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The Sun is the power-house of the Solar System.
As well as an overview of the Sun, there are descriptions of spacecraft that were launched specifically with solar research in mind, or which are in solar orbits doing other valuable work.
Click on one of the thumbnail pictures to see it enlarged.
The launch dates of Heliophysics System Observatory missions plotted on a solar cycle timeline.
These spacecraft are described on this page
List of all individual spacecraft (except Earth-orbiters) so far launched.
A solar prominence eruption exploded from the east limb (left side) of the sun on 16th April 2012. There was a simultaneous solar flare, though it was not aimed toward Earth. The event was witnessed by NASA’s Solar Dynamics Observatory satellite.
A solar storm nearly crippled satellites, power supplies, mobile phones, communications and more on 23rd July 2012. Check out this article which I have appended to my science fiction story The Green Flash.
Britain’s Cabinet Office included solar flares with terrorism, floods and pandemics in a list of potential catastrophes that threatened the country.
Sun spots are seen as the moon moves into full eclipse position in May 2012. An annular eclipse means that there is a ring of light around the Moon, so that the moon appears too small to completely cover the disc of the Sun.
See also Solar Wind.
NASA has released this footage from the Solar Dynamics Observatory that shows the sun emitting x-class solar flares. X-class flares are the most intense; however, humans are safe because the flares cannot pass through the Earth’s atmosphere, although they can disrupt communications. From The Guardian.
The clip above shows an anthelion and was published in The Guardian.
The Sun’s composition by mass is Hydrogen 73.46%, Helium 24.85%, Oxygen 0.77%, Carbon 0.29%, Iron 0.16%, Neon 0.12%, Nitrogen 0.09%, Silicon 0.07%, Magnesium 0.05%, Sulphur 0.04%...; so H and He comprise 98.31% of the mass.
The middle of the Sun, “the core”, is a very hot gas with a radius about a quarter of that of the star. It is ionized – all the electrons have been ripped away from the atomic nuclei because it is so hot. The nuclei available are mostly 1H (protons), quite a lot of 4He (helium-4), and some 2H (deuterium) and 3He (helium-3). There are a few other types too. In the core, pressures and temperatures are high enough to force fusion, that is, nuclear reactions whereby some nuclei merge to make others. Nuclear fusion does two things: it converts hydrogen into helium and it converts mass to energy. The mass-to-energy conversion is described by Einstein’s famous equation: E = mc2. Because the velocity of light is a very large number, this equation says that lots of energy can be gained from using up a modest amount of mass.
The most important reaction within the core of the Sun is the proton-proton cycle, “burning” hydrogen to make helium:
4 1H + 2 electrons → 4He + 2 neutrinos + 6 photons (the reaction actually occurs in several steps)
In this reaction, the final particles have less internal energy than the starting particles (26 MeV = 26×106 eV each time the reaction happens). Since energy is conserved, the extra energy is released as energy of motion of the nuclei and electrons in the solar gas, the production of lots of low-energy photons and, finally, the energy of the neutrinos. So the gas gets hotter and has lots of photons (and neutrinos).
The high-energy photons (gamma rays) produced by the nuclear process don’t get far; they move through the “radiative layer” surrounding the core. The photons are constantly intercepted, absorbed and re-emitted and heat the gas. This layer takes up 60% of the radius of the Sun, and it takes a million years for energy to get through this radiative layer into the “convective layer”. The helium remains in the middle of the Sun where the helium nuclei make up 62% of the mass (the rest is still hydrogen). The neutrinos easily zip out of the Sun – they easily pass through almost anything, rarely reacting with matter. The radiative and convective layers have about 72% hydrogen, 26% helium, and 2% heavier elements (by mass). The energy produced by fusion is eventually transported to the solar surface and emitted as light or ejected as high-energy particles.
By the time the energy reaches the surface of the Sun, things have cooled down to 6000 K, the temperature of the sunlight we see. By now the density of the gas is low. The energy emitted from the hot surface, on average, is near 230 million watts per square metre.
The energy created by the fusion processes within the core of the Sun exerts an outward pressure. Unless contained, such pressure would produce an explosion. The inward pressure that keeps a star from exploding is the gravitational attraction of the gas mantle surrounding the core (which is most of the volume of the Sun, and is very hot but does not burn itself). The pressure of the energy generated pushes outward and would cause the Sun to expand if it were not exactly balanced by the gravitational pressure of the outer layers.
So the Sun neither collapses nor explodes. If the fusion reactions in the core become too weak, a star can and does collapse. Such collapse can provide new conditions in a core that result in new types of fusion reactions, so that expansion follows. If fusion reactions in the core become too strong, a star can and does explode. Such events can be observed: when a star explodes it shines with extreme brightness for a while; it turns from an unnoticed to a “new” star, a nova. Stars like our Sun, where inward pressure and outward pressure is nicely balanced, fluctuate little in brightness and give off a steady stream of energy. Other stars, where the balance is not so well tuned, pulsate noticeably. In the distant future, when this balance is disturbed because most of the hydrogen is used up, the Sun will expand. This will be the end of the solar system as we know it.
So the Sun shines with a fairly steady light. However, it has changed its output over geologic time. Also, it varies a bit on a number of cycles. The most obvious manifestation of this is the so-called sunspot cycle which describes periodic changes in the abundance of spots on the face of the Sun. The abundance of spots is related to the brightness of the Sun (more spots, more brightness); the variation is less than 1%, and the mechanisms are poorly understood. It has to do with changes in the magnetic field of the Sun and with convection within the outer layer of our star (not with processes in the core). Sunspot activity is closely in phase with ejection of solar plasma (protons mainly) and the ensuing aurorae in the polar regions of Earth.
The Sun is a typical star. Evidence indicates that this happens in the middle of all stars, giving them the energy to make starlight.
Read all about the Sun in Wikipedia, NASA, National Geographic, Space•com, nineplanets•org and many others
See the transits of Venus for more pictures of the surface of the sun. (A transit is when a planet passes directly between the Earth and the Sun, rather like a solar eclipse. A transit can involve the passage of any object in front of another and is very useful in determining sizes of tiny bodies like asteroids when they pass in front of a star).
| Spacecraft | Launch Date | Operator | Mission | Outcome | Remarks |
|---|---|---|---|---|---|
| Pioneer 5 (Pioneer A) [Most other Pioneers involved lunar or outer solar system trajectories, but all are listed below.] |
11 March 1960 | NASA/DOD (United States) | Orbiter | Successful | (Delta-E launch vehicle); end of mission 30 April 1960. Measured magnetic field phenomena, solar flare particles, and ionization in the interplanetary region. |
| Pioneer 6 (Pioneer B) | 16 December 1965 | NASA (United States) | Orbiter | Successful | (Delta-E launch vehicle); Pioneer 6–9 were a network of solar-orbiting “space weather” monitors, observing solar wind, cosmic rays, and magnetic fields – Pioneer 6 could still be contacted on 8 December 2000 |
| Pioneer 7 (Pioneer C) | 17 August 1966 | NASA (United States) | Orbiter | Successful | (Delta-E launch vehicle); mission as Pioneer 6; still contactable on 31 March 1995 |
| Pioneer 8 (Pioneer D) | 13 December 1967 | NASA (United States) | Orbiter | Successful | (Delta-E launch vehicle); mission as Pioneer 6; still contactable in 2001 |
| Pioneer 9 | 8 November 1968 | NASA (United States) | Orbiter | Successful | (Delta-E launch vehicle) mission as Pioneer 6; end of mission May 1983 |
| Pioneer-E | 27 August 1969 | NASA (United States) | Orbiter | Failure | (Delta-E launch vehicle) Intended as part of the Pioneer 6–9 network; failed to reach orbit |
| Helios A (Helios 1) | 10 December 1974 | NASA (United States) /BWF (West Germany) |
Orbiter | Successful | (Titan IIIE / Centaur launch vehicle) Mission lasted from 16 January 1975 until 18 February 1985. Observations of solar wind, magnetic and electric fields, cosmic rays and cosmic dust between Earth and Sun |
| Helios B (Helios 2) | 15 January 1976 | NASA (United States) /BWF (West Germany) |
Orbiter | Successful | (Titan IIIE / Centaur launch vehicle) Mission (same as Helios A) lasted from 21 July 1976 until 23 December 1979; reached perihelion on 17 April 1976 at a record distance of 0.29 AU (43.432 million km) and a speed of 241,350 km/hr |
| ISEE-3 | 12 August 1978 | NASA (United States) | Orbiter | Successful | (Delta 2914 launch vehicle) Observed solar phenomena in conjunction with earth-orbiting ISEE-1 and ISEE-2, 1978–1982; later renamed International Cometary Explorer (ICE) and directed to Comet Giacobini-Zinner |
| Ulysses | 6 October 1990 | ESA (Europe) /NASA (United States) |
Orbiter | Successful | Launched from the Space Shuttle Discovery (STS-41). South polar observations in 1994, 2000 and 2007; north polar observations in 1995, 2001 and 2008 [only partly successful – some data returned despite failing power and reduced transmission capacity]; Communications terminated on 30 June 2009 |
| WIND | 1 November 1994 | NASA (United States) | Orbiter (at Earth–Sun L1 point) | Successful | (Delta II launch vehicle) Solar wind measurements; still returning data (as of June 2015); sister ship to GGS Polar, launched by a Delta II on 24 February 1996 into a highly elliptical Earth orbit |
| SOHO | 2 December 1995 | ESA (Europe) /NASA (United States) |
Orbiter (at Earth–Sun L1 point) | Successful | (Atlas II launch vehicle) Began serious work in May 1996, investigation of Sun's core, corona, and solar wind; comet discoveries; mission extended until 31 December 2016 |
| ACE (Advanced Composition Explorer) |
25 August 1997 | NASA (United States) | Orbiter (at Earth–Sun L1 point) | Successful | (Delta II launch vehicle) Solar wind observations; still returning data (as of June 2015) |
| Genesis | 8 August 2001 | NASA (United States) | Orbiter/sample return | Successful | (Delta II launch vehicle) Mission 2001–2004; Solar wind sample return; crash landed on return to Earth, much data salvaged |
| STEREO A (Solar Terrestrial Relations Observatory) |
26 October 2006 | NASA (United States) | Orbiter | Successful | (Delta II launch vehicle) From December 2006, stereoscopic imaging of coronal mass ejections and other solar phenomena; still returning data (as of June 2015) |
| STEREO B | 26 October 2006 | NASA (United States) | Orbiter | Successful | Twin of STEREO A, launched together; still returning data (as of June 2015) |
| DSCOVR | 11 February 2015 | NOAA (United States) | Orbiter | Successful | (SpaceX Falcon 9 launch vehicle) Solar wind and coronal mass ejection monitoring, as well as Earth climate monitoring, February 2015 – present |
| Solar Sentinels | 2018? | NASA (United States) | Multi-probe Orbiter | Planned | Six probes watching the sun |
| “Solar Orbiter” | October 2018 | ESA (Europe) | Orbiter | Planned | Solar and heliospheric physics |
| Solar Probe Plus | 2018 | NASA (United States) | Orbiter | Planned | Close-range coronal observations |
| Aditya-1 | 2019–2020 | ISRO (India) | Orbiter | Planned | To study Solar Corona |
| Intergelio-Zond | 2022 | RKA (Russia) | Orbiter | Planned | Close-range solar observations |
The Pioneer program was a series of United States unmanned space missions that was designed for planetary exploration, and were launched over a long period to learn more about escaping from the gravity of the Earth, investigating the Moon, Jupiter and Saturn, and interplanetary space. There were a number of such missions in the program, but the most notable were Pioneer 10 and Pioneer 11, which explored the outer planets and left the solar system. Each carries a golden plaque, depicting a man and a woman and information about the origin and the creators of the probes, should any extraterrestrials find them someday.
The earliest missions were attempts to achieve Earth’s escape velocity, simply to show it was feasible and study the Moon. This included the first launch by NASA which was formed from the old NACA. These missions were carried out by the US Air Force and Army.
Most missions here are listed with their most recognised name, and alternate names after in brackets.
Five years after the early Able space probe missions ended, NASA Ames Research Center used the “Pioneer” name for a new series of missions, initially aimed at the inner solar system, before the bold flyby missions to Jupiter and Saturn. While successful, the missions returned much poorer images than the Voyager’s five years later. In 1978, the end of the program saw a return to the inner solar system, with the Pioneer Venus Orbiter and Multiprobe, this time using orbital insertion rather than flyby missions.
Pioneer 6 and Pioneer 9 are in solar orbits with 0.8 AUs distance to the Sun. Their orbital periods are therefore slightly shorter than Earth’s. Pioneer 7 and Pioneer 8 are in solar orbits with 1.1 AUs distance to the Sun. Their orbital periods are therefore slightly longer than Earth’s. Because they orbit the Sun on either side of the Earth’s orbital path, some of them are, from time to time, 180° away from Earth. They can sense parts of the Sun several days before the Sun’s rotation reveals it to ground based/earth orbiting observatories. If a powerful solar magnetic storm is born, they can warn Earth in advance.
Pioneers 6, 7, 8, and 9 were space probes that together formed a series of solar-orbiting, spin-stabilized, solar-cell and battery-powered satellites designed to obtain measurements on a continuing basis of interplanetary phenomena from widely separated points in space. They were also known as Pioneer A, B, C, and D respectively. Pioneer E was lost in a launch accident.
Pioneer 6 (1965-105A) was launched on 16th December 1965 into a circular solar orbit with a mean distance of 0.8 AU. The prime travelling-wave tube (TWT) failed some time after December 1995; in July 1996 the spacecraft was commanded to the backup TWT; on 6th October 1997 it was tracked with the 70-m Deep Space Station 43 in Australia. The MIT and ARC Plasma Analyzers as well as the cosmic ray detector from University of Chicago were turned on and working; it was still in touch on 8th December 2000, with successful telemetry contact for about two hours.
Helios A and Helios B (also known as Helios 1 and Helios 2), were a pair of probes launched into heliocentric orbit for the purpose of studying solar processes – observations of the solar wind, magnetic and electric fields, cosmic rays and cosmic dust between the Earth and Sun. They were a joint venture of the Federal Republic of Germany (West Germany) and NASA.
The probes are notable for having set a maximum speed record among spacecraft at 252,792 km/h. Helios 2 flew three million km closer to the Sun than Helios 1, achieving perihelion on 17th April 1976 at a record distance of 0.29 AU (or 43.432 million km), slightly inside the orbit of Mercury. The Helios space probes completed their primary missions by the early 1980s, but they continued to send data up to 1985. The probes are no longer functional but still remain in their elliptical orbits around the Sun.
[Left]: Photo of the ISEE-3 spacecraft during test and integration at the Goddard Space Flight Center (GSFC) [Image: NASA]
The primary scientific objective of ICE was to study the interaction between the solar wind and a cometary atmosphere. After a successful thruster burn to knock it loose from its halo orbit on 1st September 1982, it used the instability of the Earth/Moon and Earth/Sun Lagrange points, making a series of lunar orbits over the next 15 months. Its last and closest pass over the Moon, on 22nd December 1983, was a mere 119.4 km above the moon’s surface; on that date it was renamed the International Cometary Explorer. By the beginning of 1984, ICE was in heliocentric orbit ahead of Earth orbit to intercept comet Giacobini-Zinner.
Comet Giacobini-Zinner encounter: After ejection from the Earth-Moon system, ICE entered a heliocentric orbit ahead of the Earth on a trajectory intercepting that of Comet Giacobini-Zinner. On 11th September 1985, the craft passed through the plasma tail of Comet Giacobini-Zinner, becoming the first spacecraft to fly through a comet’s tail passing the nucleus at a distance of approximately 7800 km. Due to the nature of its original mission, ICE carried no cameras. It instead carried instruments for measurements of energetic particles, waves, plasmas, and fields.
Comet Halley encounter: ICE transited between the Sun and Comet Halley in late March 1986 becoming the first spacecraft to investigate two comets, when other spacecraft (Giotto, Vega 1 and 2, Suisei and Sakigake) were in the vicinity of Comet Halley on their early March comet rendezvous missions (the so-called Halley Armada). ICE flew through the tail and its minimum distance to the comet nucleus was 28 million km (for comparison the Earth’s minimum distance to Comet Halley in 1910 was 20.8 million km).
Heliospheric mission: An update to the ICE mission was approved by NASA in 1991. It defines a heliospheric mission for ICE consisting of investigations of coronal mass ejections in coordination with ground-based observations, continued cosmic ray studies, and the Ulysses probe. By May 1995 ICE was being operated with only a low duty cycle, with some support being provided by the Ulysses project for data analysis.
End of mission: On 5th May 1997 NASA ended the ICE mission, and ordered the probe shut down, with only a carrier signal left operating. The ISEE-3/ICE downlink bit rate was nominally 2048 bit/s during the early part of the mission, and 1024 bit/s during the Giacobini-Zinner comet encounter. The bit rate then successively dropped to 512 bit/s (on 9th December 1985), 256 bit/s (on 5th January 1987), 128 bit/s (on 24th January 1989) and finally to 64 bit/s (on 27th December 1991).
As of January 1990, ICE was in a 355-day heliocentric orbit with an aphelion of 1.03 AU, a perihelion of 0.93 AU and an inclination of 0.1°. It may be possible to capture the spacecraft in 2014 (on 10th August), when it again makes a close approach to Earth. If the craft is recovered, it has already been donated by NASA to the Smithsonian Institution.
Reactivation: In 1999, NASA made brief contact with ICE to verify its carrier signal. There was no contact with ICE after the end of its mission in 1999 until on 18th September 2008, NASA, with the help of KinetX, it was successfully located and reactivated using the Deep Space Network. A status check revealed that all but one of its 13 experiments were still functioning, and it still has enough propellant for 150 m/s of ΔV. ICE will return to fly by the Moon on 10th August 2014, when it could be re-captured into a halo orbit and possibly sent out again to explore another comet. NASA scientists are considering reusing the probe to observe additional comets in 2017 or 2018. Such a mission, however, would delay any attempt to capture the spacecraft until the 2040s.
Ulysses (1990-090B) was launched on 6th October 1990 from the Space Shuttle Discovery (STS-41). It was intended to study the Sun, especially its polar areas. To achieve this, it had to make three sling-shot manoeuvres of Jupiter. It made its first pass of Jupiter on 8th February 1992, a flyby for gravity assistance en route to an inclined heliocentric orbit for solar south polar observations, and in 1995 for solar north polar observations. It made a second distant flyby of Jupiter in 2000 for solar south polar observations and in 2001 for solar north polar observations. It made a third distant flyby of Jupiter in 2007 to 2008, again for solar south and north polar observations.
Ulysses made several flybys of the Sun; on its first it observed the south pole of the Sun in 1994 and the north in 1995; on its second flyby it observed the south pole in 2000 and the north in 2001; on the third, the south pole in 2007 and in 2008 it had partial success observing the north pole; some data was returned and after the X-band transmitter failed and the fuel had nearly frozen. Communications terminated on 30th June 2009.
The Geotail (1992-044A) spacecraft was launched on 24th July 1992, and is observing the Earth’s magnetosphere. The primary purpose is to study the structure and dynamics of the tail region of the magnetosphere. The orbit has been designed to cover the magnetotail over a wide range of distances: 8 to 210 times the Earth’s radius from the Earth; this orbit also allows study of the boundary region of the magnetosphere as it skims the magnetopause at perigees. In the first two years the double lunar swing-by technique was used to keep apogees in the distant magnetotail.
The apogee was lowered down to 50 Earth radii in mid-November 1994 and then to 30 Earth radii in February 1995 to study substorm processes in the near-Earth tail region. The present orbit is 9 to 30 Earth radii with an inclination of -7° to the ecliptic. Geotail instruments studied electric fields, magnetic fields, plasmas, energetic particles, and plasma waves. Geotail is an active mission as of 2012.
Geotail, Wind, Polar, SOHO, and Cluster were all part of the International Solar-Terrestrial Physics (ISTP) project.
The Global Geospace Science (GGS) WIND (1994-071A) is a science spacecraft launched on 1st November 1994. The satellite is a spin stabilized cylindrical satellite with a diameter of 2.4 m and a height of 1.8 m. It was deployed to study radio and plasma that occur in the solar wind and in the Earth’s magnetosphere before the solar wind reaches the Earth. The spacecraft’s original mission was to orbit the Sun at the L1 Lagrangian point, but this was delayed when the SOHO and ACE spacecraft were sent to the same location. WIND has been at L1 continuously since 2004, and is still operating as of December 2012. It currently has enough fuel to last roughly 60 years at L1 and continues to produce relevant research as its data has contributed to over 800 publications since 2008 and nearly 2000 publications before 2008. As of 10th December 2012, the total number of publications either directly or indirectly using WIND data is about 2,795.
WIND is the sister ship to GGS Polar or just Polar (1996-013A). Polar was a science spacecraft designed to study the polar magnetosphere and aurora. It was launched on 24th February 1996 into a highly elliptical orbit with apogee at 9 earth radii and perigee at 1.8 earth radii (geocentric), 86° inclination, with a period of around 18 hours. The apogee was initially over the northern polar region, but has since been precessing south at about 16° per year. Sensors on the spacecraft gathered multi-wavelength imaging of the aurora, and measured the entry of plasma into the polar magnetosphere and the geomagnetic tail, the flow of plasma to and from the ionosphere, and the deposition of particle energy in the ionosphere and upper atmosphere. The nominal mission duration was two years, but was extended several times. Polar operations were finally terminated in April 2008; it remains in orbit, though is now inactive.
The Solar and Heliospheric Observatory (SOHO) (1995-065A) was launched on 2nd December 1995, and built by a European industrial consortium led by Matra Marconi Space (now Astrium). It is investigating the Sun’s core, corona, and the solar wind; it also discovered over 2,400 comets. It began normal operations in May 1996 as a joint project of international cooperation between the European Space Agency (ESA) and NASA. Originally planned as a two-year mission, SOHO currently continues to operate after over seventeen years in space. In November 2012, a mission extension lasting until December 2014 was approved.
In addition to its scientific mission, it is currently the main source of near-real-time solar data for space weather prediction. Along with WIND and ACE, SOHO is one of three spacecraft currently in the vicinity of the Earth–Sun L1 point, a point of gravitational balance located approximately 0.99 astronomical units (AU) from the Sun and 0.01 AU from the Earth. In addition to its scientific contributions, SOHO is distinguished by being the first three-axis-stabilized spacecraft to use its reaction wheels as a kind of virtual gyroscope; the technique was adopted after an on-board emergency in 1998 that nearly resulted in the loss of the spacecraft. It is in a halo orbit around the L1 point.
SOHO is not exactly at L1 as this would make communication difficult due to radio interference generated by the Sun, and because it is not a stable orbit. Rather it lies in the (constantly moving) plane which passes through L1 and is perpendicular to the line connecting the Sun and the Earth. It stays in this plane, tracing out an elliptical lissajous orbit centered about L1. It orbits L1 once every six months, while L1 itself orbits the sun every 12 months as it is coupled with the motion of the Earth. This keeps SOHO at a good position for communication with Earth at all times.
Advanced Composition Explorer (ACE) (1997-045A) was launched on 25th August 1997. It is a NASA Explorer program solar and space exploration mission to study energetic particles from the solar wind, the interplanetary medium, and other sources. Real-time data from ACE is used by the Space Weather Prediction Center to improve forecasts and warnings of solar storms. It is currently operating in a Lissajous orbit close to the Sun–Earth L1 Lagrange point (like SOHO). The spacecraft is still in generally good condition, and has enough fuel to maintain its orbit until 2024.
Cluster II is a space mission of the European Space Agency, with NASA participation, to study the Earth’s magnetosphere over the course of an entire solar cycle. The mission is composed of four identical spacecraft flying in a tetrahedral formation. (2000-041A, 2000-041B, 2000-045A, 2000-045B) Launch date FM6: 16 July 2000, 12:39 UTC, FM7: 16 July 2000, 12:39 UTC, FM5: 09 August 2000, 11:13 UTC, FM8: 09 August 2000, 11:13 UTC. In Geocentric orbit
WMAP (2001-027A) was launched on 30th June 2001 at 19:46 UTC, and placed into a solar orbit at the Earth’s L2 Lagrangian point (1.5 million km from Earth). It made cosmic background radiation observations until October 2010, and was then sent to a heliocentric “graveyard orbit” after 9 years of use.
The WMAP spacecraft (2001-027A) was launched on 30th June 2001, an almost perfect launch, on time to the second
WMAP measures differences in the temperature of the Big Bang’s remnant radiant heat – the Cosmic Microwave Background Radiation – across the full sky. The anisotropies then are used to measure the universe’s geometry, content, and evolution; and to test the Big Bang model, and the cosmic inflation theory. For that, the mission created a full-sky map of the CMB, with a 13 arc-minute resolution via multi-frequency observation. The map requires the fewest systematic errors, no correlated pixel noise, and accurate calibration, to ensure angular-scale accuracy greater than its resolution. The map contains 3,145,728 pixels, and uses the HEALPix scheme to pixelize the sphere. The telescope also measures the CMB’s E-mode polarization, and foreground polarization; its life was 27 months – 3 months to reach the L2 position, 2 years of observation. The WMAP mission succeeds the COBE space mission and was the second medium-class (MIDEX) spacecraft of the Explorer program. On 20th December 2012, the Nine-year WMAP data and related images were released.
WMAP’s measurements played the key role in establishing the current Standard Model of Cosmology: the Lambda–CDM model. WMAP data are very well fit by a universe that is dominated by dark energy in the form of a cosmological constant. Other cosmological data are also consistent, and together tightly constrain the Model. In the Lambda-CDM model of the universe, the age of the universe is 13.772±0.059 billion years. The WMAP mission’s determination of the age of the universe to better than 1% precision was recognized by the Guinness Book of World Records. The WMAP measurements also support the cosmic inflation paradigm in several ways, including the flatness measurement.
Genesis (2001-034A) was launched on 8th August 2001 into an orbit near the Sun–Earth L1 Lagrange point (like SOHO). It collected a sample of solar wind and returned it to Earth for analysis. It was the first NASA sample return mission to return material since the Apollo program, and the first to return material from beyond the orbit of the Moon. It crash-landed in Dugway Proving Ground, Utah on 8th September 2004, after a design flaw prevented the deployment of its drogue parachute. The crash contaminated many of the sample collectors, and although most were damaged, many of the collectors were successfully recovered. The Genesis science team demonstrated that some of the contamination could be removed or avoided, and that the solar wind could be analyzed using a variety of approaches. It is relatively easy to detect the solar wind, but the precision measurements are difficult and techniques are still being refined in laboratories worldwide. Nonetheless, in March 2008 scientists stated that they believed that all of the mission’s major science objectives would be achieved successfully.
Hinode (Solar B) (2006-041A) has been placed in a sun-synchronous orbit around Earth at an altitude of about 600 km. A sun-synchronous orbit is a special type of polar orbit in which a satellite passes over the same part of the Earth at roughly the same local time each day.
There are two advantages of Hinode’s polar orbit: the science instruments can look at the Sun all the time, and because the satellite passes over the same spot on Earth at about the same time each day, downloads of data are much easier.
Solar TErrestrial RElations Observatory (STEREO) is a solar observation mission. STEREO A (“Ahead”, 2006-047A) and STEREO B (“Behind”, 2006-047B) were launched on 26th October 2006, two nearly identical spacecraft whose orbits around the sun cause them to respectively pull farther ahead of and fall gradually behind the Earth. This enables stereoscopic imaging of the Sun and solar phenomena, such as coronal mass ejections. They were in highly elliptical geocentric orbits. The apogee reached the Moon’s orbit. On 15th December 2006, on the fifth orbit, the pair swung by the Moon for a gravitational slingshot. Because the two spacecraft were in slightly different orbits, A was ejected to a heliocentric orbit inside Earth’s orbit while B remained temporarily in a high earth orbit. B encountered the Moon again on the same orbital revolution on 21st January 2007, ejecting itself from earth orbit in the opposite direction to A. B entered a heliocentric orbit outside the Earth’s orbit. A takes 346 days to complete one revolution of the sun and B takes 388 days. The A-spacecraft/sun/earth angle will increase at 21.650°/year. The B-spacecraft/sun/earth angle will change at -21.999°/year.
Interstellar Boundary Explorer (2008-051A) (IBEX) is a NASA satellite that is making a map of the boundary between the Solar System and interstellar space. The mission is part of NASA’s Small Explorer program and launched with a Pegasus-XL rocket on 19th October 2008, at 17:47:23 UTC. It is in a very eccentric Earth orbit.
Results from IBEX have repeatedly shocked the scientific community and overturned old theories. The first shock came when it revealed a narrow ribbon of energetic neutral atom (ENA) emission. Then it showed shifts over time in this band. Another surprise came when no bow shock was found. The repercussions of overturning the bow shock theory are huge, because decades of research are based on that concept.
Kepler (2009-011A) was launched on 7th March 2009 and placed in an Earth-trailing heliocentric orbit; Kepler is expected to operate for at least 7½ years, searching for Earth-like planets orbiting other stars.
As of January 2015, Kepler and its follow-up observations had found 1,013 confirmed exoplanets in about 440 stellar systems, along with a further 3,199 unconfirmed planet candidates. In November 2013, astronomers reported, based on Kepler data, that there could be as many as 40 billion Earth-sized planets orbiting in the habitable zones of Sun-like stars and red dwarfs within the Milky Way. It is estimated that 11 billion of these planets may be orbiting Sun-like stars. The nearest such planet may be 12 light years away, according to the scientists.
In July 2012 one of the spacecraft’s stabilizing wheels stopped operating; in May 2013 a second failed. Three such wheels are necessary for Kepler to function, and now it only has two that are operational. It seems as though its planet-searching days are over. So far, the $600m (£395m) mission has confirmed 132 planets outside our solar system.
Following this severe malfunction, the spacecraft was remotely repaired, the engineers having devised a remarkable solution to use the pressure of sunlight to stabilize the spacecraft. And it worked.
The mission was reassigned the identity K2 and resumed its work.
See Kepler mission and K2 mission
As of March 2016 Kepler and K2 have found 4,696 candidate exoplanets, with 1,041 and 38 respectively confirmed, of which 12 appear to be in the small habitable zone, the “Goldilocks zone”.
In May 2016, NASA’s Kepler Mission Announced Largest Collection of Planets Ever Discovered. NASA also said that Kepler’s study of exoplanets was helping in understanding how our own solar system formed, in particular the Kepler-223 System gave clues to the migration of the planets in our own system.
Herschel (2009-026A) was launched on 14th May 2009 reaching a Lissajous orbit around the Sun–Earth Lagrange L2 point (like SOHO’s at L1), 1,500,000 km from the Earth, about two months later; it is studying the formation and evolution of galaxies and stars.
The Herschel Space Observatory is a European Space Agency space observatory sensitive to the far infrared and submillimetre wavebands (55-672 μm). It is the largest infrared space telescope ever launched, carrying a single mirror of 3.5 m in diameter. It is in its final months; it is hoped its coolant will last until March 2013. Herschel is capable of seeing the coldest and dustiest objects in space; for example, cool cocoons where stars form and dusty galaxies just starting to bulk up with new stars. The observatory will sift through star-forming clouds – the “slow cookers” of star ingredients – to trace the path by which potentially life-forming molecules, such as water, form. NASA is participating in the ESA built and operated observatory. It is the fourth “cornerstone” mission in the ESA science program, along with Rosetta, Planck, and Gaia.
Its instruments are HIFI (Heterodyne Instrument for the Far Infrared), PACS (Photodetector Array Camera and Spectrometer), and SPIRE (Spectral and Photometric Imaging Receiver). Herschel will specialise in collecting light from objects in our Solar System as well as the Milky Way and even extragalactic objects billions of light-years away, such as newborn galaxies, and is charged with four primary areas of investigation: Galaxy formation in the early universe and the evolution of galaxies; Star formation and its interaction with the interstellar medium; Chemical composition of atmospheres and surfaces of Solar System bodies, including planets, comets and moons; and Molecular chemistry across the universe.
Planck (2009-026B) was launched on 14th May 2009 into a Lissajous orbit around the Sun–Earth Lagrange L2 point (similar to that of Herschel) which it reached in July; Planck is a space observatory of the European Space Agency (ESA) and designed to observe the anisotropies of the cosmic microwave background (CMB) radiation over the entire sky, at microwave and infra-red frequencies to a high sensitivity and angular resolution. It is the third Medium-Sized Mission (M3) of the European Space Agency’s Horizon 2000 Scientific Programme.
By February 2010 it had successfully started a second all-sky survey. On 21st March 2013, the mission’s all-sky map of the cosmic microwave background was released. The mission complements and improves upon observations made by the NASA Wilkinson Microwave Anisotropy Probe (WMAP), which has measured the anisotropies at larger angular scales and lower sensitivity than Planck. Planck provides a major source of information relevant to several cosmological and astrophysical issues, such as testing theories of the early universe and the origin of cosmic structure.
The mission has a wide variety of scientific aims, including: High resolution detections of both the total intensity and polarization of the primordial CMB anisotropies; Creation of a catalogue of galaxy clusters through the Sunyaev–Zel'dovich effect, using which dense clusters of galaxies have been observed; Observations of the gravitational lensing of the CMB, as well as the integrated Sachs–Wolfe effect, in which photons from the CMB are gravitationally redshifted, causing the CMB spectrum to appear uneven; Observations of bright extragalactic radio (active galactic nuclei) and infrared (dusty galaxy) sources; Observations of the Milky Way, including the interstellar medium, distributed synchrotron emission and measurements of the Galactic magnetic field; and Studies of the Solar System, including planets, asteroids, comets and the zodiacal light.
Planck represents an advance over WMAP in several respects: It has higher resolution, allowing it to probe the power spectrum of the CMB to much smaller scales (×3); Higher sensitivity (×10); and It observes in 9 frequency bands rather than 5, with the goal of improving the astrophysical foreground models.
It is expected that most Planck measurements will be limited by how well foregrounds can be subtracted, rather than by the detector performance or length of the mission. This is particularly important for the polarization measurements. The dominant foreground depends on frequency, but examples include synchrotron radiation from the Milky Way at low frequencies, and dust at high frequencies.
The Solar Dynamics Observatory [2010-005A] is a NASA mission which has been observing the Sun since 2010. Launched on 11th February 2010, the observatory is part of the Living With a Star program. It is in a geocentric geosynchronous orbit.
Interface Region Imaging Spectrograph (2013-033A). Launch date 27th June 2013, 02:27:46 UTC. It is in a Sun-synchronous orbit.
Data collected from the IRIS spacecraft has shown that the interface region of the sun is significantly more complex than previously known. This includes features described as solar heat bombs, high-speed plasma jets, nano-flares, and mini-tornadoes. These features are an important step in understanding the transfer of heat to the corona.
The European Space Agency’s Gaia space observatory was launched [video] from the Kourou space centre in French Guiana by a Russian Soyuz ST-B/Fregat-MT rocket at 09:12:14 UTC on 19th December 2013 (COSPAR ID: 2013-074A). It was manufactured by EADS Astrium and e2v Technologies, and is planned to operate for five years.
Gaia’s mass at launch was 2,029 kg, its dry mass 1,392 kg, and its power requirement is 1,910 watts. The spacecraft’s size is 4.6 m × 2.3 m.
A second burn of the rocket inserted Gaia into a gravitationally stable Lissajous orbit at the Sun-Earth L2 Lagrangian point, with planned periapsis and apoapsis of 90,000 km and 340,000 km respectively, and a period of 180 days; this is almost a million miles from Earth which will shield it from the Sun.
From 8th January 2014 (i.e. after 20 days) Gaia started to create the most accurate map yet of the Milky Way, by pinpointing more than a billion stars in 3D with unprecedented accuracy. It is also expected to discover thousands of previously unknown objects, including exploding stars, planets orbiting other suns, and nearby asteroids. Scientists hope Gaia will yield clues about two of the universe’s biggest mysteries, Dark Matter and Dark Energy.
As it spins slowly, two telescopes will sweep across the entire sky and simultaneously focus their light on the largest digital camera ever put into space. More than a billion stars will be observed an average of 70 times each over the five-year period. Gaia will measure the position and key physical properties of every star, including its brightness, temperature and chemical composition.
More information at Gaia’s home page and at Wikipedia.
Deep Space Climate Observatory (DSCOVR) (formerly known as Triana, unofficially known as GoreSat) (2015-007) is a National Oceanic and Atmospheric Administration (NOAA) Earth observation and space weather satellite launched by SpaceX on a Falcon 9 launch vehicle on 11th February 2015 from Cape Canaveral.
DSCOVR was originally developed as a NASA satellite proposed in 1998 by then-Vice President Al Gore for the purpose of Earth observation. It is in a Lissajous orbit at the Sun-Earth L1 Lagrangian point, 1,500,000 km from Earth, to monitor variable solar wind condition, provide early warning of approaching coronal mass ejections and observe phenomena on Earth including changes in ozone, aerosols, dust and volcanic ash, cloud height, vegetation cover and climate. At this location it will have a continuous view of the Sun and the sunlit side of the Earth. The satellite is orbiting the Sun-Earth L1 point in a six-month period, with a spacecraft-Earth-Sun angle varying from 4 to 15 degrees. It will take full-Earth pictures about every two hours and be able to process them faster than other Earth observation satellites.
DSCOVR started orbiting around L1 by 8th June 2015, just over 100 days after launch.
LISA Pathfinder (“Laser Interferometer Space Antenna”) (2015-070) was launched at 04:04 UTC on 3rd December 2015 from the Kourou Guiana Space Centre. It is an ESA space probe whose mission will test technologies needed for the Evolved Laser Interferometer Space Antenna (eLISA), a gravitational wave observatory planned to be launched in 2034 to detect the gravitational waves predicted by Einstein’s General Theory of Gravitation. It was manufactured by Airbus at an estimated mission cost of €400 million, and launched on a Vega rocket by Arianespace.
It is in a Lissajous orbit at the Sun–Earth Lagrange L1 point, with periapsis and apoapsis of 500,000 km and 800,000 km respectively, and an orbital inclination of 60°.
LISA Pathfinder will place two test masses in a nearly perfect gravitational free-fall, and will control and measure their relative motion with unprecedented accuracy. The laser interferometer (operating at a wavelength of about 36.7 cm) measures the relative position and orientation of the masses 40 cm apart to an accuracy of less than 10−11 metre, a technology estimated to be sensitive enough to detect gravitational waves by the follow-on mission, eLISA.
Solar Probe Plus is due to be launched on 30th July 2018 to examine the Sun’s corona.
Solar Orbiter, due to be launched in October 2018, is intended to perform detailed measurements of the inner heliosphere and nascent solar wind.