View source for Satellite flare

{{Short description|Visual phenomenon caused by satellites}}
{{Use dmy dates|date=May 2020}}
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* Top: a simulated animation of a typical Iridium flare
* Bottom: Both images show a flare of an [[Iridium satellite constellation|Iridium satellite]]. Comet [[17P/Holmes|Holmes]] can be seen in the right image, slightly above the tree branch.</div>
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'''Satellite flare''', also known as '''satellite glint''', is a [[Artificial satellite|satellite]] [[pass (spaceflight)|pass]] [[human vision|visible]] to the [[naked eye]] as a brief, bright "[[flare]]". It is caused by the reflection toward the Earth below of sunlight incident on satellite surfaces such as [[Photovoltaic module|solar panels]] and [[Antenna (radio)|antennas]] (e.g., [[synthetic aperture radar]]).  Streaks from satellite flare are a form of [[light pollution]] that can negatively affect ground-based [[astronomy]], [[stargazing]], and [[indigenous people]].<ref>{{cite web|url=https://aas.org/satellite-constellations-1-workshop-report|title=SATCON1 Report|date=2020-08-25|archive-url=https://web.archive.org/web/20201129021515/https://aas.org/satellite-constellations-1-workshop-report|archive-date=2020-11-29}}</ref><ref>{{cite press release|url=https://noirlab.edu/public/announcements/ann20011/|date=2020-08-21|title=Media advisory: Press Conference to Unveil Conclusions from Satellite Constellations 1 (SATCON1) Workshop|archive-url=https://web.archive.org/web/20201129021144/https://noirlab.edu/public/announcements/ann20011/|archive-date=2020-11-29}}</ref><ref name="step-towards">{{cite arXiv|title=Concerns about ground based astronomical observations: A step to Safeguard the Astronomical Sky|first1=Stefano|last1=Gallozzi|first2=Marco|last2=Scardia|first3=Michele|last3=Maris|date=2020-02-04|class=astro-ph.IM |eprint=2001.10952}}</ref><ref name="global-commons"/>

Many satellites flare with magnitudes bright enough to see with the unaided eye, i.e. brighter than magnitude +6.5.<ref name="unaided-eye">{{cite journal|url=http://articles.adsabs.harvard.edu/pdf/1903LicOB...2...67C|title=On the Limits of Unaided Vision|journal=[[Lick Observatory|Lick Observatory Bulletin]]|number=38|pages=67–69|last=Curtis|first=Heber Doust|doi=10.5479/ADS/bib/1903LicOB.2.67C|publisher=[[University of California]]|year=1903|volume=2|pmid=17800603|bibcode=1903LicOB...2...67C|orig-date=1901-03-27|doi-access=free}}</ref><ref>{{cite web|url=https://www.satflare.com/home.asp|title=Satellite and Flare Tracking|access-date=2020-12-19}}</ref>  Smaller magnitude numbers are brighter, so negative magnitudes are brighter than positive magnitudes, ''i.e.'' the scale is reverse [[logarithmic scale|logarithmic]] {{xref|(see [[apparent magnitude]])}}.

The [[Iridium satellite constellation|Iridium constellation]] was one of the first anthropogenic sources of near-space light pollution to draw criticism.  Larger [[satellite constellation]]s, like [[OneWeb satellite constellation#Concerns|OneWeb]] and [[Starlink#Criticism and resistance|Starlink]], have received increased criticism.<ref name=iau>{{cite web |title=IAU's statement on satellite constellations |url=https://www.iau.org/news/announcements/detail/ann19035/ |website=International Astronomical Union |access-date=3 June 2019 |archive-date=27 May 2020 |archive-url=https://web.archive.org/web/20200527073625/https://www.iau.org/news/announcements/detail/ann19035/ |url-status=dead }}</ref><ref>{{cite web|url= http://www.astronomy.com/news/2019/06/light-pollution-from-satellites-will-get-worse-but-how-much|title=Light pollution from satellites will get worse. But how much?|date=2019-06-14|access-date=2019-11-07|website=astronomy.com}}</ref><ref>{{cite web|url=https://www.theverge.com/2019/5/29/18642577/spacex-starlink-satellite-constellation-astronomy-light-pollution|date=2019-05-29|title=SpaceX Starlink satellite constellation astronomy light pollution|archive-date=2020-11-29|archive-url=https://web.archive.org/web/20201129020343/https://www.theverge.com/2019/5/29/18642577/spacex-starlink-satellite-constellation-astronomy-light-pollution}}</ref>  Scientific and policy analyses have raised questions about which regulatory bodies hold jurisdiction over human actions that obscure starlight in ways that affect [[astronomy]],<ref>{{cite web|url=https://www.astro.princeton.edu/~gbakos/satellites/index.html|title=Light pollution from Satellites|first=Gaspar|last=Bakos}}</ref><ref>{{cite web|url=https://www.rcinet.ca/en/2020/11/18/astronomers-vs-tech-giants-in-space/|title=Astronomers vs tech giants in space|first=Marc|last=Montgomery|date=2020-11-18|archive-url=https://web.archive.org/web/20201129000829/https://www.rcinet.ca/en/2020/11/18/astronomers-vs-tech-giants-in-space/|archive-date=2020-11-29}}</ref><ref>{{cite web|url=https://aas.org/satellite-constellations-1-workshop-report|title=SATCON1 Report|date=2020-08-25}}</ref> stargazers,<ref>{{cite press release|url=https://www.darksky.org/why-do-mega-constellations-matter-to-the-dark-sky-community/|title=Why do mega constellations matter to the Dark Sky community|date=2019-12-27}}</ref><ref>{{cite web|title=SpaceX's Starlink satellites are about to ruin stargazing for everyone|url=https://theconversation.com/spacexs-starlink-satellites-are-about-to-ruin-stargazing-for-everyone-149516|date=2020-11-17|first=Samantha|last=Lawler |editor-first1=Nehal |editor-last1=El-Hadi |doi=10.64628/AAM.unhydkawg }}</ref> and [[indigenous communities]].<ref name="global-commons">{{cite journal|first1=Aparna|last1=Venkatesan|first2=James|last2=Lowenthal|first3=Parvathy|last3=Prem|first4=Monica|last4=Vidaurri|title=The impact of satellite constellations on space as an ancestral global commons|journal=Nature Astronomy|year=2020|volume=4|issue=11|pages=1043–1048|doi=10.1038/s41550-020-01238-3|bibcode=2020NatAs...4.1043V|s2cid=228975770|doi-access=free}}</ref><ref name="step-towards"/><ref>{{cite journal|date=2019-12-06|title=Towards inclusive practices with indigenous knowledge|first1=Aparna|last1=Venkatesan|first2=David|last2=Begay|first3=Adam J.|last3=Burgasser|first4=Isabel|last4=Hawkins|author4-link=Isabel Hawkins|first5=Ka’iu|last5=Kimura|first6=Nancy|last6=Maryboy|first7=Laura|last7=Peticolas|journal=Nature Astronomy|volume=3|issue=12|pages=1035–1037|doi=10.1038/s41550-019-0953-2|arxiv=2009.12425|bibcode=2019NatAs...3.1035V|s2cid=212942611}}</ref>

== Controlled satellites ==
[[File:Iridium animation.gif|thumb|Flaring from reflection of the Sun]]
The time and place of the satellite's flare can be predicted only when the satellite is controlled, and its orientation in space is known. In this case it is possible to predict the exact time of the flare, its place in the sky, the brightness and duration. 

=== Iridium flares ===
[[File:Iridium 6 & 51.jpg|right|thumb|upright=0.9|Double flare - Iridium 6 and its replacement, #51, both flare in a 21-second exposure.]]
[[File:Iridium satellite.jpg|right|thumb|upright=0.9|First-generation Iridium satellite. Antennas can be seen in front, and solar panels in the back.]]

The first generation of the [[Iridium satellite constellation|Iridium constellation]] launched a total of 95 telecommunication satellites in [[low Earth orbit]] which were known to cause '''Iridium flares''', the brightest flares of all [[Geocentric orbit|orbiting]] satellites, starting in 1997. From 2017 to 2019 they were replaced with a new generation that does not produce flares, with the first generation completely deorbited by 27 December 2019.<ref name=":0">{{Cite web |title=Catch the Iridium |url=https://catchtheiridium.com/ |website=Catch the Iridium |language=en-US |access-date=6 May 2020}}</ref><ref>{{Cite web |title=Iridium flares to cease by end of 2018 |url=http://www.skyatnightmagazine.com/news/iridium-flares-cease-end-2018 |last=Carter |first=Jamie |date=14 November 2017 |website=BBC Sky at Night Magazine |language=en |access-date=26 May 2018}}</ref>

While the first-generation Iridium satellites were still controlled, their flares could be predicted.<ref>{{Cite web |title=The end of Iridium flares? |url=https://www.heavens-above.com/IridiumDemise.aspx |last=Peat |first=Chris |date=15 May 2018 |website=Heavens-Above |location=[[Munich]] |language=en |access-date=2 November 2018}}</ref> These [[Iridium satellite constellation|Iridium communication satellites]] had three polished door-sized antennas, 120° apart and at 40° angles with the main bus. The forward antenna faced the direction the satellite is traveling. Occasionally, an antenna reflects sunlight directly down at Earth, creating a predictable and quickly moving illuminated spot on the surface below of about {{convert|10|km|abbr=on|sigfig=1}} diameter. To an observer this looks like a bright flash, or flare in the sky, with a duration of a few seconds.

Ranging up to −9.5 [[apparent magnitude|magnitude]], some of the flares were so bright that they could be seen in the daytime. This flashing caused some annoyance to [[Astronomer#Modern astronomers|astronomers]], as the flares occasionally disturbed observations.<ref>{{Cite web |title=Iridium Flare Satellite Tracker |url=https://apollosat.com/iridium-flare-satellite-tracker/ |website=Apollo Satellite Communications |language=en-US |access-date=6 November 2019}}</ref>

As the Iridium constellation consisted of 66 working satellites, Iridium flares were visible quite often (2 to 4 times per night). Flares of brightness −5 magnitude occurred 3 to 4 times per week, and −8 magnitude were visible 3 to 5 times per month for stationary observers.

Flares could also occur from solar panels, but they were not as bright (up to −3.5 magnitude). Such flares lasted about twice as long as those from the main mission antennas (MMA), because the so-called "mirror angle" for the solar panels was twice that for the MMAs. There were also rare cases of flares from MMAs and solar panels, or two MMAs (front and either right or left) of one satellite in a single [[pass (spaceflight)|pass]].

The flares were bright enough to be seen at night in big cities where light pollution usually prevents most stellar observation. When not flaring, the satellites were often visible crossing the night sky at a typical magnitude of 6, similar to a dim star.

=== Mega-constellations ===
[[File:Starlink Satellites Imaged from CTIO.jpeg|thumb|333 seconds-exposure containing 19 or more streaks due to Starlink satellites]]
[[File:StarlinkTrails Filter 1080.jpg|thumb|Starlink Satellite Trails over Brazil alongside a shooting star - NASA's Astronomy Picture of the Day, 10 December 2019.<ref>{{cite web|url=https://apod.nasa.gov/apod/ap191210.html|title=NASA's Astronomy Picture of the Day showing Starlink Satellite Trails over Brazil|first=Egon|last=Filter|publisher=[[NASA]]|date=2019-12-10}}</ref>]]
Low-orbit satellite constellations such as [[Starlink]] are a concern for [[astronomers]], stargazers, and indigenous communities because of light pollution.<ref name="global-commons"/><ref>{{Cite news |title=After SpaceX Starlink Launch, a Fear of Satellites That Outnumber All Visible Stars |url=https://www.nytimes.com/2019/06/01/science/starlink-spacex-astronomers.html |website=The New York Times |date=June 2019 |access-date=3 June 2019|last1=Hall |first1=Shannon }}</ref><ref>{{Cite news |title=The unexpected brightness of new satellites could ruin the night sky |url=https://www.economist.com/science-and-technology/2019/05/30/the-unexpected-brightness-of-new-satellites-could-ruin-the-night-sky |newspaper=The Economist |date=30 May 2019 |access-date=3 June 2019}}</ref><ref>{{Cite web |title=SpaceX's Starlink Could Change The Night Sky Forever, And Astronomers Are Not Happy |url=https://www.forbes.com/sites/jonathanocallaghan/2019/05/27/spacexs-starlink-could-change-the-night-sky-forever-and-astronomers-are-not-happy/ |website=Forbes |access-date=3 June 2019}}</ref>

In February 2020, the [[Russian Academy of Sciences]] said it would send a letter to the [[United Nations]] complaining that Starlink's satellites will damage "30-40% of astronomical images."<ref>{{cite news|url=https://ria.ru/20200227/1565298951.html|title=RAS will complain to the UN about Elon Musk over the Starlink satellite system|date=2020-02-27|publisher=RIA Novosti}}</ref><ref>{{cite news|url=https://www.newsweek.com/elon-musk-starlink-russia-united-nations-satellites-1489708|title=Light From Elon Musk's Starlink Satellites Ruins Space Photos, Says Russian Government Agency|first=Brendan|last=Cole|publisher=Newsweek|date=2020-02-28}}</ref><ref>{{cite web|url=https://www.polygraph.info/a/russia-spacex-starlink-fact-check/30467125.html|publisher=Polygraph Info|title=Russian Academy of Sciences Takes Aim at Musk's SpaceX Satellites|date=2020-03-03|first=Fatima|last=Tlis}}</ref>

Numerous satellite operators have criticized [[SpaceX]] for attempting to overwhelm the [[FCC]] with paperwork as a means to gain approval to launch 42,000 satellites,<ref>{{cite news|date=2019-10-18|title=Companies unite to reject SpaceX satellite plans|first=Kerry|last=Hebden|url=https://room.eu.com/news/companies-unite-to-reject-spacex-satellite-plans}}</ref> which has raised questions about which aspects of [[space law]] pertain to [[light pollution]] from satellites.<ref name="global-commons"/>

[[SpaceX]] and [[Elon Musk]] have asserted in meetings with the [[National Academy of Sciences]]<ref>{{cite web|title=Decadal Survey on Astronomy and Astrophysics 2020 (Astro2020): Optical Interference from Satellite Constellations Meeting|url=https://www.nationalacademies.org/event/04-27-2020/decadal-survey-on-astronomy-and-astrophysics-2020-astro2020-light-pollution-rfi-meeting|date=2020-04-27|publisher=National Academy of Science}}</ref> and in [[FCC]] filings<ref>{{cite web|url=https://fcc.report/IBFS/SAT-MOD-20200417-00037|title=Application for Fixed Satellite Service by Space Exploration Holdings, LLC SAT-MOD-20200417-00037 / SATMOD2020041700037 Space Exploration Holdings, LLC seeks to modify its Ku/Ka-band NGSO license to relocate satellites previously authorized to operate at altitudes from 1,110 km to 1,325 km down to altitudes ranging from 540 km to 570 km, and to make related changes.|date=2020-04-30|first=William|last=Wiltshire}}</ref> that "SpaceX is committed to reducing satellite brightness to allow enjoyment of the skies and not thwart scientific discovery"<ref name="musk-nas">{{cite conference|first=Elon|last=Musk|url=https://www.nationalacademies.org/event/04-27-2020/docs/D0FD9F80C81A9D3E49EB8BE599FBF8A28BB2A1F69886|title=Starlink: NAS Decadal Panel|date=2020-04-27|publisher=[[National Academy of Sciences]]}}</ref> and that its objectives are (1) to "make the satellites generally invisible to the naked eye within a week of launch", and (2) to "minimize Starlink's impact on astronomy by darkening satellites so they do not saturate observatory detectors."<ref name="musk-nas"/> There were more than 7,800 Starlink satellites in orbit as of July 2025.

[[Starlink]] satellites can flare repeatedly in an isolated area of the sky, typically directly above the sun (below the horizon) as they transit the highest latitude of their orbits. This phenomenon is most obvious when satellites are low over the horizon, and is due to the large number of Starlink satellites that are orbiting the Earth, predominantly at ~53° orbital inclination.<ref>{{Cite web |last=warnerjh |date=2023-04-24 |title=Starlink Satellites Flaring in Cassiopeia |url=https://catchingtime.com/starlink-satellites-flaring-in-cassiopeia/ |access-date=2024-03-03 |website=CatchingTime |language=en-US}}</ref>

The flares from Starlink have been misidentified as UFOs by airline pilots<ref>{{Cite web |title=Pilot Shares Videos Of Strange UFO Sightings In Skies Over The US |url=https://www.youtube.com/watch?v=iT9BtWCBbic |date=21 Oct 2022 |website=YouTube |language=en-US |access-date= 21 Oct 2022}}</ref><ref>{{Cite web |title=Why "Racetrack" UFOs are mostly Starlink Flares |url=https://www.youtube.com/watch?v=_VmrRGln1XA |date=15 May 2019 |website=Metabunk |language=en-US |access-date=23 Oct 2022}}</ref> due to their unusual repetitive nature, which is visually analogous to a car's headlights at night getting brighter (then dimmer) as it rounds a turn. At cruising altitudes for commercial aircraft at night, pilots looking toward the setting or rising sun can often see the "racetrack" reflections from multiple satellites in criss-crossing orbits.

=== Other satellite flares ===
[[File:COSMO Skymed flare.PNG|thumb|upright=0.81|A [[COSMO-SkyMed]] flare]] 
[[File:MetOp-A satellite flare May 26 2019.jpg|thumb|upright=0.81|A MetOp-A flare]]

Many other controlled satellites also flare to magnitudes visible to the naked eye, i.e. larger than +6.5.<ref name="unaided-eye"/>

[[MetOp|MetOp-B and C]], however, can produce predictable flares up to −5 magnitude (MetOp-A is no longer controlled) .<ref>{{cite web|url=http://www.satflare.com/fpas_query.asp?sat=29499&order=1&ref=1|title=FPAS Reports - DB Query for Metop-A}}</ref><ref>{{cite web|url=http://www.satflare.com/fpas_query.asp?sat=38771&order=1&ref=1|title=FPAS Reports - DB Query for Metop-B}}</ref><ref>{{cite web|url=http://tool-box.info/blog/archives/1419-MetOp-A-satellite-flare.html|title=MetOp-A satellite flare|author=Wesley|date=2014-04-09}}</ref><ref>{{cite web|url=https://sattrackcam.blogspot.com/2012/03/video-of-metop-06-044a-flaring.html|title=Video of Metop-A Flaring|date=2012-03-23}}</ref> Four [[COSMO-SkyMed]] satellites can produce flares up to -3 magnitude, and lasting much longer than the Iridium flares.<ref>{{cite web|url=http://www.satflare.com/fpas_query.asp?sat=32376&order=1&ref=1|title=FPAS Reports - DB Query for SkyMed 2}}</ref>
The Terrasar X and Tandem X also can produce predictable flares up to -3 magnitude. 

The [[International Space Station]] (ISS) is known to cause bright [[ISS flare]]s.<ref>{{Cite web |title=Watch International Space Station Flybys All Night Long |url=https://skyandtelescope.org/observing/watch-international-space-station/ |date=15 May 2019 |website=Sky & Telescope |language=en-US |access-date=6 May 2020}}</ref><ref>{{Cite web |title=The International Space Station Rides High Through the May Sky |url=https://www.universetoday.com/142187/the-international-space-station-rides-high-through-the-may-sky/ |last=Dickinson |first=David |date=14 May 2019 |website=Universe Today  |access-date=6 May 2020}}</ref><ref>{{Cite web |title=Spot the Space Station |url=https://www.space.com/5610-spot-space-station.html |last=Rao |first=Joe  |website=Space.com |date=11 July 2008 |language=en |access-date=6 May 2020}}</ref><ref>{{Cite web |title=Spot The Station |url=https://spotthestation.nasa.gov/ |website=spotthestation.nasa.gov |language=en |access-date=6 May 2020}}</ref>
[[File:Starlink Flares, 53 degree orbital inclination.jpg|alt=Starlink satellites repeatedly flaring low over horizon.|thumb|Composite image showing 27 minutes of Starlink satellite flares near the constellation Cassiopeia, 4/24/23 ]]

== Uncontrolled satellites ==

When a satellite goes out of control, it becomes possible to predict only a trajectory of its pass, while it becomes virtually impossible to accurately predict any flaring. These non-operational satellites are also described as "tumbling". This category includes many rotating rocket bodies, some failed Iridium satellites, [[ALOS]] satellite (which can produce flashes up to −10 mag), etc.
The most important and valuable information about tumbling satellites is the period of flashes. It can vary from 0.5 seconds (rapidly rotating objects) to a minute or more (slowly rotating objects). Other important characteristics are the amplitude of changes in brightness and period of repetition of these changes.{{fact|date=October 2022}}

== Observation ==
{{main|List of satellite pass predictors}}

While satellites may be seen by chance, there are websites and [[mobile app]]s which provide location-specific information as to when and where in the sky a satellite flare may be seen (for controlled satellites), or trajectory of a tumbling satellite's [[pass (spaceflight)|pass]] (for uncontrolled satellites) in the sky.

Reflections from satellites and other human space objects are sometimes reported as [[unidentified flying object]]s (UFOs),<ref name="EarthSky {{pipe}} Updates on your cosmos and world 2020">{{cite web | title=Is that a UFO?! There's probably an explanation - Human World | website=EarthSky {{pipe}} Updates on your cosmos and world | date=2020-12-15 | url=https://earthsky.org/space/if-its-not-a-ufo-what-is-it/ | access-date=2022-01-30}}</ref> and are often a result of repetitive observations in an isolated area of the sky over a short time period.

== See also ==
* [[NanoSail-D]]
* [[Humanity Star]]

== References ==
{{reflist}}

==External links==
{{commons}}
* {{URL|1=https://sourceforge.net/projects/previsat/}} – PreviSat software which calculates satellite flares (Iridium, MetOp, SkyMed)
* {{URL|1=http://www.heavens-above.com/IridiumFlares.aspx}} – schedule of upcoming Iridium flares
* {{URL|1=http://iridiumflares.sourceforge.net}} – IridiumFlares prediction software (Java application)
* {{URL|1=http://satobs.org/iridium.html}} – "Catch a Flaring Iridium" at Visual Satellite Observer
* {{URL|1=http://www.fifthstarlabs.com/}} – SkyGuide
* {{URL|1=https://www.youtube.com/watch?v=ePBE7pHzAdA}} Iridium 25 flare near the Moon
* https://www.youtube.com/watch?v=FzaPT4ahyCs Starlink satellites repeatedly flaring low on horizon

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[[Category:Satellites]]
[[Category:Visibility]]