So why is the spacecraft called ‘Rosetta’?  Just as the Rosetta Stone enabled Champollion to decipher the language of ancient Egypt (see for example
https://www.britishmuseum.org/explore/highlights/highlight_objects/aes/t/the_rosetta_stone.aspx), so the Rosetta space mission aims to probe the history of the Solar System by analysing one of its oldest remnants – a
comet.

As we said in Part 1 of this preview, the Rosetta spacecraft was launched on 2 March 2004 atop a European Ariane 5 launch vehicle. This was the first step in Rosetta’s epic odyssey to rendezvous with Comet 67P/Churnyumov-Gerasimenko in August 2014.  At the time I remember thinking, impatiently,“that’s 10 years in interplanetary space”, and that many of the people that work on the programme (and me) will be retired by the time we get to the pay-off!  However, despite this there was a lot of excitement about what Rosetta would discover once up close and personal with the comet.  But there was also an air of concern that some technical issue may occur onboard the spacecraft in the intervening 10 years that would deprive us all of the scientific revelation that would result from a successful mission.

Anyway, we are now at the other end of this long wait, and on the threshold of this revelation.  It has been a long, but eventful interplanetary journey, and the
spacecraft appears to be in good health – at least there is no indication from ESA that there is anything amiss technically with the spacecraft.  I guess that if there are technical issues which the operators believe they can work around, then it’s unlikely that the Agency or the spacecraft manufacturer (EADS Astrium) are going to be telling us about them at this stage.  So we can only assume that all is well with the mission, and we can look forward to a feast of astronautical history-making over the next few months.

After launch in 2004, the interplanetary trajectory was a complex series of gravity assist swing-by manoeuvres, three of Earth (March 2005, Nov 2007 and Nov 2009) and one of Mars (Feb 2007).  Using this technique, the spacecraft was able to acquire sufficient energy to reach the comet without having to expend excess onboard
propellant.  How this works is explained in some detail on pages 71 to 78 of the book How Spacecraft Fly, but very briefly the spacecraft executes a fly-by of the target planet.  This must be performed by flying‘behind’ the planet, with respect to the planet’s forward motion, and the direction of the outgoing trajectory of the spacecraft must be roughly aligned with the planet’s path along its orbit around the Sun.  Done in this way, the fly-by allows the spacecraft to steal a little of the planet’s momentum, which boosts the spacecraft's speed relative to the Sun. See the accompanying beautiful image of the Earth, taken by Rosetta during its third Earth gravity assist manoeuvres in November 2009.

During this phase of the mission, Rosetta also imaged 2 asteroids at close quarters –first asteroid 2867 Steins, followed by asteroid 21
Lutetia – see accompanying pictures.  The Steins encounter occurred on 5 Sept 2008, with Rosetta approaching within 800 km range at a fairly low relative speed of 8.6 km/sec.  As can be seen, Steins is an irregularly shaped object. 
It is also quite a small object with a maximum dimension of around 5 km.  The second asteroid encounter (Lutetia) occurred on 10 July 2010.  Lutetia is a significantly larger object, compared to Steins, being about 100 km across.  This time Rosetta passed by at a minimum range of about 3,170 km with a relative speed of 15 km/sec, resulting in images of this irregularly-shaped asteroid with a maximum resolution of approximately 60 m/pixel.

Watch out for the next episode of this Rosetta mission preview when we’ll take a look at the ‘deep-space hibernation’ phase of the mission – a bold and risky operation, and another ‘first’ for the Rosetta programme and the European Space Agency.

If all goes well with the European Space Agency (ESA) Rosetta mission, 2014 will be an historic year for the Agency and for astronautics in general.

The objectives of the mission are extremely ambitious, and it’s all about to happen this year and next. 
So I thought it a good idea to prime everyone with a timely preview of
what’s on the ‘menu’ of events for 2014/15.

The mission objectives of the Rosetta spacecraft are to chase a comet, rendezvous with it, orbit it and deploy an instrument package to land on its surface.  The comet orbiting and landing phases have never been attempted before, and if ESA can pull this off it will be a truly historic and remarkable achievement. 

You may ask the obvious question ‘Why go to a comet anyway?’.  Effectively comets are
giant dirty snowballs (typically 1 to 10 km in diameter), usually in highly eccentric closed orbits or open near-parabolic trajectories around the Sun.   So why is a load of dusty
iceballs so interesting? This is because they are generally believed to be the remnants of material left over from the formation of the Sun and planets – and as such are samples of very old (and generally uncontaminated) material left over from the original solar nebula dating from about 5 billion years ago. Consequently, analysis of the comet’s composition will hopefully tell us a good deal about the beginning of the Solar System.  Also it is generally assumed that comets were the original source of the abundance of water found on planet Earth.  During the formative years of the Earth, the Solar System was full of debris (including comets), and all of the larger bodies in the Solar System were subjected to violent bombardment.  Obvious evidence of
this period can be seen on the cratered surfaces of many planets and moons throughout the Solar System.  The
majority of the craters formed on the Earth during this period have not survived the extensive processes of erosion that occur on Earth, but the ocean’s of water are believed to be evidence of cometary impacts over time. 
Also long-range analysis of comets shows evidence of organic molecules, which raises the question about whether comets had anything to do with the rise of life on planet Earth.  All of these issues, and many others, will be addressed by the instrument package to be deployed on the comet’s surface.

Rosetta (see picture, and image heading up the ‘External links’ page of this website) is a large spacecraft, with a box-like central structure of approximate dimensions 2.8 m x 2.1 m x 2.0 m. Stretching either side of this structure are two large solar array panels with an area of 64 square meters presented to the Sun to raise power (840 Watts at a distance from the Sun of 3.4 AU – you may recall from the book How Spacecraft Fly that an Astronomical Unit (AU) is the average Earth-Sun distance of around 150 million km).  This gives a 32 meter total span across the spacecraft.  Communications with Earth are facilitated by a 2.2 m high-gain antenna.  The
total launch mass of Rosetta is about 3,000 kg, of which 2,900 kg comprises the comet orbiter and 100 kg the comet lander.  To undertake the various manoeuvres required during the mission, this mass budget includes 1,670 kg of rocket propellant.
 
The Rosetta was to be lofted by the European Ariane 5 launch vehicle, with the launch scheduled for January 2003.  At that time the target comet was identified as 46P/Wirtanen.  However, due to an Ariane 5 launch failure during 2002, the Rosetta launch was delayed to March 2004, and a new target comet had to be selected.  Consequently, the comet now subject to Rosetta’s scrutiny is 67P/Churnyumov-Gerasimenko.

The table below gives a concise summary of the main Rosetta mission events.  Watch this space for Part 2 of the Rosetta mission preview.

Recent images, from the Hubble Space Telescope, show evidence that water is erupting from the surface of Jupiter’s frozen moon Europa.  
 
Europa is one of the four largest moons of Jupiter, called the ‘Galilean moons’ as they were discovered by Galileo when he turned the first telescope on to the giant planet in 1610. Observations by various telescopes and passing spacecraft show Europa as an ‘ice planet’, having the appearance of a white snooker cue ball.  However closer examination principally by the spacecraft Galileo, which orbited the planet from 1995 until its suicidal dive into the planets atmosphere in 2003, shows the surface to be crazed with cracks and streaks.  It also apparent that surface craters are rare, suggesting a geologically young
surface.

Galileo’s (the spacecraft) observations of Europa raised huge interest in the late 1990s as they suggested that the moon may have an ocean of liquid water beneath the
outer ice crust.  Some background to this discovery is discussed briefly on pages 231-232 of the book How Spacecraft Fly.   The reason why scientists got so excited about this was the unlikely prospect that Europa may be the only place in the Solar System (other than the Earth) to host life.  The logic of the argument is something along the lines of the following.   Firstly, the existence of a sub-ice ocean suggests there must be some source of heat energy coming from below the surface to produce and maintain water in a liquid state.  The mechanism for this, it was realised, comes from consideration of Europa’s orbit around Jupiter which is elliptical.  Consequently, the distance varies by about 13,500 km every orbit period
of 3.55 days.  Because Jupiter is so massive, it causes significant tides in the solid structure of the moon.  So as Europa orbits, and its distance from Jupiter varies, it is squashed and stretched by Jupiter’s tidal forces.  This in turn produces an internal heat source capable of causing volcanic activity in the moon’s core.

So the current view is that beneath Europa’s icy surface there is a liquid water ocean maintained by tidal heating, with thermal vents in the ocean floor. 
These vents are likely to be similar to the thermal vents found in the deep ocean trenches here on Earth, where scientists have found unique forms of life energised not by the Sun but by geothermal energy (or volcanism).  So all this adds up to the idea that a
similar thing may be happening on Jupiter’s frozen moon.

The water spouts recently observed by Hubble gives further evidence to the notion of a sub-ice liquid ocean on Europa.  The water vapour plumes have been seen to rise to a height of 200 km above the surface, and it is estimated that about 7 tonnes of water per second is being ejected from the surface at about 700 metres per second.  Despite the vigour of these emissions, the water has insufficient energy to escape Europa’s gravity field, and the water falls back onto the moon’s surface.  Another characteristic is that the water erupts for around 7 hours at a time, and peaks when the moon is furthest from Jupiter, reinforcing the theory that the source of energy is derived from tidal effects in the moon.

This recent evidence is significant for three main reasons:
-        it reinforces the theory that a subsurface ocean exists,
-        it shows that the sub-ice ocean may be easily accessible from the surface,
-        and thirdly, there may be organic molecules on Europa’s surface.

Clearly, since the findings of the Galileo spacecraft, there has been a lot of interest in sending robotic spacecraft to the surface of Europa to investigate the intriguing idea of life on Europa.  It was believed that the ice crust may be kilometres thick, so that landing a sufficient source of energy to melt an access tunnel would be required to investigate the sub-ice ocean. Also, due to the moon’s relatively close proximity to Jupiter, it is very difficult to land a large payload mass on the surface of the moon, compounding this problem. However, the recent discovery of water vents suggests that the ocean may be more accessible than previously thought.