11.1 Exploration of the Outer Planets

1. Composition of the Giant Planets

• The giant planets hold most of the mass in our solar system, with Jupiter alone exceeding the mass of all other planets combined.

• Their material can be classified into three types:

  • "Gases": Primarily hydrogen (75% by mass) and helium (24% by mass), the most abundant elements in the universe.

  • "Ices": Compounds formed from oxygen, carbon, and nitrogen. Common examples include water,methane , ammonia (${NH}_3$ ), carbon monoxide, and carbon dioxide.

  • Water (${H}_2{O}$ ) makes up approximately 0.6% by mass.

  • Methane (${CH}_4$ ) makes up approximately 0.4% by mass.

  • Ammonia (${NH}_3$ ) makes up approximately 0.1% by mass.

  • "Rocks": Less abundant than ices, including elements like magnesium (Mg), silicon (Si), iron (Fe), and others, making up approximately 0.3% by mass.

• Categorization of Giant Planets:

  • Gas Giants: Jupiter and Saturn are dominated by gases in their composition.

  • Ice Giants: Uranus and Neptune contain significantly more "ice" components in their interiors than the gas giants.

• Atmospheric Chemistry: The atmospheres of all four giant planets are dominated by hydrogen, leading to a reducing chemistry where other elements primarily combine with hydrogen (e.g., to form $ext{H}<em>2 ext{O}$, $ext{CH}</em>4$, $ext{NH}<em>3$, ethane ($ext{C}</em>2 ext{H}<em>6$), and acetylene ($ext{C}</em>2 ext{H}_2$)).

2. Robotic Exploration of the Outer Solar System

• Nine spacecraft have ventured beyond the asteroid belt to explore the giant planets.

• Challenges of Outer Solar System Exploration:

  • Long Flight Times: Missions require years to decades to reach the giant planets, compared to months for Venus or Mars.

  • Communication Delays: Messages can take hours to travel between Earth and the spacecraft, demanding highly reliable and autonomous designs.

  • Power Sources: Spacecraft must carry their own power or use very large solar arrays due to the Sun's distance.

  • Temperature Control: Heaters are necessary to maintain optimal operating temperatures for instruments.

  • Data Transmission: Powerful radio transmitters are required to send data back to distant Earth.

• Pioneer Missions (Pioneer 10 and 11):

  • Launched in 1972 and 1973, these were the first pathfinders to Jupiter.

  • Objectives: Navigate the asteroid belt and measure radiation hazards in Jupiter’s magnetosphere.

  • Findings: Successfully passed the asteroid belt; Jupiter’s magnetic field particles almost destroyed electronics, providing crucial design information for future missions.

  • Pioneer 10: Flew past Jupiter in December 1973, then sped toward the solar system limits.

  • Pioneer 11: Flew past Jupiter in December 1974, then used Jupiter's gravity to reach Saturn in September 1979.

• Voyager Missions (Voyager 1 and 2):

  • Launched in 1977, carrying 11 scientific instruments (cameras, spectrometers, magnetosphere devices).

  • These spacecraft are currently the most distant ever launched by humanity, still operating.

  • Voyager 1: Reached Jupiter in March 1979, then used a gravity assist to reach Saturn in November 1980.

  • Voyager 2: Arrived at Jupiter in July 1979, then followed a unique "Grand Tour" trajectory:

  • Saturn: August 1981

  • Uranus: January 1986

  • Neptune: August 1989

  • Grand Tour: Made possible by a rare alignment of the four giant planets (occurring once every ~175 years), allowing a single spacecraft to visit all of them using gravity-assisted flybys.

  • Voyager 2 at Neptune: Faced challenges due to age (arthritic arm, hard-of-hearing radio, memory loss, dwindling energy) and environmental factors (sunlight 900 times weaker than Earth, necessitating complex camera maneuvers at high speed).

3. Missions Sent to Orbit the Gas Giants

• Unlike flyby missions, orbiters are designed for detailed, long-term studies of a planet and its system.

• To date, no orbiter missions have been sent to Uranus and Neptune.

• Galileo Mission to Jupiter:

  • Launched in 1989, arrived in 1995.

  • Entry Probe: Deployed into Jupiter’s atmosphere for direct study:

  • Entered at a speed of 50 kilometers per second, the highest entry speed for any probe.

  • Heat shield endured temperatures up to $15,000 ext{ °C}$.

  • After slowing, a parachute deployed, allowing the probe to operate for an hour, descending 200 kilometers into the atmosphere before vaporizing.

  • Orbiter: Main spacecraft subsequently entered orbit to study Jupiter’s large moons.

• Cassini Mission to Saturn:

  • A joint venture between NASA and the European Space Agency.

  • Launched in 1997, arrived in 2004, and orbited Saturn.

  • Huygens Probe: Deployed an entry probe in January 2005 that successfully landed on the surface of Saturn’s large moon, Titan.

  • Conducted extensive studies of Saturn’s rings, moons, and the planet itself.

• Juno Mission to Jupiter:

  • Arrived at Jupiter in July 2016.

  • Objectives: Primarily to study Jupiter’s magnetosphere.

  • Orbit: Features a highly elongated (eccentric) 55-day orbit, ranging from 4,000 to 76,000 kilometers above the cloud tops, taking it over Jupiter’s poles for unique close-up views.

  • Camera: A simple downward-looking color camera was added, and raw images are explicitly shared for processing by “citizen scientists” to generate dramatic views of Jupiter.