The European Space Agency's Mission to Send the Rosetta Satellite to the Comet Wirtanen

Ben Thoma

In 2003, the European Space Agency (ESA) will launch the Rosetta Satellite on a ten-year mission to land on the comet Wirtanen. Scientists at ESA hope to gather information to explain some of the phenomena of comets, Earth, and the Universe. There are four main stages that must be completed for the mission to be successful: launching of the satellite, voyaging to the comet, orbiting the comet, and landing on the comet's surface. Upon reaching the comet, the satellite will perform several experiments on the comet's atmosphere and surface. This article assesses what scientists know about comets, describes the four main stages of the mission, and gives an overview of the experiments that will be performed.


In the quest to explore space, man has landed on the moon, brought a mechanical rover down on Mars, and last October launched Cassini, a satellite headed to visit a Saturn moon [Ahlstrom, 1997]. Coming next will be an achievement of a different sort: sending a satellite to rendezvous with a comet and dropping a lander on its dirt-and-ice surface. In 2003, the European Space Agency (ESA) will launch the Rosetta Satellite on a ten-year mission to reach and to land on the comet Wirtanen.
Scientists have waited a long time for the chance to get a closer glimpse of a comet. Although there is a great deal of information that is known of comets, there remain many unanswered questions pertaining to their creation and existence. Comets are believed to be made of the material, originally from the outer part of the solar system, that didn't get incorporated into the planets [Hamilton, 1997]. The fact that comets are composed of unchanged primitive material makes them extremely interesting to scientists who wish to learn about conditions during the earliest period of our solar system. Scientists working at ESA hope to gather information with Rosetta to gain further insight to the origin of comets, Earth, and the Universe.
The purpose of this report is to present an overview of the Rosetta Mission to the comet Wirtanen. The four primary stages of the mission are the launch from Earth, the voyage to the comet, the orbiting about the comet, and the landing on the comet's surface. Discussion is limited to the satellite's voyage--beginning with its launch. Since the mission is still several years away, there are many aspects of the mission that have not yet been determined. As a result, this report will not discuss the development of the satellite or the telecommunication centers and their operational links with the satellite.
To show the importance of this mission, the first section of this report will summarize current knowledge of comets as well as discuss what scientists hope to learn about Wirtanen. The next section of the report outlines each of the four main stages during Rosetta's mission. Finally, the report presents some of the experiments that will be performed when the satellite is in orbit about Wirtanen and when the satellite's lander is on the comet's surface.

Knowledge of Comets

Comets are some of the oldest untouched objects in the solar system [Kronk, 1997]. Recent years have revealed that comets may have shaped the progress of life as we know it here on Earth. Many astronomers are convinced that early collisions between Earth and comets brought the vast amounts of water that make up our oceans. Comets may also be responsible for the mass extinction of the dinosaurs. What would life on Earth be like today if these suspected collisions had never occurred? To help understand the importance and potential of the Rosetta Mission, this report will begin by discussing comets. This section of the report will describe what scientists currently know of comets and explain what scientists hope to learn about comets upon completion of this mission.

What Scientists Currently Know of Comets

A comet is basically a ball of dust and ice. Most of a comet's existence is spent frozen solid in the far reaches of the solar system [Kronk, 1997]. Only when comets are relatively close to the Sun do they become the familiar ball-and-tail wonders. Just as with the nine planets in our solar system, comets orbit about the Sun. However, while the orbits of comets are quite elongated, the orbits of the planets are nearly circular. Unlike Earth, which takes only one year to orbit the Sun, comets may need thousands or even hundreds of thousands of years to make one complete orbit around the Sun [Hamilton, 1997]. Scientists not only possess knowledge on the motion of comets, but they also possess knowledge on the different components of a comet.
At the heart of a comet is its nucleus. The surface of the nucleus is usually described as a black crust [Kronk, 1997]. Underneath this crust, the nucleus contains many frozen gases, the most common being water. Scientists believe that water makes up 75-80 percent of the volatile material in most comets [Hamilton, 1997]. Other frozen gases include carbon monoxide, carbon dioxide, methane, ammonia, and formaldehyde. As a comet approaches the Sun, it begins to warm, causing the frozen gases to sublime. Pressure under the nucleus's crust begins to build until it erupts and shoots outward like a geyser--referred to by astronomers as a jet. Dust at the surface is mixed and blown out with the erupting gases. As more and more jets appear, a dust shell is formed around the nucleus. This dust shell is called the coma. Shown in Figure 1 are the different components of a comet.

Figure 1. Different components of a comet [Hamilton, 1997]. The nucleus is at the heart of the comet. As the comet nears the Sun, gases and dust explode out from the surface forming the visible coma and dust tail. The ion tail blows back opposite the Sun because of the Sun's radiation pressure and solar winds.

Along with the nucleus and coma, a comet possesses a dust tail and an ion tail. As the comet flies through the solar system at velocities up to several miles per second, the coma is blown backwards forming the comet's dust tail [Ahlstrom, 1997]. This backward blowing is similar to the situation when a flag attached to a car is blown backwards in the opposite direction of the car's motion. A comet's coma and dust tail are seen because they reflect sunlight toward Earth [Hamilton, 1997]. The ion tail is formed when the Sun's radiation pressure and solar winds accelerate ions away from the comet. Unlike the dust tail, the ion tail is blown in the opposite direction of the Sun regardless of the direction that the comet is travelling. If the comet is travelling away from the Sun, the dust tail is blown back behind the comet towards the Sun, and the ion tail is blown in front of the comet in the direction the comet is moving. When this situation occurs, the two tails are pointing in opposite directions from one another.
Although comets vary in size, a typical comet's nucleus is around 10 kilometers in diameter, and its coma is several thousand kilometers in diameter [Kronk, 1997]. One of the largest comets in history was the Great Comet of 1811. Its nucleus has been estimated to be between 30 and 40 kilometers in diameter with a coma that reached a diameter roughly equivalent to that of the Sun. A comet's dust and ion tails can extend for millions of kilometers. The current record holder for longest tail length is the Great Comet of 1843, which had a tail that extended for more than 250 million kilometers. If the comet's nucleus were placed at the center of the Sun, the comet's tail would have extended beyond the orbit of Mars.
Scientists theorize that comets originate from a cloud of dust and gas referred to as the Oort Cloud [Kronk, 1997]. The Oort Cloud was first theorized in 1950 by the Dutch astronomer Jan Oort. After studying comets with very long orbital periods, he concluded that a large "cloud" of comets existed far outside the solar system, possibly within the range of 5 trillion to 8 trillion kilometers from the Sun. It is believed that the objects within this cloud are occasionally ejected by collisions with one another or by the gravitational forces of stars. The Oort Cloud is only a theory because it has never been directly detected.

What Scientists Hope to Learn of Comets

Despite already knowing a great deal about comets, scientists believe that there is still much to learn. The primary scientific objective of the Rosetta Mission is to study the origin of comets [Schwehm, 1998]. Scientists hope to determine the relationship between cometary and interstellar material and its implications with regard to the origin of Earth and the solar system.
Some scientists theorize that comets have both created and destroyed life on Earth. They believe that impacts of comets on primordial Earth delivered amino acids, the chemical building-blocks of life [Ahlstrom, 1997]. Additionally, there are scientists who believe that collisions of comets with Earth are responsible for the extinction of the dinosaurs. Upon completion of the Rosetta Mission, scientists hope to determine whether these theories are true. Also, an analysis of the comet may tell us whether dying comets simply evaporate and disappear or whether they lose their supply of dust and ice and become an invisible and inactive mass of tar and minerals. If the latter is true, Earth may be threatened by collisions with these dark masses that are circulating in Earth-crossing orbits.

Mission Stages

The Rosetta Satellite will undergo four main stages during its mission: launching from Earth, voyaging to the comet, orbiting the comet, and landing on the comet's surface [Schwehm, 1998]. The French satellite launcher Ariane-5 will launch the Rosetta Satellite in January 2003. The satellite will travel through space for eight years before reaching comet Wirtanen in 2011. During this period, the satellite will pass once by Mars, go twice by Earth, and fly by two asteroids (Mimistrobell and Rodari) for close observation. The Rosetta Satellite will then orbit the comet Wirtanen for one year before the Rosetta Lander (RoLand) delivers the Surface Science Package (SSP) in 2012. After analyzing the cometary nucleus for an additional year, the Rosetta Mission will end in July 2013.

Launching the Satellite

Ariane-5, the latest French satellite launcher, is scheduled to launch the Rosetta Satellite on January 21, 2003, from Kourou in French Guiana, South America [Schwehm, 1998]. Ariane-5 undergoes several steps when launching a satellite out of Earth's atmosphere [Ariane-5, 1998]. The launch begins with the ignition of the main stage booster, shown in Figure 2 (left). Upon confirmation that the main stage engine is operating properly, the two solid boosters are ignited to achieve lift-off. After the solid boosters have exhausted all their fuel, at an altitude of 60 kilometers, they are jettisoned and Ariane-5 continues its flight through the upper atmosphere propelled only by the main stage booster, as shown in Figure 2 (center). The fairing is jettisoned as soon as the atmosphere is thin enough for the satellite not to need the fairing's protection, as shown in Figure 2 (right). After the launcher adjusts its trajectory, the launcher releases the satellite to begin its eight-year voyage to Wirtanen. The launch trajectory of Ariane-5 is designed to ensure that the main stage engine, the two solid boosters, and the fairing fall safely into the Pacific Ocean for recovery.

Figure 2. Launch stages of Ariane-5 ["Ariane-5," 1998]. The left picture shows the main stage engine and solid booster stages igniting to achieve lift-off; the center picture shows the side boosters being jettisoned at an altitude of sixty kilometers; and the right picture shows the fairing being jettisoned and the satellite being deployed once the atmosphere is thin enough.

Voyaging to the Comet

Once launched, the Rosetta Satellite begins its eight-year voyage to the comet Wirtanen. Although there are long periods of inactivity during these eight years, the satellite will experience periods of immense activity. The voyage begins immediately after being released by the Ariane-5 launcher when the satellite deploys its solar panels, which span 32 meters across [Schwehm, 1998]. Before reaching Wirtanen, Rosetta will orbit past Mars once, twice by Earth, and past two asteroids, Mimistrobell and Rodari.
The satellite's first goal is to reach the planet Mars. Once the satellite is correctly on course to Mars, the satellite will switch off its scientific instruments [Schwehm, 1998]. The satellite will be in a hibernation mode for the 950-day flight to Mars. Once at Mars, Rosetta will perform one of three gravity-assist maneuvers around a planet. A gravity-assist maneuver occurs when a satellite (or a spacecraft) approaches a planet and uses the planet's gravitational force to accelerate and redirect itself. The satellite approaches Mars from one side and travels part way around the planet before it is thrown in a different direction away from the planet. Rosetta will perform two more gravity-assist maneuvers with Earth before reaching the comet. Without these gravity-assist maneuvers, the satellite would be unable to attain the velocities required to reach and to match speeds with the comet. These gravity-assist maneuvers are nothing new to the aerospace industry. The Voyager spacecraft, which has visited all nine planets (except Pluto) in our solar system, has depended upon this maneuver to reach the planets [Hamilton, 1997]. Recently, the Cassini Satellite performed its first gravity-assist maneuver around Venus to boost its speed to reach Saturn ["Cassini Mission," 1998].
Two months after swinging by Mars in 2005, the Rosetta Satellite will perform its second gravity-assist maneuver, this time around Earth [Schwehm, 1998]. During this short interplanetary phase, the satellite will be kept in an active cruise phase; it will not be put into hibernation.
After being in an active mode to perform the two gravity-assist maneuvers, the satellite enters another hibernation phase during its year-long voyage to the asteroid Mimistrobell [Schwehm, 1998]. Three months prior to reaching the asteroid, Rosetta's systems will be powered up to correctly fly by the asteroid. The aim is to pass the asteroid at a minimum distance of 600 kilometers on the sunward side. From this distance the satellite will be able to capture extremely close-up images of the asteroid. These images along with other scientific data will be recorded and transmitted back to Earth after the flyby. Once the satellite has finished analyzing the asteroid, it will perform orbit correction maneuvers required to put itself on course for the second Earth gravity-assist maneuver.
The next cruise phase, from Mimistrobell to Earth, lasts about 400 days [Schwehm, 1998]. Here the satellite will once again be placed in an energy-saving hibernation mode. The operations required to perform this last gravity-assist maneuver are essentially the same as the previous two. Similarly, the operations to reach and analyze the asteroid Rodari are essentially the same as they were for the asteroid Mimistrobell. The only major difference is that Rosetta will pass Rodari from a minimum distance of 1580 kilometers. Rosetta will spend the majority of its 1200-day journey from Rodari to Wirtanen in a hibernation mode. During this period, the satellite will reach its aphelion (farthest distance the satellite will ever be from the Sun) at a distance of 6.2 AU (1.2 billion miles) from the Sun and 5.2 AU (1.0 billion miles) from Earth. When the Rosetta Satellite finally reaches the comet Wirtanen 3443 days after being launched, it will be at 2.6 AU (502 million miles) from Earth.

Orbiting the Comet

Several steps must be taken before the satellite is in orbit about its host comet [Schwehm, 1998]. To begin this process, the satellite must decrease the relative velocity between itself and the comet to about 25 meters per second. As the satellite approaches Wirtanen, it must choose a suitable location to begin orbit. The primary criteria are to avoid any cometary debris and to achieve good comet illumination conditions. Large amounts of the comet's surface may be ejected into its coma by the jets of gases erupting at the surface. The satellite must choose a location where the erupting debris poses no danger. The satellite will also want to select a position on the sunward side of the comet. It is necessary for the comet's coma and nucleus to be illuminated for the satellite to navigate correctly and to map the comet's surface.
Once the satellite has slowed its relative velocity, it will begin the preliminary survey of the comet's surface. This survey is known as global mapping [Schwehm, 1998]. During global mapping, it may be determined that the surface is irregularly shaped. This irregular shaping would cause the gravity field of the nucleus to be irregularly shaped as well. If this is the case, a stable orbit, one that will not cause the satellite to impact the comet's surface, is still possible. At this point, Rosetta will be placed in orbit about the comet. While in orbit, the satellite will continue the global mapping operation, as depicted in Figure 3, at a distance between five and twenty-five nucleus radii. Once the comet's surface has been mapped, a site will be chosen for the Surface Science Package (SSP), which is contained within the Rosetta Lander (RoLand).

Figure 3. Artist's rendition of Rosetta Satellite orbiting the comet [Schwehm, 1998].

Landing on the Comet's Surface

One year after arriving at the comet's coma, more than nine years after its launch from Earth, the Rosetta Satellite will release the RoLand onto Wirtanen's surface [Schwehm, 1998]. As stated earlier, not all orbits are perfectly circular. A non-circular orbit will be chosen to bring the satellite within close range (as low as 1 kilometer) of the comet's surface. This "elongated" orbit will be chosen for two reasons. First, the elongated orbit minimizes the time spent at low altitudes to reduce the likelihood of being hit by debris, dust, and gas jets from the surface. Second, the elongated orbit makes communication easier. When the satellite is at its aphelion (farthest point from the comet), it will be in a better position to communicate information with scientists back on Earth. Then, when the satellite is back at its perihelion (closest point to the comet), it will be in a good position to transmit data to and from the SSP on the comet's surface.
An ejection mechanism separates the RoLand from the Rosetta Satellite with a maximum relative velocity of 1.5 meters per second [Schwehm, 1998]. The lander's three legs unfold from its sides as it falls to the comet's surface ["Surface Science Package," 1998]. If the cometary surface is uneven, the lander may tip over upon landing on the surface. In this case, the lander is equipped with an erection procedure to properly stabilize itself in an upright position. As soon as the lander has landed and is fully upright, its anchoring system is activated. An anchoring harpoon is shot into the surface and tightened to secure a firm fixation to the cometary ground. Once secured to the surface, the SSP will begin performing experiments to determine the nucleus's composition.

Summary of Events

There will be several major events during Rosetta's mission to the Comet Wirtanen (see Table 1). After being launched by Ariane-5 in 2003, the Rosetta Satellite will perform three gravity-assist maneuvers and fly past two asteroids during its eight-year voyage to the comet Wirtanen. In 2012, after orbiting the cometary nucleus for one year, the SSP will be lowered to the surface by the RoLand. The SSP will perform compositional experiments for one year before shutting down all of Rosetta's systems in July 2013. Table 1 summarizes these major events during the Rosetta Mission. Each major event is followed by the nominal date, the elapsed time since launch, the distance between Rosetta and the respective object, and the distance between Rosetta and Earth.

Table 1. Summary of Major Events of the Rosetta Mission [Schwehm, 1998]
Event Date Days Object Distance
Earth Distance
(km or AU*)
Launch from Earth 1/21/2003 0 0 0 km
Mars gravity assist 8/26/2005 948 200 0.69 AU
First Earth gravity assist 11/26/2005 1040 3332 3332 km
Mimistrobell flyby 9/15/2005 1333 600 2.34 AU
Second Earth gravity assist 11/26/2007 1770 2315 2315 km
Rodari flyby 5/4/2008 1930 1580 1.46 AU
Orbiting about Wirtanen 8/24/2011 3136 4-18 4.81 AU
Delivery of Roland 8/22/2012 3443 1 2.60 AU
Shutdown of systems 7/10/2013 3768 0 1.06 AU

astronomical unit (AU) represents 93 million miles.

Mission Experiments

Although reaching and landing on a comet is an enormous task, it is not the primary goal of the Rosetta Mission. The primary goal is to perform a series of experiments to learn more about comets, their origin, and their relation to life on Earth. Scientists have planned several experiments that the Rosetta Satellite will perform while in orbit about the comet and while the RoLand is on the comet's surface. These experiments may be divided into two major groups: atmospheric and surface.

Atmospheric Experiments

While in orbit about the comet Wirtanen, the Rosetta Satellite will perform several atmospheric experiments [Schwehm, 1998]. As the satellite approaches the comet for the first time, it will perform several experiments to determine the best path toward the nucleus. Once in orbit, Rosetta will map the topography of the comet's surface. This information will serve two major functions. First, scientists will need to select an appropriate landing site for the Rosetta Lander (RoLand). Second, scientists will use this information to determine an orbit free of debris, dust, and gas jets. After the mapping is complete, the satellite will release the RoLand onto the cometary surface. Once RoLand has been released, the Rosetta Orbiter (the part of the satellite that remains in orbit about the comet will begin a new phase of experiments on the comet's atmosphere.
While orbiting through the coma (the comet's atmosphere), the orbiter will analyze the composition of its surroundings [Schwehm, 1998]. For each orbit that the satellite completes, it will pass once through the ion tail and the dust tail of the comet (refer back to Figure 1). Scientists hope to study the effect of the solar winds on the comet's ion tail. Scientists will also analyze the elements that exist in the comet's dust tail in hopes of acquiring unknown facts. Since these elements have been locked away beneath the comet's frozen surface, they are untouched materials that are as old as time.

Surface Experiments

The surface experiments will be carried out when the RoLand delivers the Surface Science Package (SSP) to Wirtanen's surface [Schwehm, 1998]. The SSP consists of a wide range of experiments. These experiments may be classified as follows: time-dependent and non-time-dependent.
Some experiments require more time than others do. For example, a long period of time will be necessary to analyze the time-dependence of chemical and physical characteristics at the surface of the nucleus ["Surface Science Package," 1998]. A significant amount of time will also be required to study the time-dependent characteristics of temperature, thermal conductivity, and electrical conductivity. Medium-duration measurements include an acoustic and seismic investigation of the surface.
There are also several experiments that are not as dependent upon time. For example, samples of the nucleus will be taken from different depths for chemical analysis ["Surface Science Package," 1998]. A drill-like apparatus will remove samples from depths of at least 0.2 meters. Other instruments will investigate the elemental, molecular, and mineralogical composition and the morphology of the nucleus's material. Scientists are particularly interested in the results from these investigations because the material has been preserved in the cometary nucleus since its creation, which scientists believe occurred at the same time as the creation of the planets and the solar system.
All of the information gathered from both the SSP and the orbiter will continuously be transmitted to awaiting scientists back on Earth [Schwehm, 1998]. These scientists will be in constant communication with Rosetta until the last experiment is performed. The last command from Earth--shut down all systems--will leave the satellite as a piece of space junk orbiting about the comet, with its lander anchored to the surface below [Ahlstrom, 1997].


After ten years of development, the European Space Agency (ESA) will realize its goal of landing a probe onto a comet's surface when it launches the Rosetta Satellite on a ten-year mission to the comet Wirtanen. Scientists have been baffled as to the origin and make-up of comets since their discovery nearly 3000 years ago [Kronk, 1997]. Scientists at ESA believe that they will gain insight to these questions when their Rosetta Lander (RoLand) sends back the results from its atmospheric and surface experiments.
After being launched in August 2003 by the satellite launcher Ariane-5, the Rosetta Satellite will undergo three gravity-assist maneuvers--once by Mars and twice by Earth--during its eight-year voyage to reach Wirtanen. Also during its voyage, the comet will pass by asteroids Rodari and Mimistrobell. Scientists hope to acquire close-up images and gain new information from these asteroids.
Once Rosetta reaches Wirtanen in August 2011, it will orbit the comet for one year. During this time, Rosetta will perform remote-sensing experiments to map the surface's topography. The information gathered will be used to determine an acceptable landing site for the RoLand. In August 2012, more than nine years after its launch from Earth, the Rosetta Satellite will release the RoLand, which will secure itself to Wirtanen's surface. For the last eleven months of the Rosetta Mission, the RoLand will perform several experiments on the comet's surface. During this time, the Rosetta Satellite will also perform several experiments analyzing the comet's atmosphere, ion tail, and dust tail. Finally, in July 2013 Rosetta will receive its final command to shut down all systems.
Not only will scientists gain insight into the origin of comets and their composition, but they also hope to discover information revealing the secrets surrounding the creation of the Earth, the solar system, and the universe. These celestial bodies consist of material that has been preserved in the comet's nucleus since the dawn of time. The atmospheric and surface experiments could provide results that are instrumental to scientists who wish to learn of the prehistoric conditions during the creation of our solar system and of the universe. The overlying question is, What astonishing information will be revealed upon the completion of the Rosetta Mission?


aphelion: the point of a planet's or comet's orbit most distant from the sun. (Back)

astronomical unit (AU): unit of distance equal to the average distance between Earth and the Sun, approximately 93 million miles. (Back)

gravity-assist maneuver: process by which a satellite or spacecraft is accelerated and redirected around a planet. (Back)

gravity field: space surrounding an object, often a planet or sun, that causes a force of attraction on other objects. (Back)

interstellar: situated or occurring between the stars. (Back)

morphology: the branch of biology that deals with the form and structure of organisms. (Back)

orbital period (period): amount of time to complete one orbit. (Back)

perihelion: the point of a planet's or comet's orbit nearest the sun. (Back)

solar wind: the radial outflow of charged particles, mainly electrons and protons, from the sun. (Back)

sublime: to pass directly from the solid state to the vapor state without passing through the liquid state. (Back)

volatile: easily passing off by evaporation; being readily vaporized at relatively low temperatures. (Back)


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Ahlsrom, Dick, "To Catch a Comet," The Philadelphia Inquirer (20 October 1997), sec. D, pp. 1, 4.

Bradlow, "Equations for an Ellipse," (Urbana-Champaign: University of Illinois, May 1998).

"Cassini Mission Status," (Pasadena: Jet Propulsion Laboratory, January 1998).

Hamilton, Calvin, "Views of the Solar System," (Honolulu: Hawaiian Astronomical Society, 1997).

Kronk, Gary, "The Comet Primer," (1997).

Kronk, Gary, "46P/Wirtanen," (1998).

Schwehm, "The International Rosseta Mission," (Frascati, Italy: European Space Agency, March 1998).

"Surface Science Package for ROSETTA Mission," (Germany: Max- Planck Institut fuer Aeronomie, February 1998).

Warhaut, "Rosetta: ESA's Rendezvous Mission with a Comet," (Paris: European Space Agency, August 1997).

Author's Note: When he wrote this report, Ben Thoma was a senior in Mechanical Engineering at the University of Wisconsin. After finishing his degree, he began working at the Jet Propulsion Laboratory in Pasadena, California. Ben also studied engineering at Ecole Centrale Paris and interned in Paris at the automotive manufacturer, Renault. (Back to Beginning)

Last Updated: 11/98

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