speed at which any probe has entered a planet's atmosphere, placing extreme demands on its heat shield.
Atmospheric friction slowed the probe within 2 minutes, generating temperatures up to 15,000^\circC at the front of the heat shield.
At a speed of 2500 kilometers per hour, the glowing heat shield was jettisoned, and a parachute deployed.
Operated for an hour, descending 200 kilometers into the atmosphere.
The polyester parachute melted a few minutes after the hour.
Within a few hours, the probe's main aluminum and titanium structure vaporized.
Data from the probe was relayed to Earth via the main Galileo spacecraft.
Main Spacecraft (Orbiter): After receiving probe data, it fired retro-rockets to enter orbit around Jupiter.
Primary objectives were to study Jupiter's large and often puzzling moons.
Cassini Mission (Saturn Orbiter)
Cooperative Venture: Between NASA and the European Space Agency (ESA).
Launch and Arrival: Launched in 1997, arrived at Saturn in 2004.
Dual Approach: Orbiter and entry probe.
Orbiter Objectives: Began extensive studies of Saturn's rings, moons, and the planet itself.
Huygens Probe: In January 2005, Cassini deployed the Huygens entry probe into the atmosphere of Saturn's large moon, Titan, successfully landing on its surface.
Juno Mission (Jupiter Orbiter)
Arrival: Arrived at Jupiter in July 2016.
Primary Objectives: To study Jupiter's magnetosphere, building upon previous missions which focused on moons and atmosphere.
Orbit: Has a highly elongated (eccentric) 55-day orbit.
This orbit takes it from 4,000 kilometers above the cloud tops out to 76,000 kilometers.
The orbit passes over Jupiter's poles, providing unique close-up views of these regions (previous spacecraft primarily viewed lower latitudes).
Camera and Citizen Science:
Juno was initially designed without a camera, but a simple downward-looking color camera was fortunately added.
Raw images are posted online, encouraging "citizen scientists" to process them.
This initiative has yielded many dramatic, brightly colored views of Jupiter's polar storms and clouds. (Figure 11.3b)
Other Missions Utilizing Giant Planets
Ulysses Spacecraft: Designed to study the Sun, it flew past Jupiter in February 1992 for a gravity assist.
New Horizons Spacecraft: Designed to study Pluto, it flew past Jupiter in February 2007 for a gravity assist.