GC Columns Travel to Space for Comet Rendezvous

Editor's note: On November 12, 2014, the comet probe Philae touched down on the surface of comet 67P. The probe is now photographing and testing the surface of the comet.

Gas chromatography (GC) is a powerful analytical tool, and it’s getting out of the lab and traveling millions of miles into space as part of the Rosetta mission, a project initiated by the European Space Agency (ESA) in 1999 with the aim of achieving the first-ever landing of a manmade spacecraft on a comet. The ESA, an intergovernmental organization created in 1975, is helping to shape the development of European space capability and seeks to ensure that the investment in space delivers benefits to people worldwide.

Comets are time capsules comprised of primitive material left over from the period when the sun and its planets formed. Insight into what happens beyond our atmosphere will be made possible by studying the structure, dust and gas of the nucleus and organic materials associated with the comet via remote and in situ observations.

Current progress

Launched in 2004 from Kourou, French Guiana, the goal of the Rosetta expedition was to be the first mission to rendezvous with comet 67P/Churyumov-Gerasimenko, escort it as it orbits the sun, and deploy a lander. Before being sent into the outer solar system, the three-ton spacecraft, which is controlled by the European Space Operations Centre (ESOC), was inserted into a parking orbit because no existing rocket had the ability to send such a large spacecraft directly to 67P. During its decade-long journey, Rosetta has circled the sun almost four times and passed two asteroids, 2867 Steins in 2008 and 21 Lutetia, in 2010.

The Rosetta remained in close proximity to the icy nucleus of the comet as it moved closer to the sun. Shortly after arriving at the comet, a small robotic lander (Philae) was dispatched from Rosetta for the first controlled touchdown on a comet’s nucleus in history. The Rosetta lander’s instruments are now obtaining the first images from a comet’s surface and making the first in situ analysis to find out what it is made of.

The final decision on a landing site for the lander was made in October 2014.1 The choice of where to land was dependent on which site posed the least risk. The probe had one chance to dispatch its Philae contact robot to 67P’s icy surface, on November 12, 2014. The entire process was heavily reliant on fully automated commands sent days in advance. Key considerations for the landing point included the selection of sites that were free from rocks and boulders. In addition, the potential landing site needed to experience a day/night cycle. This is to provide the lander with protection from too much sun, which can cause it to overheat, or too little light, which can make it difficult to charge its batteries.

Background of the Rosetta mission

The probe is named after the well-known Egyptian Rosetta stone, which features a decree in three individual languages. The lander is named after the Nile island Philae, where an obelisk was discovered with inscriptions that, when compared with the Rosetta Stone, provided a greater understanding of the Egyptian writing system. Similarly, it is hoped that the spacecraft will further our knowledge of comets and the early solar system.

Mission objectives

Comets are thought to preserve the most unspoiled material in the solar system due to their formation from dust particles and because their interiors remain cold. When they do get closer to the sun, volatile components and dust particles are released, which creates the opportunity to obtain cometary data. An understanding of the physicochemical structure of comets may make it possible to gain insight into the development of the earth and the initial conditions that prompted the start of life on earth.

The main objective of the Rosetta mission is to study the origin of comets and the relationship between cometary and interstellar material in order to better understand the origins of the solar system. The following measurements will be made as part of the study:

  • Global characterization of the nucleus and the determination of its dynamic properties, surface morphology and composition
  • Determination of the chemical, mineralogical and isotopic compositions of volatiles and refractories in a cometary nucleus
  • Determination of the physical properties and interrelation of volatiles and refractories in a cometary nucleus
  • Study of the development of cometary activity as well as the processes in the surface layer of the nucleus and the inner coma
  • Global characterization of asteroids, including the determination of dynamic properties, surface morphology and composition.2

The Philae lander and its instruments

Figure 1 ‒ Rosetta arrives at comet.

The Rosetta mission spacecraft (see Figure 1) consists of the Rosetta space probe orbiter, which features 12 instruments, and the Philae robotic lander, which consists of 10. The lander’s instruments weigh 26.7 kg (59 lb), making up nearly one-third of its mass. These include:

  1. APXS (Alpha Proton X-ray Spectrometer) to analyze the chemical element composition of the surface below the lander.
  2. COSAC (the Cometary Sampling and Composition) combined gas chromatograph and time-of-flight mass spectrometer, which will perform analysis of soil samples and determine the content of volatile components.
  3. PTOLEMY gas chromatograph analyzer.
  4. MUPUS (Multi-Purpose Sensors for Surface and Sub-Surface Science).
  5. ROMAP (Rosetta Lander Magnetometer and Plasma Monitor).3

Gas chromatography is a vital part of the Rosetta project because it enables the mission to characterize, identify and quantify volatile cometary compounds, including larger organic molecules, through in situ measurements of surface and subsurface samples.4,5 As shown in Figure 2, the Philae lander counts two gas chromatographs (COSAC and PTOLEMY) among its equipment. COSAC consists of eight capillary GC columns, while PTOLEMY is a GC/MS system with three columns.

Figure 2 ‒ Philae instruments.

As with every piece of equipment on the lander, these needed to be specifically chosen to meet the unique task of carrying out GC in a completely foreign environment, away from the support of laboratory personnel. Agilent Technologies supplied two capillary GC columns to COSAC—an Agilent J&W Carbobond (specially made for this mission) and an Agilent J&W CP-Chirasil-DEX CB, plus an additional three columns to PTOLEMY—an Agilent J&W CP-PoraPLOT Q, Agilent J&WCP-Molsieve 5A, and Agilent J&W CP-Sil 8 CB, for use within the Philae lander.

GC history in the making

The circumstances of the analysis meant that reliable equipment capable of sending back accurate information to scientists on the ground had to be chosen. There were some major obstacles to be considered when deciding on which instrumentation to send up to the comet. These included the lack of access after launch, the durability required for such a long mission, and the broad range of potential samples.

Of key importance to the mission is using columns comprehensively tested for column bleed, inertness, efficiency, and consistent reproducibility. Optimal peak shape symmetry with low level response contributes greatly to the reliability and efficiency of the instrumentation to deliver accurate results to the analysts on the ground. In this way, the ESA researchers are assured that the information they are receiving is dependable.

Column selection

The three types of Agilent J&W GC columns provided for use within the Philae lander have a variety of features that make them particularly well suited to meeting this challenge. The Agilent J&W CP-PoraPLOT Q column is capable of analyzing both polar and nonpolar volatile compounds, giving it broad applicability; this is of great advantage when trying to limit the amount of instrumentation on the lander, while still ensuring that all needed measurements can be carried out. Q type porous polymer columns with repeatable retention times for long-term stability are especially important for ensuring efficiency and reliability throughout the mission.

This focus on efficiency is mirrored in the other GC columns installed in the Philae robotic lander. The Agilent J&W CP-Molsieve 5Å was selected for separation of permanent gasses such as helium, hydrogen, xenon, and argon. With its high efficiency and symmetrical peak shape performance, this PLOT column plays an important role in accurately monitoring the composition of these noble gases on the comet surface and in the atmosphere.

The third column, an Agilent J&W CP-Sil 8 CB, has a general-purpose 5% phenyl stationary phase, which provides selectivity and temperature limit that has been classically used for a broad range of gas-phase separations including environmental semi-volatile compounds.

The fourth column, the Agilent J&W Carbobond, specially designed for this mission, is an Ultimetal-treated stainless-steel column, and is able to separate the typical permanent gases we find in our atmosphere with the addition of some light hydrocarbons as well as carbon monoxide and carbon dioxide.

Finally, the fifth GC column, the Agilent J&W CP-Chirasil-DEX CB, incorporates a uniquely bonded chiral phase to separate and identify compounds in racemic mixtures (like mirrored images or the left hand to the right hand), which often occur in nature.

All the columns provided to the Rosetta mission by Agilent are produced with the sole aim of delivering accurate data quickly while requiring little to no maintenance. This range of column chemistries and selectivities, combined with the high level of quality and reliability, is an essential element reflected throughout the design of Rosetta and Philae. They are capable of undertaking delicate, sensitive readings in foreign environments, and deliver precisely the data required to draw real conclusions.

Conclusion

Figure 3 ‒ Philae touchdown.

The Rosetta mission is the first project of its kind, and may soon provide researchers with valuable information that will assist in understanding the early stages of the earth’s development (see Figure 3). This would not be possible without accurate, efficient equipment that the ESA can use with confidence.

Agilent J&W GC columns have long set a standard for a high level of performance and reliability, due to constant improvement, years of experience, and industry-leading testing   to ensure column-to-column consistency. It is these standards that make them so well suited for this unique project and, in November 2014, will play a role in generating the first set of highly anticipated data.

For more information on Agilent J&W GC columns, please visit www.agilent.com/chem/GCcolumns. Keep up to date with the Rosetta mission at http://sci.esa.int/rosetta/

References

  1. Amos, J. BBC News Science & Environment. http://www.bbc.co.uk/news/science-environment-28923010; accessed 05/09/2014.
  2. ESA Rosetta Mission. http://sci.esa.int/rosetta/; accessed 05/09/2014.
  3. Rosetta (spacecraft). http://en.wikipedia.org/wiki/Rosetta_%28spacecraft%29; accessed 05/09/2014.
  4. Goesmann, F.; Rosenbauer, H. et al. COSAC, the cometary sampling and composition experiment on Philae.  Space Science Reviews  2007, 128, 257–80.
  5. Wright, I.P.; Barber, S.J. An instrument to measure stable isotopic ratios of key volatiles on a cometary nucleus. Space Science Reviews  2007, 128, 363–81.

Dr.-Ing. Norbert Reuter is Channels Support Organization Manager, and Gary Lee is GC Columns Product Marketing Manager, Agilent Technologies; www.agilent.com. Images reproduced with permission from ESA/ATG Medialab.