Interstellar Boundary Explorer (IBEX or Explorer 91 or SMEX-10) is a

NASA The National Aeronautics and Space Administration (NASA ) is an independent agencies of the United States government, independent agency of the federal government of the United States, US federal government responsible for the United States ...

satellite in

Earth Earth is the third planet from the Sun and the only astronomical object known to Planetary habitability, harbor life. This is enabled by Earth being an ocean world, the only one in the Solar System sustaining liquid surface water. Almost all ...

orbit that uses

energetic neutral atom Energetic Neutral Atom (ENA) imaging is a technology used to create global images of otherwise invisible phenomena in the magnetospheres of planets and throughout the heliosphere. Charged particles— protons, electrons, and various nuclei— ...

s (ENAs) to image the interaction region between the

Solar System The Solar SystemCapitalization of the name varies. The International Astronomical Union, the authoritative body regarding astronomical nomenclature, specifies capitalizing the names of all individual astronomical objects but uses mixed "Sola ...

and interstellar space. The mission is part of NASA's Small Explorer program and was launched with a Pegasus-XL

launch vehicle A launch vehicle is typically a rocket-powered vehicle designed to carry a payload (a crewed spacecraft or satellites) from Earth's surface or lower atmosphere to outer space. The most common form is the ballistic missile-shaped multistage ...

on 19 October 2008. The mission is led by Dr. David J. McComas (IBEX principal investigator), formerly of the

Southwest Research Institute Southwest Research Institute (SwRI), headquartered in San Antonio, Texas, is an independent and nonprofit applied research and development (R&D) organization. Founded in 1947 by oil businessman Tom Slick, it provides contract research and deve ...

(SwRI) and now with

Princeton University Princeton University is a private university, private Ivy League research university in Princeton, New Jersey, United States. Founded in 1746 in Elizabeth, New Jersey, Elizabeth as the College of New Jersey, Princeton is the List of Colonial ...

. The

Los Alamos National Laboratory Los Alamos National Laboratory (often shortened as Los Alamos and LANL) is one of the sixteen research and development Laboratory, laboratories of the United States Department of Energy National Laboratories, United States Department of Energy ...

and the

Lockheed Martin The Lockheed Martin Corporation is an American Arms industry, defense and aerospace manufacturer with worldwide interests. It was formed by the merger of Lockheed Corporation with Martin Marietta on March 15, 1995. It is headquartered in North ...

Advanced Technology Center built the IBEX-Hi and IBEX-Lo sensors respectively. The

Orbital Sciences Corporation Orbital Sciences Corporation (commonly referred to as Orbital) was an American company specializing in the design, manufacture, and launch of small- and medium- class space and launch vehicle systems for commercial, military and other governmen ...

manufactured the

satellite bus A satellite bus (or spacecraft bus) is the main body and structural component of a satellite or spacecraft, in which the payload and all scientific instruments are held. Bus-derived satellites are less customized than specially-produced satelli ...

and was the location for spacecraft environmental testing. The nominal mission baseline duration was two years after commissioning, and the prime ended in early 2011. The spacecraft and sensors are still healthy and the mission is continuing in its extended mission. IBEX is in a

Sun The Sun is the star at the centre of the Solar System. It is a massive, nearly perfect sphere of hot plasma, heated to incandescence by nuclear fusion reactions in its core, radiating the energy from its surface mainly as visible light a ...

-oriented spin-stabilized orbit around the Earth. In June 2011, IBEX was shifted to a new, more efficient, much more stable orbit. It does not come as close to the

Moon The Moon is Earth's only natural satellite. It Orbit of the Moon, orbits around Earth at Lunar distance, an average distance of (; about 30 times Earth diameter, Earth's diameter). The Moon rotation, rotates, with a rotation period (lunar ...

in the new orbit, and expends less fuel to maintain its position. The spacecraft is equipped with two large aperture imagers which detect ENAs with energies from 10 eV to 2

keV In physics, an electronvolt (symbol eV), also written electron-volt and electron volt, is the measure of an amount of kinetic energy gained by a single electron accelerating through an electric potential difference of one volt in vacuum. When us ...

(IBEX-Lo) and 300 eV to 6 keV (IBEX-Hi). The mission was originally planned to be a 24-month operations period. The mission has since been extended, with the spacecraft still in operation .

Spacecraft

The spacecraft is built on an octagonal base, roughly high and across. The dry mass is and the instrument payload comprises . The fully fueled mass is , and the entire flight system launch mass, including the ATK Star 27

solid rocket motor A solid-propellant rocket or solid rocket is a rocket with a rocket engine that uses solid propellants (fuel/oxidizer). The earliest rockets were solid-fuel rockets powered by gunpowder. The inception of gunpowder rockets in warfare can be cre ...

, is . The spacecraft itself has a

hydrazine Hydrazine is an inorganic compound with the chemical formula . It is a simple pnictogen hydride, and is a colourless flammable liquid with an ammonia-like odour. Hydrazine is highly hazardous unless handled in solution as, for example, hydraz ...

attitude control system Spacecraft attitude control is the process of controlling the orientation of a spacecraft (vehicle or satellite) with respect to an inertial frame of reference or another entity such as the celestial sphere, certain fields, and nearby objects, ...

. Power is produced by a

solar array A photovoltaic system, also called a PV system or solar power system, is an electric power system designed to supply usable solar power by means of photovoltaics. It consists of an arrangement of several components, including solar panels to abs ...

with a capability of 116

watt The watt (symbol: W) is the unit of Power (physics), power or radiant flux in the International System of Units (SI), equal to 1 joule per second or 1 kg⋅m2⋅s−3. It is used to quantification (science), quantify the rate of Work ...

s, and nominal power use is 66 W (16 W for the payload). Communications are via two hemispherical antennas with a nominal downlink data rate of 320 kbps and an uplink rate of 2 kbps.

Science goal

The Interstellar Boundary Explorer (IBEX) mission science goal is to discover the nature of the interactions between the

solar wind The solar wind is a stream of charged particles released from the Sun's outermost atmospheric layer, the Stellar corona, corona. This Plasma (physics), plasma mostly consists of electrons, protons and alpha particles with kinetic energy betwee ...

and the

interstellar medium The interstellar medium (ISM) is the matter and radiation that exists in the outer space, space between the star systems in a galaxy. This matter includes gas in ionic, atomic, and molecular form, as well as cosmic dust, dust and cosmic rays. It f ...

at the edge of the Solar System. IBEX has achieved this goal by generating full sky maps of the intensity (integrated over the line-of-sight) of ENAs in a range of energies every six months. Most of these ENAs are generated in the

heliosheath The heliosphere is the magnetosphere, astrosphere, and outermost atmospheric layer of the Sun. It takes the shape of a vast, tailed bubble-like region of space. In plasma physics terms, it is the cavity formed by the Sun in the surrounding in ...

, which is the region of interaction.

Mission

Launch

The IBEX satellite was mated to its Pegasus XL launch vehicle at

Vandenberg Air Force Base Vandenberg may refer to: * Vandenberg (surname), including a list of people with the name * USNS ''General Hoyt S. Vandenberg'' (T-AGM-10), transport ship in the United States Navy, sank as an artificial reef in Key West, Florida * Vandenberg S ...

California California () is a U.S. state, state in the Western United States that lies on the West Coast of the United States, Pacific Coast. It borders Oregon to the north, Nevada and Arizona to the east, and shares Mexico–United States border, an ...

, and the combined vehicle was then suspended below the

Lockheed L-1011 The Lockheed L-1011 TriStar (pronounced "El-ten-eleven") is an American medium-to-long-range, wide-body trijet airliner built by the Lockheed Corporation. It was the third wide-body airliner to enter commercial operations, after the Boeing 74 ...

'' Stargazer'' mother airplane and flown to

Kwajalein Atoll Kwajalein Atoll (; Marshallese language, Marshallese: ) is part of the Marshall Islands, Republic of the Marshall Islands (RMI). The southernmost and largest island in the atoll is named Kwajalein Island, which its majority English-speaking re ...

in the central

Pacific Ocean The Pacific Ocean is the largest and deepest of Earth's five Borders of the oceans, oceanic divisions. It extends from the Arctic Ocean in the north to the Southern Ocean, or, depending on the definition, to Antarctica in the south, and is ...

. ''Stargazer'' arrived at Kwajalein Atoll on 12 October 2008. The IBEX satellite was carried into space on 19 October 2008, by the Pegasus XL launch vehicle. The launch vehicle was released from ''Stargazer'', which took off from Kwajalein Atoll, at 17:47:23

UTC Coordinated Universal Time (UTC) is the primary time standard globally used to regulate clocks and time. It establishes a reference for the current time, forming the basis for civil time and time zones. UTC facilitates international communica ...

. By launching from this site close to the

equator The equator is the circle of latitude that divides Earth into the Northern Hemisphere, Northern and Southern Hemisphere, Southern Hemispheres of Earth, hemispheres. It is an imaginary line located at 0 degrees latitude, about in circumferen ...

, the Pegasus launch vehicle lifted as much as more mass to orbit than it would have with a launch from the

Kennedy Space Center The John F. Kennedy Space Center (KSC, originally known as the NASA Launch Operations Center), located on Merritt Island, Florida, is one of the NASA, National Aeronautics and Space Administration's (NASA) ten NASA facilities#List of field c ...

Florida Florida ( ; ) is a U.S. state, state in the Southeastern United States, Southeastern region of the United States. It borders the Gulf of Mexico to the west, Alabama to the northwest, Georgia (U.S. state), Georgia to the north, the Atlantic ...

Mission profile

The IBEX satellite initially launched into a highly elliptical transfer orbit with a low

perigee An apsis (; ) is the farthest or nearest point in the orbit of a planetary body about its primary body. The line of apsides (also called apse line, or major axis of the orbit) is the line connecting the two extreme values. Apsides perta ...

and used a solid fuel rocket motor as its final boost stage at

apogee An apsis (; ) is the farthest or nearest point in the orbit of a planetary body about its primary body. The line of apsides (also called apse line, or major axis of the orbit) is the line connecting the two extreme values. Apsides perta ...

in order to raise its perigee greatly and to achieve its desired high-altitude elliptical orbit. IBEX is in a highly eccentric elliptical terrestrial orbit, which ranges from a perigee of about to an apogee of about . Its original orbit was about — that is, about 80% of the distance to the

— which has changed primarily due to an intentional adjustment to prolong the spacecraft's useful life. This very high orbit allows the IBEX satellite to move out of the Earth's

magnetosphere In astronomy and planetary science, a magnetosphere is a region of space surrounding an astronomical object in which charged particles are affected by that object's magnetic field. It is created by a celestial body with an active interior Dynamo ...

when making scientific observations. This extreme altitude is critical due to the amount of charged-particle interference that would occur while taking measurements within the magnetosphere. When within the magnetosphere of the Earth (), the satellite also performs other functions, including

telemetry Telemetry is the in situ collection of measurements or other data at remote points and their automatic transmission to receiving equipment (telecommunication) for monitoring. The word is derived from the Greek roots ''tele'', 'far off', an ...

downlinks.

Orbit adjusted

In June 2011, IBEX shifted to a new orbit that raised its perigee to more than . The new orbit has a period of one third of a lunar month, which, with the correct phasing, avoids taking the spacecraft too close to the Moon, whose gravity can negatively affect IBEX's orbit. The now spacecraft uses less fuel to maintain a stable orbit, increasing its useful lifespan to more than 40 years.

Instruments

The heliospheric boundary of the Solar System is being imaged by measuring the location and magnitude of charge-exchange collisions occurring in all directions. The satellite's payload consists of two energetic neutral atom (ENA) imagers, IBEX-Hi and IBEX-Lo. Each consists of a

collimator A collimator is a device which narrows a beam of particles or waves. To narrow can mean either to cause the directions of motion to become more aligned in a specific direction (i.e., make collimated light or parallel rays), or to cause the spat ...

that limits their fields of view (FoV) a conversion surface to convert neutral

hydrogen Hydrogen is a chemical element; it has chemical symbol, symbol H and atomic number 1. It is the lightest and abundance of the chemical elements, most abundant chemical element in the universe, constituting about 75% of all baryon, normal matter ...

and

oxygen Oxygen is a chemical element; it has chemical symbol, symbol O and atomic number 8. It is a member of the chalcogen group (periodic table), group in the periodic table, a highly reactivity (chemistry), reactive nonmetal (chemistry), non ...

into

ions An ion () is an atom or molecule with a net electrical charge. The charge of an electron is considered to be negative by convention and this charge is equal and opposite to the charge of a proton, which is considered to be positive by convent ...

, an

electrostatic analyzer An electrostatic analyzer or ESA is an instrument used in ion optics that employs an electric field to allow the passage of only those ions or electrons that have a given specific energy. It usually also focuses these particles (concentrates them) ...

(ESA) to suppress

ultraviolet Ultraviolet radiation, also known as simply UV, is electromagnetic radiation of wavelengths of 10–400 nanometers, shorter than that of visible light, but longer than X-rays. UV radiation is present in sunlight and constitutes about 10% of ...

light and to select ions of a specific energy range, and a detector to count particles and identify the type of each ion. Both of these sensors are a single-pixel camera with a field of view of roughly 7° x 7°. The IBEX-Hi instrument is recording particle counts in a higher energy band (300 eV to 6 keV) than the IBEX-Lo energy band (10 eV to 2 keV). The scientific payload also includes a Combined Electronics Unit (CEU) that controls the voltages on the collimator and the ESA, and it reads and records data from the particle detectors of each sensor.

Communication

Compared to other space observatories, IBEX has a low data transfer rate due to the limited requirements of the mission.

Data collection

IBEX is collecting energetic neutral atom (ENA) emissions that are traveling through the Solar System to Earth and cannot be measured by conventional telescopes. These ENAs are created on the boundary of the Solar System by the interactions between solar wind particles and interstellar medium particles. On average IBEX-Hi detects about 500 particles per day, and IBEX-Lo, less than 100. By 2012, over 100 scientific papers related to IBEX were published, described by the principal investigator as "an incredible scientific harvest".

Data availability

As the IBEX data is validated, the IBEX data is made available in a series of data releases on the SwRI IBEX Public Data website. In addition, the data is periodically sent to the NASA Space Physics Data Facility (SPDF), which is the official archive site for IBEX data. SPDF data can be searched at the Heliophysics Data Portal.

Science results

Initial data revealed a previously unpredicted "very narrow ribbon that is two to three times brighter than anything else in the sky". Initial interpretations suggest that "the interstellar environment has far more influence on structuring the

heliosphere The heliosphere is the magnetosphere, astrosphere, and outermost atmospheric layer of the Sun. It takes the shape of a vast, tailed bubble-like region of space. In plasma physics terms, it is the cavity formed by the Sun in the surrounding ...

than anyone previously believed". It is unknown what is creating the energetic neutral atoms (ENA) ribbon. The Sun is currently traveling through the

Local Interstellar Cloud The Local Interstellar Cloud (LIC), also known as the Local Fluff, is an interstellar cloud roughly across, through which the Solar System is moving. This feature overlaps with a region around the Sun referred to as the solar neighborhood. It i ...

, and the heliosphere's size and shape are key factors in determining its shielding power from

cosmic ray Cosmic rays or astroparticles are high-energy particles or clusters of particles (primarily represented by protons or atomic nuclei) that move through space at nearly the speed of light. They originate from the Sun, from outside of the ...

s. Should IBEX detect changes in the shape of the ribbon, that could show how the heliosphere is interacting with the Local Fluff. It has also observed ENAs from the Earth's

. In October 2010, significant changes were detected in the ribbon after six months, based on the second set of IBEX observations. It went on to detect neutral atoms from outside the Solar System, which were found to differ in composition from the Sun. Surprisingly, IBEX discovered that the heliosphere has no

bow shock In astrophysics, bow shocks are shock waves in regions where the conditions of density and pressure change dramatically due to blowing stellar wind. Bow shock occurs when the magnetosphere of an astrophysical object interacts with the nearby fl ...

, and it measured its speed relative to the local

(LISM) as , improving on the previous measurement of by '' Ulysses''. Those speeds equate to 25% less pressure on the Sun's heliosphere than previously thought. In July 2013, IBEX results revealed a 4-lobed tail on the Solar System's heliosphere.