Launched on July 23, 2014, the Photochemistry on Space Station (PSS) experiment is an astrochemistry and exobiology experiment, selected by the European Space Agency (ESA) and funded by CNES, the CNRS as well as universities and other partners. It is part of the EXPOSE R2 campaign which gathers two biology experiments (Boss and Biomex) and a chemistry experiment (PSS). The latter consists in exposing solid samples of organic molecules and gaseous mixtures in orbit, and in bringing them back to Earth for analyses.
Though the principle seems simple, the challenges at stake are not trivial: the goal of PSS is to learn about the origin of life on Earth and whether it could appear elsewhere in the Universe.
EXPOSING MOLECULES TO SPACE CONDITIONS
Hervé Cottin, from the LISA (Laboratoire Interuniversitaire des Systèmes Atmosphériques, CNRS/Université Paris Est Créteil/Université Paris-Diderot) explained the little experiment’s expedition since the beginning of sample preparation in February 2014. “The PSS experiment consists in exposing organic molecules (composed of Carbon, Hydrogen and potentially Nitrogen and Oxygen) to space conditions in order to discover, for example, if the comet chemistry is an heritage of the interstellar medium chemistry, to solve the mechanisms that initiate the complex chemistry of Titan’s atmosphere, and to identify the most stable molecules on Mars.”
Samples FROM ALL OVER THE EARTH
To answer the international invitation to tender, various European laboratories organized a consortium. Samples were prepared in Netherlands by Leiden University, in Italy by Catane University, in France by the CBM (Centre de Biophysique Moléculaire) in Orléans, LATMOS (Laboratoire Atmosphères, Milieux, Observations Spatiales) in Guyancourt and LISA at Paris Est Créteil University, which coordinated the operations. Biochips were designed and produced by Montpellier and Bordeaux Universities, then embarked as samples for the PSS experiment to test new methods for chemical analysis which may be used in future Solar System exploration missions.
Sample carrier for PSS was designed to receive up to 25 exposure cells. They can accommodate open cells (grey) RUAG closed cells (purple) or CNES closed cells (yellow, more easy to see on the right panel). Two layers of samples are flown at the same time: one layer exposed to space, and one layer right below acting as a flight control layer. Pictures courtesy of RUAG/Kayser-Threde GmbH.
OFF TO Créteil
LISA received all the samples at the beginning of May 2014. Its work can be divided into two steps: first, everything must be controlled and measured: “Overlooking the exact characteristics of the samples before their departure was inconceivable, otherwise how could we study the modifications induced by the space conditions?” said Hervé Cottin. The 375 samples were submitted to a series of tests with the same tools as the ones that will be used when they will be back, i.e., infra-red tests, ultraviolet tests, etc.
The second step consists in distributing meticulously the samples in 5 layers. The first layer (75 samples) is destined to be carried out in microgravity; it will be submitted to solar rays. The second (75 samples) will also be submitted to space conditions but it will be protected from solar rays. The last three layers will stay on Earth, where they will be precociously kept at DLR in Cologne.
The EXPOSE facility (480 x 520 x 327,5 mm) is made of three experiment trays into which four square sample carriers (77 x 77 x 26 mm) are fitted. Pictures courtesy of RUAG/Kayser-Threde GmbH.
Cologne takes over
The five layers arrived in Cologne on June 2, 2014, the first two for their launch, while the three others will be compared to the samples placed in microgravity. One will be kept in vacuum at 5°C until the return of the space mission. Another will also be kept in vacuum but it will be submitted to the same conditions of temperature as if it was in space (measurements are regularly sent to DLR from the International Space Station). The last layer will receive an additional dose of photons, as if it was exposed to solar rays.
The sketch of the experimental breakdown of samples between the ISS and DLR Cologne, and the environmental conditions to which they are exposed. In the recent case of the PSS experiment a total of 75 samples can be exposed directly in LEO, with 75 dark controls directly below them (A). The same disposition is reproduced in an irradiation chamber at DLR Cologne (B), with an additional series of samples that will be kept in vacuum, in the dark, at 5° C(C).
Expose-R lift-off from Baikonur on July 23, 2014
Arrival at the International Space Station
At first, the samples stayed inside the station and waited for an astronaut to make an extra-vehicular operation.
Outside the International Space Station
On August 18, 2014, the cosmonauts Alexander Skvortsov and Oleg Artemyev installed EXPOSE-R2 outside the Station on the Russian part of the orbital complex. To prevent sample contamination produced by the combination of outgassing of the EXPOSE R2 facility and the solar rays, the samples exposed to vacuum and cold, were nonetheless protected from the solar rays by a cover for two months. Then the cosmonauts Alexander Samokutyaev and Maxim Suraev made a new extra-vehicular mission on October 22, 2014 to take the cover off. The samples were then passively submitted to all the space conditions for a while.
BACK TO THE STATION AND TO THE SOYUZ CAPSULE
On February 4, 2016, the Russian cosmonauts Yuri Malenchenko and Sergei Volkov took the EXPOSE-R2 facility back inside the station. It was then placed inside the Soyuz capsule, ready to get back to Earth.
Return to its starting point
The Soyuz capsule landed in Kazakhstan on March 2, 2016. The samples were sent back to Cologne, then to Créteil. They will be extracted from the sample carriers and referenced before being sent again to other European cities and their laboratories of origin. They will be analyzed in the same conditions as before their departure and compared to the samples that stayed on Earth. All tests are expected to be over by October 2016.
The scientists will then interpret the results and write the publications.