CoRoT


CoRoT was a space telescope mission which operated from 2006 to 2013. The mission's two objectives were to search for extrasolar planets with short orbital periods, particularly those of large terrestrial size, and to perform asteroseismology by measuring solar-like oscillations in stars. The mission was led by the French Space Agency in conjunction with the European Space Agency and other international partners.
Among the notable discoveries was CoRoT-7b, discovered in 2009 which became the first exoplanet shown to have a rock or metal-dominated composition.
CoRoT was launched at 14:28:00 UTC on 27 December 2006, atop a Soyuz 2.1b rocket, reporting first light on 18 January 2007. Subsequently, the probe started to collect science data on 2 February 2007. CoRoT was the first spacecraft dedicated to the detection of transiting extrasolar planets, opening the way for more advanced probes such as Kepler and TESS. It detected its first extrasolar planet, CoRoT-1b, in May 2007, just 3 months after the start of the observations. Mission flight operations were originally scheduled to end 2.5 years from launch but operations were extended to 2013. On 2 November 2012, CoRoT suffered a computer failure that made it impossible to retrieve any data from its telescope. Repair attempts were unsuccessful, so on 24 June 2013 it was announced that CoRoT had been retired and would be decommissioned; lowered in orbit to allow it to burn up in the atmosphere.

Overview

Spacecraft design

The CoRoT optical design minimized stray light coming from the Earth and provided a field of view of 2.7° by 3.05°. The CoRoT optical path consisted of a diameter off-axis afocal telescope housed in a two-stage opaque baffle specifically designed to block sunlight reflected by the Earth and a camera consisting of a dioptric objective and a focal box. Inside the focal box was an array of four CCD detectors protected against radiation by aluminum shielding 10mm thick. The asteroseismology CCDs are defocused by 760μm toward the dioptric objective to avoid saturation of the brightest stars. A prism in front of the planet detection CCDs gives a small spectrum designed to disperse more strongly in the blue wavelengths.
The four CCD detectors are model 4280 CCDs provided by . These CCDs are frame-transfer, thinned, back-illuminated designs in a 2,048 by 2,048 pixel array. Each pixel is in size which corresponds to an angular pixel size of 2.32 arcsec. The CCDs are cooled to. These detectors are arranged in a square pattern with two each dedicated to the planetary detection and asteroseismology. The data output stream from the CCDs are connected in two chains. Each chain has one planetary detection CCD and one asteroseismology CCD. The field of view for planetary detection is 3.5°.
The satellite, built in the Cannes Mandelieu Space Center, had a launch mass of 630 kg, was 4.10 m long, 1.984 m in diameter and was powered by two solar panels.

Mission design

The satellite observed perpendicular to its orbital plane, meaning there were no Earth occultations, allowing up to 150 days of continuous observation. These observation sessions, called "Long Runs", allowed detection of smaller and long-period planets. During the remaining 30 days between the two main observation periods, CoRoT observed other patches of sky for a few weeks long "Short Runs", in order to analyze a larger number of stars for the asteroseismic program. After the loss of half the field of view due to failure of Data Processing Unit No. 1 in March 2009, the observation strategy changed to 3 months observing runs, in order to optimize the number of observed stars and detection efficiency.
In order to avoid the Sun entering in its field of view, during the northern summer CoRoT observed in an area around Serpens Cauda, toward the Galactic Center, and during the winter it observed in Monoceros, in the Galactic anticenter. Both these "eyes" of CoRoT have been studied in preliminary observations carried out between 1998 and 2005, allowing the creation of a database, called CoRoTsky, with data about the stars located in these two patches of sky. This allowed selecting the best fields for observation: the exoplanet research program requires a large number of dwarf stars to be monitored, and to avoid giant stars, for which planetary transits are too shallow to be detectable. The asteroseismic program required stars brighter than magnitude 9, and to cover as many different types of stars as possible. In addition, in order to optimize the observations, the fields had to not be too sparse – fewer targets observed – or too crowded – too many stars overlapping.
Several fields were observed during the mission:
  • IRa01, from 18 January 2007 to 3 April 2007 – 9,879 stars observed;
  • SRc01, from 3 April 2007 to 9 May 2007 – 6,975 stars observed;
  • LRc01, from 9 May 2007 to 15 October 2007 – 11,408 stars observed;
  • LRa01, from 15 October 2007 to 3 March 2008 – 11,408 stars observed;
  • SRa01, from 3 March 2008 to 31 March 2008 – 8,150 stars observed;
  • LRc02, from 31 March 2008 to 8 September 2008 – 11,408 stars observed;
  • SRc02, from 8 September 2008 to 6 October 2008 – 11,408 stars observed;
  • SRa02, from 6 October 2008 to 12 November 2008 – 10,265 stars observed;
  • LRa02, from 12 November 2008 to 30 March 2009 – 11,408 stars observed;
  • LRc03, from 30 March 2009 to 2 July 2009 – 5,661 stars observed;
  • LRc04, from 2 July 2009 to 30 September 2009 – 5,716 stars observed;
  • LRa03, from 30 September 2009 to 1 March 2010 – 5,289 stars observed;
  • SRa03, from 1 March 2010 to 2 April 2010;
  • LRc05, from 2 April 2010 to 5 July 2010;
  • LRc06, from 5 July 2010 to 27 September 2010;
  • LRa04, from 27 September 2010 to 16 December 2010;
  • LRa05, from 16 December 2010 to 5 April 2011;
  • LRc07, from 5 April 2011 to 30 June 2011;
  • SRc03, from 1 July 2011 to 5 July 2011 – a run made to reobserve the transit of CoRoT-9b;
  • LRc08, from 6 July 2011 to 30 September 2011;
  • SRa04, from 30 September 2011 to 28 November 2011;
  • SRa05, from 29 November 2011 to 9 January 2012;
  • LRa06, from 10 January 2012 to 29 March 2012 – a run dedicated to reobservation of CoRoT-7b;
  • LRc09, from 10 April 2012 to 5 July 2012;
  • LRc10, from 6 July 2012 to 1 November 2012 - interrupted by the fatal failure which ended the mission.
The spacecraft monitored the brightness of stars over time, searching for the slight dimming that happens in regular intervals when planets transit their host star. In every field, CoRoT recorded the brightness of thousands stars in the V-magnitude range from 11 to 16 for the extrasolar planet study. In fact, stellar targets brighter than 11 saturated the exoplanets CCD detectors, yielding inaccurate data, whilst stars dimmer than 16 do not deliver enough photons to allow planetary detections. CoRoT was sensitive enough to detect rocky planets with a radius two times larger than Earth, orbiting stars brighter than 14; it is also expected to discover new gas giants in the whole magnitude range.
CoRoT also studied asteroseismology. It can detect luminosity variations associated with acoustic pulsations of stars. This phenomenon allows calculation of a star's precise mass, age and chemical composition and will aid in comparisons between the sun and other stars. For this program, in each field of view there was one main target star for asteroseismology as well as up to nine other targets. The number of observed targets have dropped to half after the loss of Data Processing Unit No. 1.
The mission began on 27 December 2006 when a Russian Soyuz 2-1b rocket lifted the satellite into a circular polar orbit with an altitude of 827 km. The first scientific observation campaign started on 3 February 2007.
The mission's cost amounted to, of which 75% was paid by the French space agency CNES and 25% was contributed by Austria, Belgium, Germany, Spain, Brazil and the European Space Agency.

Development

The primary contractor for the construction of the CoRoT vehicle was CNES, to which individual components were delivered for vehicle assembly. The CoRoT equipment bay, which houses the data acquisition and pre-processing electronics, was constructed by the LESIA Laboratory at the Paris Observatory and took 60 person-years to complete. The design and building of the instruments were done by the Laboratoire d'études spatiales et d'instrumentation en astrophysique de l'Observatoire de Paris, the Laboratoire d'Astrophysique de Marseille, the Institut d'Astrophysique Spatiale from Orsay, the Centre spatial de Liège in Belgium, the IWF in Austria, the DLR in Germany and the ESA Research and Science Support Department. The 30 cm afocal telescope Corotel has been realized by Alcatel Alenia Space in the Centre spatial de Cannes Mandelieu.

Potential

Before the beginning of the mission, the team stated with caution that CoRoT would only be able to detect planets few times larger than Earth or greater, and that it was not specifically designed to detect habitable planets. According to the press release announcing the first results, CoRoT's instruments are performing with higher precision than had been predicted, and may be able to find planets down to the size of Earth with short orbits around small stars.
The transit method requires the detection of at least two transits, hence the planets detected will mostly have an orbital period under 75-day. Candidates that show only one transit have been found, but uncertainty remains about their exact orbital period.
CoRoT should be assumed to detect a small percentage of planets within the observed star fields, due to the low percentage of exoplanets that would transit from the angle of observation of the Solar System. The chances of seeing a planet transiting its host star is inversely proportional to the diameter of the planet's orbit, thus close in planets detections will outnumber outer planets ones. The transit method is also biased toward large planets, since their very depth transits are more easily detected than the shallows eclipses induced by terrestrial planets.