Rosetta

The mission

Rosetta is the European Space Agency's first cornerstone mission in planetary exploration. Its mission is to orbit and land on a comet, following gravity assist flybys of inner planets (Earth three times and Mars once) and scientific flybys of asteroids. Launched successfully 2 March 2004 to comet 67P/Churyumov-Gerasimenko.

Science objectives

Rosetta is an ambitious and comprehensive mission consising of two spacecraft - an orbiter and a lander. The orbiter will be delivered to a comet, where it will become the first spacecraft to go into orbit around a cometary nucleus. The first encounter will be far out in the comet's orbit where it is cold and inactive. The lander will be deployed soon afterwards, to make the first ever measurements on the surface of a comet.

The orbiter will travel with the target comet as it nears the Sun, the nucleus warms, material boils away and activity develops. Close to the Sun, comets have two tails - a dust tail and a plasma tail which can be millions of km long. Rosetta will make detailed close-up studies of the comet's nucleus, including the development of activity, imaging studies, composition measurements and dust.

Comets are important to study as they are ancient objects, almost unchanged since the beginning of the solar system 4.6 billion years ago. They are often called the building blocks of the outer solar system, and are the nearest surviving objects to the early 'planetesimals'. Comets are thought to be stored in two 'reservoirs': the Oort cloud and the Kuiper belt. Every so often comets are nudged inwards in their orbits, passing through the inner solar system and in some cases being trapped in a lower solar orbit by Jupiter. Some comets, likely to be from the Oort cloud, have orbits with 'long' periods (eg Halley 76 years, Hale Bopp thousands of years) and others such as Wirtanen (Rosetta's original target) and Grigg-Skjellerup have interacted significantly with Jupiter and have 'short' periods near 5 years.

Comets are also important due to their role early in the solar system of bombarding the Earth's and other atmospheres with volatile substances like water and carbon based compounds. There is evidence that at the end of the solar system's early bombardment phase, 4.6 to 3.8 billion years ago, the compositition of Earth's atmosphere bore some similarities to the composition of comets. Comets therefore may have played an important role in bringing water and other volatiles to Earth and other inner solar system objects, in addition to outgassing of volatile material from the forming planets. They are also important because of collisions which can still happen, as seen with comet Shoemaker-Levy 9 hitting Jupiter in 1994.

In space exploration terms the missions to Giacobini-Zinner (ICE), Halley (Giotto, Vega-1 and 2, Suisei and Sakigake) and Grigg-Skjellerup (Giotto) started cometary exploration in the mid-1980s and early 1990s. The 2001 encounter with Borrelly (Deep Space 1) provided some more data, and Stardust flew by comet Wild-2 in 2004. Stardust images showed only the third cometary nucleus, after Halley and Borrelly, seen by humankind, and will return some cometary dust to Earth in 2006. The imminent NASA mission to Tempel 1 (Deep Impact, launch January 2005) will also provide vital and complementary data. But ESA's Rosetta has a relatively broad suite of scientific objectives compared to these smaller missions with more tightly focussed sceintific aims. Rosetta will take cometary exploration to the next stage by orbiting and landing on a comet.

At MSSL we have studied data from our Johnstone Plasma Analyser on Giotto, which visited Halley and Grigg Skjellerup, providing key information on how the ion tail forms, via 'ion pickup', at two different comets with a factor 100 difference in gas production rate. This process is also important in many other solar system contexts, including the 'scavenging' of the Mars and Venus atmospheres by the solar wind, and even in fusion experiments on Earth.

MSSL role in Rosetta

Andrew Coates of MSSL is a co-investigator in the Rosetta Plasma Consortium, and leads the science team for this set of five complementary sensors on the Rosetta Orbiter. The sensor teams are led by IRF-Uppsala and Kiruna (S), T.U.Braunschweig (D), SwRI (USA), and LPCE (F); the plasma interface unit is from IC (UK). We expect that the RPC instrument will be one of the first to detect the signs of activity as the comet warms. The UK involvement in RPC consists of Imperial College (plasma interface unit hardware) and UCL-MSSL (science team lead).

Scientific goals of the RPC include:

  • Study of onset of activity of a comet, and development of its interaction with the solar wind, as the spacecraft approaches the Sun. The gas production will change over many orders of magnitude during the mission, changing the activity from a quiescent, bare nucleus to a complex interaction region, with a plasma tail

  • Interaction between the nucleus and its environment: studies of conductivity, magnetization, sputtering, charging, dust levitation

  • First exploration of the detailed structure of the inner coma of a comet

  • First exploration of plasma tail formation region and determination of its relationship to other plasma boundaries

  • Determination of the role of tail rays, long observed at comets but role in tail formation and relation to in-situ data not understood as yet

  • Determination of permanence of boundaries in the comet-solar wind interaction

  • Determination of particle acceleration processes near the comet

  • Study of interaction between gas expansion, ionization and photochemical processes


    • Further Information

    The Rosetta homepage at ESA


    11 January 2005
    Andrew Coates
    ajc@mssl.ucl.ac.uk