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 US federal government responsible for the civil List of government space agencies, space program ...

satellite in

Earth Earth is the third planet from the Sun and the only astronomical object known to harbor life. While large volumes of water can be found throughout the Solar System, only Earth sustains liquid surface water. About 71% of Earth's surfa ...

orbit that uses energetic neutral atoms (ENAs) to image the interaction region between the

Solar System The Solar System Capitalization 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 "Solar ...

and

interstellar space Outer space, commonly shortened to space, is the expanse that exists beyond Earth and its atmosphere and between celestial bodies. Outer space is not completely empty—it is a near-perfect vacuum containing a low density of particles, predo ...

. The mission is part of NASA's

Small Explorer program The Explorers program is a NASA exploration program that provides flight opportunities for physics, geophysics, heliophysics, and astrophysics investigations from space. Launched in 1958, Explorer 1 was the first spacecraft of the United Stat ...

and was launched with a Pegasus-XL

launch vehicle A launch vehicle or carrier rocket is a rocket designed to carry a payload ( spacecraft or satellites) from the Earth's surface to outer space. Most launch vehicles operate from a launch pads, supported by a launch control center and ...

on 19 October 2008. The mission is led by Dr. David J. McComas (IBEX principal investigator), formerly of the Southwest Research Institute (SwRI) and now with

Princeton University Princeton University is a private research university in Princeton, New Jersey. Founded in 1746 in Elizabeth as the College of New Jersey, Princeton is the fourth-oldest institution of higher education in the United States and one of the ...

. The

Los Alamos National Laboratory Los Alamos National Laboratory (often shortened as Los Alamos and LANL) is one of the sixteen research and development laboratories of the United States Department of Energy (DOE), located a short distance northwest of Santa Fe, New Mexico, ...

and the Lockheed Martin 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 opposed to specially produced satellites. Bus- ...

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 center of the Solar System. It is a nearly perfect ball of hot plasma, heated to incandescence by nuclear fusion reactions in its core. The Sun radiates this energy mainly as light, ultraviolet, and infrared radi ...

-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 is the fifth largest satellite in the Solar System and the largest and most massive relative to its parent planet, with a diameter about one-quarter that of Earth (comparable to the width of ...

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 Kev can refer to: Given name * Kev Adams, French comedian, actor, screenwriter and film producer born Kevin Smadja in 1991 * Kevin Kev Carmody (born 1946), Indigenous Australian singer-songwriter * Kev Coghlan (born 1988), Scottish Grand Prix moto ...

(IBEX-Lo) and 300 eV to 6 keV (IBEX-Hi). The mission was originally planned for a 24 month operations period. The mission has since been extended, with the spacecraft still in operation as of July 2022.

Spacecraft

The spacecraft is built on an octagonal base, roughly high and across. The dry mass is of which 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; they were used in warfare by the Arabs, Chinese, Persia ...

, is . The spacecraft itself has a hydrazine

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

. Power is produced by a

solar array A photovoltaic system, also 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 absorb and ...

with a 116

watt The watt (symbol: W) is the unit of 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 quantify the rate of energy transfer. The watt is named after James ...

s capability, 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 upper atmosphere of the Sun, called the corona. This plasma mostly consists of electrons, protons and alpha particles with kinetic energy between . The composition of the sol ...

and the interstellar medium at the edge of our 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, which is the region of interaction.

Mission

Launch

The IBEX satellite was mated to its

Pegasus XL Pegasus is an Air launch to orbit, air-launched launch vehicle developed by Orbital Sciences Corporation (OSC) and now built and launched by Northrop Grumman. Capable of carrying small payloads of up to into low Earth orbit, Pegasus first fl ...

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 Sp ...

California California is a state in the Western United States, located along the Pacific Coast. With nearly 39.2million residents across a total area of approximately , it is the most populous U.S. state and the 3rd largest by area. It is also the m ...

, and the combined vehicle was then suspended below the

Lockheed L-1011 The Lockheed L-1011 TriStar, also known as the L-1011 (pronounced "El-ten-eleven") and TriStar, 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 comme ...

'' Stargazer'' mother airplane and flown to Kwajalein Atoll in the central

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

. ''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. By launching from this site close to the equator, 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 National Aeronautics and Space Administration's (NASA) ten field centers. Since December 1968 ...

Florida Florida is a state located in the Southeastern region of the United States. Florida is bordered to the west by the Gulf of Mexico, to the northwest by Alabama, to the north by Georgia, to the east by the Bahamas and Atlantic Ocean, and to ...

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. For example, the apsides of the Earth are called the aphelion and perihelion. General description There are two apsides in any ell ...

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. For example, the apsides of the Earth are called the aphelion and perihelion. General description There are two apsides in any ell ...

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 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'', "remote", 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 spati ...

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

hydrogen Hydrogen is the chemical element with the symbol H and atomic number 1. Hydrogen is the lightest element. At standard conditions hydrogen is a gas of diatomic molecules having the formula . It is colorless, odorless, tasteless, non-toxic ...

and

oxygen Oxygen is the chemical element with the symbol O and atomic number 8. It is a member of the chalcogen group in the periodic table, a highly reactive nonmetal, and an oxidizing agent that readily forms oxides with most elements as ...

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 conven ...

, 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 the ...

(ESA) to suppress

ultraviolet Ultraviolet (UV) is a form of electromagnetic radiation with wavelength from 10 nm (with a corresponding frequency around 30 PHz) to 400 nm (750 THz), shorter than that of visible light, but longer than X-rays. UV radiation ...

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 that cannot be measured by conventional telescopes. These ENAs are created on the boundary of our Solar System by the interactions between solar wind particles and interstellar medium particles. On the 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, bubble-like region of space. In plasma physics terms, it is the cavity formed by the Sun in the surrounding interstell ...

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

cosmic ray Cosmic rays 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 Solar System in our own ...

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 magnetosphere. 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, a bow shock occurs when the magnetosphere of an astrophysical object interacts with the nearby flowing ambient plasma such as the solar wind. For Earth and other magnetized planets, it is the boundary at which the speed of th ...

, and it measured its speed relative to the local interstellar medium (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.