ESA's Rosetta Mission: A decade long quest for the keys to the universe

Recently a space mission called Rosetta, sent by the European Space Agency(ESA) more than ten years ago, has created a lot of excitement amongst scientists. It is extraordinary for its technological audacity and its brilliant performance as well as its promise of some truly magnificent data about the solar system in general and comets in particular. 

It was a probe built and launched by the European Space Agency to study comet 67P/Churyumov–Gerasimenko by orbiting and landing on its nucleus. It consists of two parts; an orbiter and a lander module Philae. The spacecraft is a box 2.8 x 2.1 x 2.0 metres in size. With its two 14-metre-long solar panels, from tip to tip, the spacecraft spans 32 metres, enough space for 16 people. This robotic spacecraft had a weight of 2,900 kg at launch, of which 100 kg was the weight of Philae. A further 1,670 kg was fuel and 1,230 kg was the instruments. The Rosetta space probe will orbit the comet and has 12 instruments, while Philae landed on the comet with nine additional instruments. It has been estimated that in the decade preceding 2014, some 2,000 people assisted in the mission in some capacity and it cost about 1.3 billion Euros.

Like most comets, comet 67P/Churyumov–Gerasimenko is named after its discoverers, Soviet astronomers Klim Ivanovych Churyumov and Svetlana Ivanovna Gerasimenko, who first observed it on photographic plates in 1969. It takes 6.45 years to go around the Sun and rotates on its axis with a period of approximately 12.4 hours. 67P is about 4.1 km by 4.3 km at its widest and longest dimensions. It is shaped like two large blobs connected by some gravel. Comet 67P loops around the Sun between the orbits of Jupiter and Earth, that is, between about 800 million and 186 million kilometres from the Sun. When close to the Sun, its maximum velocity will be 135,000 km/h (38 km/s). It will reach closest to the Sun on 13 August 2015. 

It took more than 10 years and 8 months for Rosetta to reach its goal; from launching on March 2, 2004 to reaching Comet 67P on August 6, 2014. We do not have rockets capable of directly sending a mission into such an orbit, and the team had to utilise Earth's gravity (2005, 2007 and 2009) and Mars' as well (2007) four times on a long circuitous trip that took ten years to complete.

Even before reaching comet 67P, the spacecraft flew past two asteroids on flyby missions on its way to the comet. In 2007, Rosetta also flew past Mars. It even flew past the Earth thrice in 2005, 2007 and 2009. The craft flew past asteroid 2867 Šteins in September 2008 and 21 Lutetia in July 2010. On January 20, 2014, Rosetta was taken out of a 31-month hibernation mode and continued towards the comet. In all this period, the mission had travelled a total distance of 6.4 billion kilometres before reaching 67P. Rosetta met 67P when it was still in the cold regions of the Solar System 673 million kilometres from the Sun, when the comet and Rosetta were on their return journey back into the inner Solar System.

The sequence of events that brought Rosetta close to the comet are very impressive. Beginning May 7, Rosetta began orbital correction manoeuvres to bring itself into orbit around 67P. At the time of the first deceleration burn Rosetta was approximately 2 million km away from 67P and had a relative velocity of 775 m/s (2,800 km/hr). It was then aligned to 67P by 18 June and its velocity was reduced to 91 m/s (300 km/hr) on June 18. It was further slowed down and, by July 23, the distance had been reduced to just over 4,000 km with a relative velocity of +7.9 m/s (28 km/hr). On July14, Rosetta returned images of Comet 67P which confirmed the irregular shape of the comet. On August 6 Rosetta arrived at 67P, approaching to within 100 km, and reduced its relative velocity to 1 m/s (3.6 km/hr). At this stage scientists began searching for the the optimum location for the Philae landing. On September 4, the first scientific study showed that the comet is unusually dark in ultraviolet wavelengths. It also showed that hydrogen and oxygen are present in the coma. It found no significant areas of water-ice on the comet's surface. On September10, 2014, Rosetta began orbiting 67P at an altitude of 29 km. On November 12, 2014 it released Philae from a height of 22.5 km. After a 7-hour descent Philae landed on the surface of 67P. Unfortunately, on landing, it bounced twice in the weak gravity of the comet and eventually landed with one of its legs not touching the ground. How inclined is the sitting position is not yet known.  It also landed in a shadow area and hence its Solar Panels are not receiving enough sunlight to charge the batteries. 

Even then, as scheduled, in the first 30 hours it completed most of its immediate tasks. Within these 30 hours Philae sent some excellent pictures of the surface of the comet, managed to put in harpoons into its soil and drill a hole to look at the inner material in the comet’s nucleus. 
It analysed the composition and structure of the comet’s surface and subsurface material. A drilling system obtained samples down to 23 cm below the surface and fed these to the spectrometers for analysis to determine the chemical composition. Other instruments measured properties such as near-surface strength, density, texture, porosity, ice phases and thermal properties. Microscopic studies of individual grains were also done. Having run out of its battery power, Philae is now waiting for sunlight to light up its solar panels so that it can begin working again. Although the future of the lander Philae is uncertain, the orbiter Rosetta is the workhorse of the mission and its work will carry on. The Rosetta mission will orbit 67P for 17 months and is designed to complete the most detailed study of a comet ever attempted. 

In order to study the comet, Rosetta orbiter has several cameras, spectrometers, a number of sensors, and experiments that work at different wavelengths – infrared, ultraviolet, microwave, and radio. They will provide, among other things, very high-resolution images and information about the shape, density, temperature, and chemical composition of the comet. Rosetta’s instruments will analyse the gases and dust grains in the coma that forms when the comet becomes hot in intense sunlight and begins to loose gaseous material in interaction with the solar wind. Rosetta is expected to send data till December 2015, when the comet has passed through the point of closest approach to the Sun and is on its way back to deep space. 

You can find more information on the official website of Rosetta. 

 This collage of navcam imagery from Rosetta was taken on Aug. 1, 2, 3 and 4.

So the question is, why should one take so much trouble to land on such a small piece of rock that involves so much technical difficulty?

Comets are the residues of the formation of the solar system. About 4,500 million years ago, a large rotating disc of gaseous matter (itself a residue from an earlier stellar explosion) slowly coalesced and the central region became heavy and heated up to a level where its internal heat ignited the hydrogen fuel in producing fusion reactions. This became the Sun. The remaining matter in the surrounding region then slowly coalesced into smaller pieces of rock. These formed the 8 planets of the solar system. The planets inside the present day orbit of Jupiter were warmed significantly by the newly born Sun and, depending on how far they were from the Sun, they lost a large fraction of their volatile gaseous material and had larger quantities of harder material like iron. From the inner planets to planets further out, the amount of volatiles found on the planets increases until you cross the ice line beyond Mars when planets became fluffy gaseous planets with large amounts of gas. That is a brief story of the formation of the solar system. But, far too many details are unclear. Much can be discerned by studying the planets but they, being large bodies often have an atmosphere and other interactions with the Sun as a result of which they have been worked on by heat and winds and their original stories have been re-written over time.

This is where comets come in. These are objects that have spent much of their time as small pieces of rock far away from the Sun and beyond the ice line where they have suffered little damage over time and retain the pristine material from the early solar system. Hence they carry a lot of information about the early solar system. In fact, the mission is called Rosetta in the hope that it will allow us to unlock the mysteries of the oldest building blocks of our Solar System. The comet’s composition will provide us with information on the composition of the pre-solar nebula out of which the Sun and the planets of the solar system formed. Comets have also played a crucial role in alterations in the solar system. For example, the water that we take for granted on Earth was not available when the Earth was born. It was transported to the Earth by a couple of comets after the Sun (and Earth) had cooled a little and liquid water could be sustained on Earth. There are some 200 known comet impact craters on Earth and more than 6 lakh such craters on Mars. Rosetta will provide crucial detailed information about the nature of comets and the history of the solar system.

To make matters interesting, Rosetta will travel with the comet as it gets close to the Sun, heats up and produces the characteristic tail, and we will have a ring side view of how this happens. In addition, comets are also known to be rich in organic molecules that are crucial to the evolution of life. So it is important to know what kinds of organic molecules they contain and how they were formed. 

Why is the knowledge of the early solar system important? Well for one, we do not know how it formed and how life evolved on it. As we begin to systematically study planetary systems around other stars, the questions of how they form and how they evolve life may not remain a purely academic curiosity. It will provide us with important information about how planetary systems form and also about what is the nature of organic matter in early solar system and gaseous clouds that form the planetary systems around other stars. Rosetta therefore will provide a lot of very important data and we can only wish a happy and successful life with its lifelong companion, 67P/Churyumov–Gerasimenko.

Dr. Mayank N. Vahia, Department of Astronomy & Astrophysics, Tata Institute of Fundamental Research