Outer Planets

Overview

The outer solar system is by far the least-studied part of the solar system. The planets that this includes are Jupiter, Saturn, Uranus, Neptune, and Pluto. The first crafts to visit were Pioneer 10 and 11, followed by Voyager 1 and 2, together making a sweep of the four gas giants. The next probe to an outer planet was Galileo, which studied Jupiter in depth for several years. Now, NASA has sent a probe to study the next planet, Saturn, in depth for at least four years. Currently in orbit, Cassini is studying Saturn, and it launched the ESA-sponsored Huygens Titan probe in December of 2004, which landed on Titan in January 2005.

Cassini

There are two main purposes to this mission. The first is to deliver the Huygens probe to the Saturnian moon Titan. The second is to have the Cassini Orbiter explore Saturn, its rings, and satellites. The main objectives of the orbiter are as follows:

• to determine the three-dimensional structure and dynamical behavior of the rings
• determine the composition of the satellite surfaces and the geological history of each object
• determine the nature and origin of the dark material on Iapetus' leading hemisphere
• measure the three-dimensional structure and dynamical behavior of the magnetosphere
• study the dynamical behavior of Saturn's atmosphere at cloud level
• study the time variability of Titan's clouds and hazes
• characterize Titan's surface on a regional scale

The Cassini project is in orbit of Saturn, having been launched on October 15, 1997, and achieving orbit on June 30, 2004. After leaving Earth, it made two flybys of Venus, one more of Earth, and then of Jupiter (an image it took of Jupiter is to the right, and was taken on December 8, 2000, and also shows Ganymede; the image to the left was the first image Cassini took of Saturn (taken on October 21, 2002), and includes the moon Titan). This trajectory is commonly referred to as a VVEJGA (Venus-Venus-Earth-Jupiter Gravity Assist).

In December 2004, the Huygens probe separated and headed towards Titan, entering Titan's atmosphere in January 2005. The purpose of this probe was to learn about the characteristics (density, pressure, temperature, etc.) of Titan's atmosphere, measure the chemical make-up of the atmosphere - especially with regard to organic molecules, characterize the weather of Titan - particularly with respect to cloud physics, lightning discharges, and general circulation. Besides these atmospheric inquiries, the Huygens probe will also examine the physical state, topography, and composition of the surface of Titan.

To accomplish this, Huygens entered Titan's atmosphere. For two full minutes after entry, a heat shield protected the probe from the heat of entry. After this, a parachute deployed and the shield was jettisoned. The probe then started to take its data, and it represents the farthest location from Earth that a successful touch-down has occurred. The probe continued to take readings from the surface of Titan after it landed.

Cassini's mission is to orbit Saturn at least 30 times in loose elliptical orbits, each targeted for different purposes. Besides simple visual imaging, Cassini will map in radar, infrared, and ultraviolet frequencies. Cassini will also look for cosmic dust, conduct a radio and plasma wave experiment, and take magnetic measurements. It has a communications antenna to communicate with Earth, but it also has other transmitters to make observations of the atmospheres of Titan and Saturn and to measure the gravity fields of the planet and its satellites.