Why is the mission called Rosetta?

It is named after the Rosetta Stone, a slab of volcanic basalt found near Rashid (Rosetta), Egypt, in 1799. This stone tablet revolutionised our understanding of the past. By comparing the carved inscriptions on the stone (written in two forms of Egyptian and Greek), historians were able to decipher the mysterious hieroglyphics - the written language of ancient Egypt. The Rosetta Stone provided the key to an ancient civilisation. ESA’s Rosetta spacecraft will allow scientists to unlock the mysteries of the oldest building blocks of our Solar System - the comets.

What are the mission’s objectives?

Rosetta's main objective is to help understand the origin and evolution of the Solar System. A comet’s composition reflects the composition of the pre-solar nebula, out of which the Sun and the planets of the Solar System developed, more than 4600 million years ago. So, an in-depth analysis of Comet 67P/Churyumov-Gerasimenko by Rosetta will provide essential information to understand how the Solar System formed.

What makes the Rosetta mission so special?

Rosetta will be undertaking several ‘firsts’ in space exploration. It will be the first mission to orbit and land on a comet. That makes Rosetta one of the most complex and ambitious missions ever undertaken. Scientists had to plan in advance a ten-year trip through the Solar System. Approaching, orbiting, and landing on a comet require delicate and spectacular manoeuvres. We know very little about its actual surface properties, so we will only be able to choose a safe landing scenario when we get there.

Rosetta is very special because of the unique science it will perform. No other previous mission has had the potential to look back to the infancy of our Solar System and investigate the role that comets may have played in the beginnings of life on Earth. Rosetta will be the first spacecraft to witness, at close proximity, how a comet changes as it approaches the increasing intensity of the Sun’s radiation. Rosetta’s lander will obtain the first images from a comet’s surface and make the first in-situ subsurface analysis of its composition.

Why is it so important to study comets?

Comets are very interesting to scientists because they are most probably the most primitive bodies in the Solar System, preserving the earliest record of material from the nebula out of which our Sun and planets were formed. Planets have gone through chemical transformations, but comets have remained almost unchanged. There is convincing evidence that comets played a key role in the evolution of planets, because cometary impacts are known to have been much more common in the early Solar System than today. Comets, for example, probably brought much of the water in our oceans today. They could even have provided the complex organic molecules that may have played a crucial role in the evolution of life on Earth.

How much do we already know about how the Solar System formed?

The Solar System formed about 5000 million years ago when a cloud of gas and dust – called the ‘pre-solar nebula’ started to collapse due to gravitational forces. A disc of leftover material - made of the same gas and dust present in the primaeval cloud - formed around the still-forming Sun. After the Sun ‘ignited’ due to its own gravitational energy and began its life as a star, most of the particles in this disc collided and stuck to one another, growing in size until they became the planets and the other Solar System bodies.

However, it took some time before the Solar System became the way it is now. About 4500 million years ago, it was still 'under construction'. The interplanetary space was still littered with clouds of dust particles. Most of these chunks hit and merged with the planets, but thousands of millions of them survived – they are the comets and probably many of the asteroids we know today.

What will Rosetta look for to find out about the beginnings of life on Earth?

Previous studies by ESA’s Giotto spacecraft and ground-based observatories have shown that comets contain complex organic molecules. These are compounds that are rich in carbon, hydrogen, oxygen and nitrogen. Intriguingly, these are the elements that make up nucleic acids and amino acids, essential ingredients for life as we know it. Although Rosetta may not give us a definitive answer to whether life on Earth began with ‘comet seeding’, it will provide a wealth of information. For example, the mass spectrometers on the orbiter and the lander will analyse, more precisely than ever before, the kind of organic molecules present in the comet. Laboratory simulations of interstellar processes showed that such instruments can detect a variety of amino acids.

How exactly will Rosetta find out if comets brought the water to Earth?

Comets may have delivered much of the water of the Earth's hydrosphere, the surface water on Earth. Rosetta will investigate this by analysing the ‘isotopic abundances’ in the cometary ices. The 'isotopes' of a certain chemical element are atoms of the same kind that differ slightly in weight. Deuterium, for example, is an isotope of hydrogen. If the hydrogen-to-deuterium ratio in our oceans is similar to that in the cometary ice, it may strongly support the theory that a fraction of Earth's water has its origin in space.

How long will the spacecraft operate?

Rosetta’s planned lifetime is about 12 years. The (nominal) mission ends in December 2015, after the comet has reached its closest point to the Sun and is on its way back towards the outer Solar System.

How long will the lander operate on the comet nucleus?

The Rosetta lander will touch down on the comet's surface in November 2014. The science observations will start immediately. During the first 65 hours– the minimum mission target – a first run of the most important scientific measurements will be completed. During this phase the lander can operate on battery power, should this be necessary. In a second phase that is meant to last up to three months, a secondary set of observations will be conducted, using the remaining battery power and the energy from the solar cells on the lander. However, no one knows precisely how long the lander will survive on the comet.

Will the activity on the comet's surface 'kill' the lander?

Survival of the lander depends on a number of factors, such as power supply, temperature and surface activity on the comet.

What scientific instruments are on board the spacecraft and what will they do?

Rosetta's goal is to examine the comet in great detail. The instruments on the orbiter include several cameras, spectrometers and a number of sensors and experiments that work at different wavelengths - infrared, ultraviolet, microwave and radio. The instruments on the lander will do an on-the-spot analysis of the composition and structure of the comet’s surface and subsurface material. A drilling system will take samples down to 30 centimetres below the surface and will feed these to the ‘composition analysers’. See http://www.esa.int/rosetta.

How were these instruments selected?

The most important factors in the selection of each instrument were their expected scientific performance and their technical feasibility. How the instruments fitted together was another consideration, as well as the experience of the team proposing the instrument. This selection was done on the basis of the ‘Announcement of Opportunity (AO)’ open competition issued by ESA to the scientific community. This defines the mission scientific objectives and requirements, and the scientific community had to be compliant with these when submitting their proposals.

Why is ESA a pioneer in cometary exploration? Which other countries are working in this field too?

In 1986, ESA’s Giotto spacecraft performed the closest comet fly-by ever at a distance of 200 kilometres. It sent back pictures and data showing that comets contain complex organic molecules. Giotto continued its successful journey and in 1992 flew within 200 kilometres of Comet Grigg-Skjellerup. Now scientists will be eagerly waiting to be able to answer some of the new intriguing questions that arose from analysing the exciting results from Giotto.

Other past missions that have flown by a comet were: NASA’s ICE mission in 1985, the two Russian Vega spacecraft and the two Japanese spacecraft Susei and Sagigake that were part of the armada that visited Comet Halley in 1986. In 2001, NASA’s Deep Space 1 went to Comet Borelly and in January 2004 NASA’s Stardust spacecraft flew by Comet Wild 2 and will return samples of the comet’s coma in 2006. Unfortunately, NASA’s Contour launched in 2002 failed when it was inserted onto its interplanetary trajectory. Later this year we will see the launch of NASA’s Deep Impact, a spacecraft that will shoot a massive block of copper into a comet nucleus.

What is known about the comet that Rosetta will land on?

Comet 67P/Churyumov-Gerasimenko is a large, dirty snowball whose orbit around the Sun takes 6.6 years. Ground-based telescopes have observed 67P/Churyumov-Gerasimenko during almost all its appearances since its discovery in 1969. To acquire as much information as possible about the comet, ESA made a rigorous series of ground and space-based observations. These observations have provided us with a fairly reliable estimate of the comet’s size and shape - about four kilometres in diameter.

How and where will Comet 67P/Churyumov-Gerasimenko be at the time of the rendezvous?

Rosetta will meet 67P/Churyumov-Gerasimenko when it is still in the cold regions of the Solar System at around 675 million kilometres from the Sun.

When was the last time Comet 67P/Churyumov-Gerasimenko came close to the Sun? Was it visible with the naked eye from Earth?

Comet 67P/Churyumov-Gerasimenko last passed by its perihelion - its closest point to the Sun – in April 2002. Even at that point, when its brightness was at its maximum, it was impossible to see it with the naked eye. Only medium or large telescopes can observe it.

What is the gravity on Comet 67P/Churyumov-Gerasimenko?

Comet 67P/Churyumov-Gerasimenko is so small that its gravitational pull is millions of times weaker than on Earth. For this reason, the Rosetta lander will touch down at no more than walking pace. It will need its ‘harpoon’ system to safely anchor it to the comet’s surface and prevent it from bouncing back into space.

Why is the comet called 67P/Churyumov-Gerasimenko?

Comet 67P/Churyumov-Gerasimenko is named after its discoverers, Klim Churyumov and Svetlana Gerasimenko, who ‘spotted’ the comet for the first time in 1969. The ‘P’ means it is a short-period comet, so its orbit around the Sun is well established and it takes less than 200 years to fulfil one revolution. The number 67 refers to Churyumov-Gerasimenko's position in the list of catalogued periodic comets.

How many comets are there?

There are thousands of millions of them. Most of them are located in one of the following two regions. One is the ‘Oort cloud’, at the edge of the Solar System. It is 100 000 times more distant from the Sun than the Earth and astronomers estimate it contains about 12 000 million comets. The second region is the ‘Kuiper belt’ formed by thousands of millions of comets just beyond Neptune’s orbit. The Kuiper belt is 30 to 50 times more distant than the Earth from the Sun.

Some comets escape from these regions and come to the inner Solar System. Scientists do not know how many comets come towards the Sun. About 880 of them have been catalogued so far, of which around 150 are periodic, meaning their orbits will repeat. In 2002 over 150 new comets were discovered (mostly ‘sun grazers’) by the ESA/NASA SOHO spacecraft.

What is the difference between asteroids and comets?

Comets are called 'dirty ice-balls', whereas asteroids, or minor planets, are ‘rocks in space’. The size of asteroids typically ranges from a metre to several hundred kilometres across. One of the main differences is that asteroids do not contain ‘volatiles’ (substances that ‘sublimate’ i.e. they pass directly from the solid to the gaseous state when heated) or other frozen material. Therefore asteroids do not develop a tail when they approach the Sun. The comet’s tail is due to the frozen gas vaporizing and dragging small dust grains with it into the surrounding space. In this way the comet’s atmosphere, the ‘coma’ is formed and evolves, and the ion and dust tails form. There is evidence that some asteroids are ‘dead comets’, comets that have lost their volatile materials after many approaches to the Sun.

Were new inventions developed for Rosetta?

The solar cells in Rosetta's solar panels are based on a completely new technology. Thanks to it, Rosetta will become the first space mission to journey beyond the main asteroid belt relying solely on solar cells for power generation. Until now, deep-space missions have used nuclear energy. The new solar cells allow Rosetta to operate over 800 million kilometres from the Sun, where levels of sunlight are only 4% those on Earth. This technology can be used in future deep-space missions.

Some of the systems to control the temperature inside the spacecraft are another example of new technological developments. Temperature control is a critical requirement for Rosetta. When the spacecraft is near the Sun, overheating has to be prevented by using radiators. However, in the outer Solar System, the hardware must be kept warm. Scientists achieve this by placing louvres over the radiators, among other techniques. Rosetta’s louvres are also a new European development.

What challenges will Rosetta have to face during its long trip?

Ensuring that the spacecraft survives the hazards of travelling through deep space for more than 10 years is one of the great challenges. Temperature control is very important. Rosetta will be travelling from the safer environment of near-Earth space to the cold regions beyond the asteroid belt. Experts conducted some very tough pre-launch tests to confirm Rosetta's endurance. For example, they heated the outside surfaces to more than 150 °C, and then cooled them to –180 °C in the next test. The instruments, kept insulated inside, survived undamaged.

Does Rosetta 'know' where to go and what to do, or will you have to send commands from the ground?

Rosetta will be operated from the ground. It is impossible to have the manoeuvres for the whole mission programmed before the launch because adjustments will have to be made at each stage of the journey. At the appropriate time, commands will be sent from Earth to adjust the spacecraft’s trajectory. But instructions from the ground can take up to 50 minutes to reach the spacecraft, so to overcome the delay, Rosetta must have the ‘intelligence’ to look after itself. This is done by its on-board computers, whose tasks include data management and attitude and orbit control. If any problems arise during the cruise, back-up systems will ensure that the spacecraft can remain operational during critical mission phases. For example, the spacecraft will automatically position itself with the solar panels facing the Sun, to avoid it becoming powerless.

Rosetta will be flying past the Earth in March 2005, November 2007 and November 2009. What are the dangers of crashing back to Earth?

The chances of Rosetta hitting the Earth are millions to one. Many spacecraft have previously flown this manoeuvre and none of them have ever gone off course and crashed. During the first Earth fly-by, Rosetta will pass no closer than 4500 kilometres – 10 times further than the ISS. The second fly-by will be at a distance of 1400 kilometres – three times the altitude of the ISS. The spacecraft’s position will be monitored continually and carefully adjusted before both fly-bys. There will also be plenty of fuel on board for orbital manoeuvres. If by any extremely remote chance an atmospheric entry did occur, it would disintegrate and burn up in the atmosphere, causing no damage on Earth.

What are the chances that Comet 67P/Churyumov-Gerasimenko changes course drastically before Rosetta reaches it? Very, very low. There would need to be a very unlikely dramatic event to alter its orbit.

Why does Rosetta need to be kept in hibernation during some of its trip?

To limit its consumption of power and fuel, and to minimise operating costs. When the spacecraft is hibernating, it spins once per minute while it faces the Sun, so that its solar panels can receive as much sunlight as possible. Almost all of the electrical systems are switched off, with the exception of the radio receivers, command decoders, and power supply.

How far will Rosetta get from the Earth and by when?

Between July 2011 and January 2014, Rosetta will record its maximum distances from the Sun: about 800 million kilometres, and Earth: about 1000 million kilometres.

Why is the mission so long?

The spacecraft has to go to 5 AU (Earth distances) from the Sun to rendezvous with Comet 67P/Churyumov-Gerasimenko. No current launch vehicle is powerful enough to insert Rosetta directly into this orbit, so we need gravity assists from Earth and Mars during four flybys in order to reach the comet.

What will happen with Rosetta once the mission is finished?

Rosetta's mission will end in December 2015 after a total lifetime of 12 years. There could be an extension, provided Rosetta ‘survives’ and there is power left. It is very likely that the spacecraft will remain close to the comet after the mission ends.

A lot has been said about saving money by reusing existing hardware and/or technology. Was any this done during the development of Rosetta?

Yes, much of the technology used for Rosetta is standard space technology, which means that it has been used already in other missions and will certainly be used again in the future. Some technologies were first developed specially for Rosetta, and have since been used on Mars Express and will be used again on Venus Express.

Why doesn’t Rosetta need to be shielded against impacting dust grains from Comet 67P/Churyumov-Gerasimenko?

Close to the Sun, comets release large quantities of dust and gas to make the magnificent tails that stretch through space and can sometimes be seen from Earth. When ESA’s Giotto mission crossed the path of Comet Halley in 1986, it was travelling at 68 kilometres per second. At that speed it was ‘sandblasted’ by dust grains. The spacecraft was struck 12 000 times, each one eroded a little more of the spacecraft’s shielding until 7.6 seconds before closest approach, the spacecraft plunged into a particularly dense region. The collision of a ‘large’ one-gram particle sent it spinning, temporarily breaking contact with Earth.

Rosetta will not experience this violence because, rather than making a headlong rush at Churyumov-Gerasimenko, it will fall gently into step with it. By keeping pace with the comet, the impact velocity of the dust grains is very much reduced and so Rosetta does not need the bulky shielding that Giotto required.

What efforts are being made to learn more about Comet 67P/Churyumov-Gerasimenko, while Rosetta is still en route?

Currently, an extraordinary ground-based effort is taking place to understand the physical condition of the comet. The Hubble Space Telescope has observed the comet, as has the European Southern Observatory’s New Technology Telescope. From now on, the comet will be tracked from Earth all the time, in an effort to learn as much as possible about its behaviour and composition. At present, the Rosetta science team believe that the comet is about four kilometres in diameter and almost as black as coal, reflecting only about 5% of the sunlight that strikes its surface.

They will soon be observing the comet with the Spitzer Space Telescope (SIRTF), using its infrared capabilities to learn more. In particular, a key parameter to be determined is the density of the comet. Once this is known, in conjunction with its size, astronomers will be able to calculate its gravitational field. This is an essential parameter that must be known before Rosetta’s lander ‘Philae’ is released, as it affects the speed with which the lander will strike the surface. The Rosetta spacecraft itself will make the final, precise determination of all these parameters once it is in orbit around Churyumov-Gerasimenko.