View source for Envisat

{{Short description|ESA Earth observation satellite (2002–2012)}}
{{Use dmy dates|date=October 2019}}
{{Use British English|date=January 2014}}
{{Infobox spaceflight
| name = Envisat
| image = Envisatmod.jpg
| image_caption = Model of Envisat
| mission_type = [[Earth observation satellite|Earth observation]]
| operator = [[European Space Agency|ESA]]
| website = {{url|http://envisat.esa.int/}}
| COSPAR_ID = 2002-009A
| SATCAT = 27386
| mission_duration = Planned: 5 years <br /> Final: {{time interval|1 March 2002 01:07:59|8 April 2012|show=ymd|sep=,}}
| spacecraft_bus = 
| manufacturer = [[Astrium]]
| launch_mass = {{convert|8211|kg|lb|abbr=on}}
| dimensions = {{convert|26|xx|10|xx|5|m|ft|abbr=on}}
| power = 6,500 watts
| instruments = {{Hlist | ASAR | RA-2 | MWR | MIPAS | [[MERIS]] | [[AATSR]] | [[DORIS (geodesy)|DORIS]] | [[GOMOS]] | [[SCIAMACHY]] }}
| launch_date = {{start-date|1 March 2002, 01:07:59|timezone=yes}}&nbsp;UTC
| launch_rocket = [[Ariane 5|Ariane 5G]] V-145
| launch_site = [[Guiana Space Centre|Kourou]] [[ELA-3]]
| launch_contractor = [[Arianespace]]
| last_contact = {{end-date|8 April 2012}} <br /> (spacecraft failure)
| declared = {{end-date|9 May 2012}}
| disposal_type = None
| decay_date = ~150 years
| orbit_epoch = 15 December 2013, 03:07:00&nbsp;UTC<ref name="n2yo">{{cite web|url=http://www.n2yo.com/satellite/?s=27386|title=ENVISAT Satellite details 2002-009A NORAD 27386|work=N2YO|date=15 December 2013|access-date=15 December 2013}}</ref>
| orbit_reference = [[geocentric orbit|Geocentric]]
| orbit_regime = [[Polar orbit|Polar]] [[Low Earth orbit|low Earth]]
| orbit_semimajor = {{convert|7144.9|km|mi|abbr=on}}
| orbit_periapsis = {{convert|772|km|mi|abbr=on}}
| orbit_apoapsis = {{convert|774|km|mi|abbr=on}}
| orbit_inclination = 98.40&nbsp;degrees
| orbit_repeat = 35&nbsp;days
| orbit_period = 100.16&nbsp;minutes
| orbit_eccentricity = 0.00042
| apsis = gee
}}

'''Envisat''' ("'''Environmental Satellite'''") is a large [[Earth observation satellite|Earth-observing satellite]] which has been inactive since 2012. It was launched on 1 March 2002 aboard an [[Ariane 5]] from the [[Guyana Space Centre]] in [[Kourou]], [[French Guiana]], into a [[Sun synchronous]] [[polar orbit]] at an altitude of 790&nbsp;±&nbsp;10&nbsp;km. 

Operated by the [[European Space Agency]] (ESA), it was the world's largest civilian Earth observation satellite.<ref>[https://earth.esa.int/web/guest/missions/esa-operational-eo-missions/envisat EarthNet Online]</ref> Its objective was to support the continuity of [[European Remote-Sensing Satellite]] missions, providing additional observations to improve environmental studies. To accomplish the global and regional objectives of the mission, numerous scientific disciplines used the data acquired from the sensors on the satellite to study [[atmospheric chemistry]], [[ozone depletion]], [[biological oceanography]], ocean temperature and colour, [[wind waves]], [[hydrology]] ([[humidity]], [[flood]]s), agriculture and [[arboriculture]], natural hazards, [[digital elevation model]]ling (using [[interferometry]]), monitoring of maritime traffic, [[atmospheric dispersion modelling]] (pollution), [[cartography]] and [[snow]] and [[ice]].

Envisat malfunctioned before it could be [[deorbit]]ed,<ref name="Foust2020"/> and after losing contact with the satellite on 8 April 2012, ESA formally announced the end of Envisat's mission on 9 May 2012.<ref name="missionend">{{cite web | url=http://www.esa.int/esaCP/SEM1SXSWT1H_index_0.html | title=ESA declares end of mission for Envisat | publisher=ESA | date=9 May 2012}}</ref> The defunct satellite is considered [[space debris]] and [[orbit]]s the [[Earth]] in about 101 minutes, with a repeat cycle of 35 days. An analysis that determined the 50 "statistically most concerning" debris objects in low Earth orbit ranked Envisat in 21st place.<ref name="Foust2020"/>

Envisat was able to operate five years beyond its planned mission lifetime, delivering over a [[petabyte]] of data.<ref name="missionend"/> At the time of its failure, Envisat had cost a total of 2.5 billion euro to develop, launch, and operate.<ref name="BBC2012"/> The mission has been replaced by the [[Sentinel (satellite)|Sentinel]] series of satellites. The first of these, [[Sentinel 1]], has taken over the radar duties of Envisat since its launch in 2014.

== Mission ==
Envisat was launched as an [[Earth observation satellite]]. Its objective was to support the continuity of [[European Remote-Sensing Satellite]] missions, providing additional observations to improve environmental studies.

To accomplish the global and regional objectives of the mission, numerous scientific disciplines used the data acquired from the sensors on the satellite to study [[atmospheric chemistry]], [[ozone depletion]], [[biological oceanography]], ocean temperature and colour, [[wind waves]], [[hydrology]] ([[humidity]], [[flood]]s), agriculture and [[arboriculture]], natural hazards, [[digital elevation model]]ling (using [[interferometry]]), monitoring of maritime traffic, [[atmospheric dispersion modelling]] (pollution), [[cartography]] and [[snow]] and [[ice]].

==Specifications==
;Dimensions
{{convert|26|m|ft|abbr=on}} × {{convert|10|m|ft|abbr=on}} × {{convert|5|m|ft|abbr=on}} in orbit with the solar array deployed.<ref>{{Cite web|url=https://directory.eoportal.org/web/eoportal/satellite-missions/e/envisat|title=EnviSat (Environmental Satellite)|website=eoPortal}}</ref>

;Mass
{{convert|8211|kg|lb|abbr=on}}, including {{convert|319|kg|lb|abbr=on}} of fuel and a {{convert|2118|kg|lb|abbr=on}} instrument payload.<ref>[https://earth.esa.int/web/guest/missions/esa-operational-eo-missions/envisat/satellite/space-segment Envisat – Overall configuration]</ref>
;Power
[[Solar array]] with a total load of 3560 [[Watt|W]].

== Instruments ==
[[File:Envisat instruments ESA194759.jpg|thumb|right|Instruments carried by Envisat.]]{{More citations needed section|date=August 2020|find=Envisat instruments}}
Envisat carries an array of nine Earth-observation instruments that gathered information about the Earth (land, [[water]], [[ice]], and [[atmosphere]]) using a variety of measurement principles. A tenth instrument, DORIS, provided [[guidance, navigation, and control|guidance and control]]. Several of the instruments were advanced versions of instruments that were flown on the earlier [[ERS-1]] and [[ERS 2]] missions and other satellites.

=== MWR ===
MWR ([[Microwave]] Radiometer) was designed for measuring [[water vapour]] in the [[Earth's atmosphere|atmosphere]].

=== AATSR ===
{{Main|AATSR}}
AATSR (Advanced Along Track Scanning [[Radiometer]]) can measure the [[sea]] surface temperature in the [[visible spectrum|visible]] and [[infrared spectrum|infrared spectra]]. It is the successor of ATSR1 and ATSR2, payloads of [[ERS 1]] and [[ERS 2]]. AATSR can measure Earth's surface temperature to a precision of {{convert|0.3|K-change|sigfig=2}}, for [[climatology|climate research]]. Among the secondary objectives of AATSR is the observation of environmental parameters such as water content, biomass, and vegetal health and growth.

=== MIPAS ===
MIPAS (Michelson [[Interferometer]] for Passive [[Atmospheric sounding|Atmospheric Sounding]]) is a [[Fourier transform]]ing infrared spectrometer which provides pressure and temperature profiles, and profiles of trace gases nitrogen dioxide ({{chem|NO|2}}), [[nitrous oxide]] ({{chem|N|2|O}}), methane ({{chem|CH|4}}), nitric acid ({{chem|HNO|3}}), ozone ({{chem|O|3}}), and water ({{chem|H|2|O}}) in the [[stratosphere]]. The instrument functions with high spectral resolution in an extended spectral band, which allows coverage across the Earth in all seasons and at equal quality night and day. MIPAS has a vertical resolution of {{convert|3|to|5|km|mi|abbr=on|sigfig=1}} depending on altitude (the larger at the level of the upper stratosphere).

=== MERIS ===
{{Main|MERIS}}

MERIS (MEdium Resolution Imaging [[Spectrometer]]) measures the reflectance of the Earth (surface and atmosphere) in the solar spectral range (390 to 1040&nbsp;[[Nanometre|nm]]) and transmits 15 spectral bands back to the [[ground segment]]. MERIS was built at the [[Cannes Mandelieu Space Center]].

=== SCIAMACHY ===
{{Main|SCIAMACHY}}

SCIAMACHY (SCanning Imaging Absorption spectroMeter for Atmospheric CHartographY) compares light coming from the sun to light reflected by the Earth, which provides information on the atmosphere through which the Earth-reflected light has passed.

SCIAMACHY is an image spectrometer with the principal objective of mapping the concentration of trace gases and aerosols in the troposphere and stratosphere. Rays of sunlight that are reflected transmitted, backscattered and reflected by the atmosphere are captured at a high spectral resolution (0.2 to 0.5&nbsp;nm) for wavelengths between 240 and 1700&nbsp;nm, and in certain spectra between 2,000 and 2,400&nbsp;nm. Its high spectral resolution over a wide range of wavelengths can detect many trace gases even in tiny concentrations. The wavelengths captured also allow effective detection of aerosols and clouds.
SCIAMACHY uses 3 different targeting modes: to the nadir (against the sun), to the limbus (through the atmospheric corona), and during solar or lunar eclipses. SCIAMACHY was built by Netherlands and Germany at [[Netherlands Organisation for Applied Scientific Research|TNO]]/TPD, [[SRON]] and [[Airbus Defence and Space]] Netherlands.<ref>{{Cite web|url=https://www.sron.nl/experimenten-50-jaar-ruimteonderzoek-2820/i-scimachy|title=I - Sciamachy}}</ref>

=== RA-2 ===
RA-2 ([[Radar]] [[Altimeter]] 2) is a dual-[[frequency]] [[Nadir]] pointing Radar operating in the [[Ku band|K<sub>u</sub> band]] and [[S band]]s, it is used to define [[ocean]] [[topography]], map/monitor [[sea ice]] and measure land heights.

Mean sea level measurements from Envisat are continuously graphed at the Centre National d'Etudes Spatiales web site, on the Aviso page.

=== ASAR ===
ASAR (Advanced Synthetic Aperture Radar) operates in the [[C band (IEEE)|C band]] in a wide variety of modes. It can detect changes in surface heights with sub-[[millimeter]] precision. It served as a data link for [[ERS 1]] and [[ERS 2]], providing numerous functions such as observations of different [[polarization (waves)|polarities of light]] or combining different polarities, angles of incidence and spatial resolutions.
{| class="wikitable"
|-
! Mode
! Id
! Polarisation
! Incidence
! Resolution
! Swath
|-
| Alternating polarisation
| AP
| HH/VV, HH/HV, VV/VH
| 15–45°
| 30–150&nbsp;m
| 58–110&nbsp;km
|- style="background:#efefef;"
| Image
| IM
| HH, VV
| 15–45°
| 30–150&nbsp;m
| 58–110&nbsp;km
|-
| Wave
| WV
| HH, VV
|
| {{0}}400 m
| 5&nbsp;km × 5&nbsp;km
|- style="background:#efefef;"
| Suivi global (ScanSAR)
| GM
| HH, VV
|
| 1000&nbsp;m
| 405&nbsp;km
|-
| Wide Swath (ScanSAR)
| WS
| HH, VV
|
| {{0}}150&nbsp;m
| 405&nbsp;km
|}

These different types of raw data can be given several levels of treatment (suffixed to the ID of the acquisition mode: IMP, APS, and so on):
* RAW (raw data, or "Level 0"), which contains all the information necessary to create images.
* S (complex data, "Single Look Complex"), images in complex numeric form, the real and imaginary parts of the output of the compression algorithm
* P (precision image), amplified image with constant pixel width (12.5&nbsp;m for IMP)
* M (medium precision image), amplified radiometry image with a resolution greater than P
* G (geocoded image), amplified image to which simple geographical transforms have been applied to show relief.

Data capture in WV mode is unusual in that they constitute a series of 5&nbsp;km × 5&nbsp;km spaced at 100&nbsp;km.

=== DORIS ===
{{Main|DORIS (geodesy)}}

DORIS ([[Doppler effect|Doppler]] Orbitography and Radiopositioning Integrated by Satellite) determines the satellite's orbit to within {{convert|10|cm|in|abbr=on|sigfig=1}}.

=== GOMOS ===
{{Main|GOMOS}}

GOMOS (Global [[Ozone]] Monitoring by Occultation of [[Star]]s) looks at stars as they descend through the Earth's atmosphere and change colour, allowing measurement of [[gas]]es such as ozone ({{chem|O|3}}), including their vertical distribution.

GOMOS uses the principle of [[occultation]]. Its sensors detect light from a star traversing the Earth's atmosphere and measures the depletion of that light by trace gases nitrogen dioxide ({{chem|NO|2}}), nitrogen trioxide, ({{chem|NO|3}}), {{chem|OClO}}), ozone ({{chem|O|3}}) and aerosols present between about {{convert|20|to|80|km|mi|abbr=on}} altitude. It has a resolution of {{convert|3|km|mi|abbr=on}}.

== End of mission ==

=== Loss of contact ===
[[ESA]] announced on 12 April 2012 that they lost contact with Envisat on Sunday, 8 April 2012, after 10 years of service, exceeding the initially planned life span by 5 years.  The spacecraft was still in a stable orbit, but attempts to contact it were unsuccessful.<ref>{{cite web| url=http://www.spaceflightnow.com/news/n1204/12envisat/ |title=Breaking News {{pipe}} Flagship Envisat satellite stops communicating |publisher=Spaceflight Now |access-date=21 April 2012}}</ref><ref>{{cite web| url=http://www.esa.int/esaCP/SEMQ2EHWP0H_index_0.html |title=ESA Portal – Envisat services interrupted |publisher=Esa.int |access-date=21 April 2012}}</ref> Ground-based radar and the French [[Pleiades (satellite)|Pleiades]] Earth probe were used to image the silent Envisat and look for damage.<ref>[http://www.space.com/15369-mysteriously-silent-envisat-satellite-space-photos.html Huge, Mysteriously Silent Satellite Spotted by Another Spacecraft] Space.com article, 20 April 2012</ref> ESA formally announced the end of Envisat's mission on 9 May 2012.<ref name="missionend" />

Envisat was launched in 2002 and it operated five years beyond its planned mission lifetime, delivering over a [[petabyte]] of data.<ref name="missionend"/> ESA was planning to operate the spacecraft until 2014.<ref name="BBC2012">[https://www.bbc.co.uk/news/science-environment-17692027 Contact lost with flagship Envisat spacecraft] 12 April 2012.</ref>

=== Space safety ===
[[File:Debris-GEO1280.jpg|thumb|[[Space debris]] populations seen from outside [[geosynchronous orbit]] (GEO). Note the two primary debris fields, the ring of objects in GEO, and the cloud of objects in [[low Earth orbit]] (LEO).]]
The defunct Envisat satellite is considered [[space debris]] and [[orbit]]s the [[Earth]] in about 101 minutes, with a repeat cycle of 35 days. It poses a hazard because of the risk of collisions. Given its orbit and its area-to-mass ratio, it will take about 150 years for the satellite to be gradually pulled into the Earth's atmosphere.<ref>[http://spacenews.com/envisat-pose-big-orbital-debris-threat-150-years-experts-say/ "Envisat To Pose Big Orbital Debris Threat for 150 Years, Experts Say."] ''SpaceNews''. Retrieved: 27 September 2015.</ref> Envisat is currently orbiting in an environment where two catalogued space debris objects can be expected to pass within about {{convert|200|m|ft|abbr=on}} of it every year, which would likely trigger the need for a manoeuvre to avoid a possible collision.<ref name=Kessler>{{cite news | first = Andrea | last = Gini | title = Don Kessler on Envisat and the Kessler Syndrome | date = 25 April 2012 | url = http://www.spacesafetymagazine.com/2012/04/25/don-kessler-envisat-kessler-syndrome/ | work = Space Safety Magazine | access-date = 9 May 2012}}</ref> A collision between a satellite the size of Envisat and an object as small as 10&nbsp;kg could produce a very large cloud of debris, initiating a self-sustaining chain-reaction of collisions and fragmentation with production of new debris, a phenomenon known as the [[Kessler Syndrome]].<ref name=Kessler/> An analysis that determined the 50 "statistically most concerning" debris objects in low Earth orbit ranked Envisat in 21st place.<ref name="Foust2020">{{cite web |last1=Foust |first1=Jeff |title=Upper stages top list of most dangerous space debris |url=https://spacenews.com/upper-stages-top-list-of-most-dangerous-space-debris/ |website=Space News |date=13 October 2020 |access-date=12 December 2022}}</ref>

Envisat was a candidate for a mission to remove it from orbit, called [[e.Deorbit]]. The spacecraft sent to bring down Envisat would itself need to have a mass of approximately 1.6 tonnes.<ref name="ESAconference1">{{cite web|url=https://indico.esa.int/indico/event/46/|title=e.Deorbit Symposium|publisher=ESA|date=6 May 2014|access-date=2 June 2015|archive-date=1 July 2015|archive-url=https://web.archive.org/web/20150701150318/https://indico.esa.int/indico/event/46/|url-status=dead}}</ref>

==See also==
{{Portal|Spaceflight}}
*[[List of European Space Agency programmes and missions]]
*[[List of heaviest spacecraft]]
*[[List of large reentering space debris]]

==References==
{{Reflist}}

==External links==
* [https://earth.esa.int/eogateway/missions/envisat/ Envisat homepage at ESA]
* [http://www.esa.int/SPECIALS/Operations/SEMOZY8L6VE_0.html Envisat operations page at ESA]
* [https://web.archive.org/web/20090220130752/http://miravi.eo.esa.int/ Miravi] – Meris Image Rapid Visualization. MIRAVI shows the gallery of images generated on the Level0 (raw data) Meris Full Resolution (300m) products, few seconds after their availability.
* [https://web.archive.org/web/20090224193538/http://srrs.chelys.it/ SRRS] – Satellite Rapid Response System. Like MIRAVI but including also ASAR, MERIS Full and Reduced Resolution and ALOS AVNIR2 images.
* [http://www.eosnap.com Earth Snapshot] – Web Portal dedicated to Earth Observation. Includes commented satellite images, information on storms, hurricanes, fires and meteorological phenomena.
* [http://step.esa.int/main/ ESA SNAP Toolbox] for viewing, calibrating and analyzing Envisat ASAR Level 1 data and higher
* [http://www.physorg.com/news11276.html Four years on, Envisat hailed for its contribution to Earth science] Physorg.com (2006-02-28)
* [https://web.archive.org/web/20111104060106/http://dwlinkdvb.esrin.esa.it/DDS/welcome.html ESA Envisat DDS (Data Dissemination System)]
* [http://www.gcs-salzburg.at/ gcs Global Communication & Services] – Manufacturer of the Envisat DDS Commercial Receiver Kit for [https://web.archive.org/web/20120306005518/http://dwlinkdvb.esrin.esa.it/DDS/docs/Public_Docs/EnviHam_abstract.pdf ENVIHAM Home Users]
* [https://web.archive.org/web/20120723025123/http://www.astrium.eads.net/en/dossiers/envisat-ten-years-of-climate-and-environmental-research.html Dossier Envisat: 10 years of climate and environmental research at Astrium]
* [https://www.eoportal.org/satellite-missions/envisat Envisat article on eoPortal by ESA]

{{ESA projects}}
{{Orbital launches in 2002}}
{{Authority control}}

[[Category:Earth observation satellites of the European Space Agency]]
[[Category:Derelict satellites orbiting Earth]]
[[Category:Space synthetic aperture radar]]
[[Category:Spacecraft launched in 2002]]
[[Category:Spacecraft launched by Ariane rockets]]
[[Category:Earth satellite radar altimeters]]
[[Category:Satellites in low Earth orbit]]