<div><div>Payload refers to the elements or parts of the <a href="https://spacesecuritylexicon.org/common-definition/spacecraft">spacecraft</a> that perform the desired functions of the <a href="https://spacesecuritylexicon.org/common-definition/space-object">space object</a>. It can also refer to the cargo of the <a href="https://spacesecuritylexicon.org/common-definition/space-vehicle">space vehicle</a>, which can be, for example, humans headed for the International Space Station (ISS), or a <a href="https://spacesecuritylexicon.org/common-definition/satellite">satellite</a>.</div></div>

Payload

<div><div>A body, either natural or artificial, which orbits another body in space. Artificial satellites are placed in orbit around planets for many purposes, including collecting information, navigation, or communication. Natural satellites are celestial bodies that orbit planets, other celestial bodies or stars.</div></div>

Satellite

<div><div>Also known as space junk or space waste, this term encompasses both natural meteoroid and artificial (human-made) orbital debris. Human-made debris is also known as ‘orbital debris’, as it is found orbiting the Earth. Orbital debris refers to any human-made object that no longer serves a useful function that was previously in orbit, including non-functional <a href="https://spacesecuritylexicon.org/common-definition/spacecraft">spacecraft</a>, expended launch vehicle stages, mission-related debris, and fragmentation debris from kinetic counterspace activities.</div></div>

Space Debris

<div><div>A space launch vehicle is a rocket-propelled vehicle that is used to carry <a href="https://spacesecuritylexicon.org/common-definition/payload-2">payloads</a> from Earth’s surface to space, usually to Earth orbit or beyond. Concerns have been raised regarding the similarities between SLVs and ballistic missiles, and in fact some States developed their SLVs from ballistic missile technology, while others have used elements of SLV programmes to develop ballistic missiles. While these technologies are similar, they also have some key differences, such as their propellants, guidance systems or their use of re-entry vehicles.</div><div>A space launch vehicle is different from a <a href="https://spacesecuritylexicon.org/common-definition/space-vehicle">space vehicle</a>. The former is used to carry payloads to space, whereas the latter is used to transport those payloads in space or on celestial bodies.</div></div>

Space Launch Vehicle (SLV)

<div><div>Space object refers to any object launched into orbit from the Earth, the Moon, or other celestial bodies to travel to, in or through outer space. The term ‘space object’ includes component parts of a space object as well as its launch vehicle and parts thereof (see Liability Convention, article I.d). It should not be confused with <a href="https://spacesecuritylexicon.org/common-definition/space-system">space systems</a>, which also include segments not located in space. </div></div>

Space Object

<div><div>Space vehicle refers to a spacecraft that is used to transport crew or a separate <a href="https://spacesecuritylexicon.org/common-definition/payload-2">payload</a> in space, as well as on celestial bodies. A space vehicle is different from a <a href="https://spacesecuritylexicon.org/common-definition/space-launch-vehicle-slv">space launch vehicle</a>, as the former is used to transport payloads in space and on celestial bodies, whereas the latter is used to carry those payloads into space.</div><div>Sometimes the term ‘space vehicle’ used as a synonym for ‘<a href="https://spacesecuritylexicon.org/common-definition/spacecraft">spacecraft</a>’, however it is generally understood that space vehicle is a more specific term, used for <a href="https://spacesecuritylexicon.org/common-definition/space-object">space objects</a> that transport payloads, whereas spacecraft refers to any human-made machine designed to operate in space.</div></div>

Space Vehicle

<div><div>A human-made vehicle or machine designed to operate, with or without a crew, beyond the major portion of the Earth’s atmosphere, in outer space. The nature, complexity, and capabilities of spacecraft are diverse. Spacecraft can operate in Earth’s orbit or beyond it. Sometimes used as a synonym for <a href="https://spacesecuritylexicon.org/common-definition/space-vehicle">space vehicle</a>, the term ‘spacecraft’ however is generally understood to be less specific than the term ‘space vehicle’, and to refer to any human-made machine designed to operate in space.</div></div>

Spacecraft

<div><div>The region of space that exists between the Earth and the Moon, including the Moon’s own orbit.</div></div>

Cislunar Space

<div><div>Deep space generally refers to areas beyond Earth’s orbit and <a href="https://spacesecuritylexicon.org/common-definition/cislunar-space">cislunar space</a>, specifically space at distances from the Earth equal to, or greater than, 2×106 km, although some definitions also consider the Moon to be part of deep space.</div></div>

Deep Space

<div><div>Geostationary orbit is a specific <a href="https://spacesecuritylexicon.org/common-definition/geosynchronous-orbit-gso">geosynchronous orbit</a> (GSO), whose circular and direct orbit lies in the plane of the Earth’s equator which differentiates it from other GSOs, where <a href="https://spacesecuritylexicon.org/common-definition/satellite">satellites</a> can have any inclination. Satellites in this orbit revolve around Earth, above the equator from west to east, at the same rate as the Earth rotates. This makes them appear stationary above Earth. GEO satellites are placed at an altitude of approximately 35,786 km (22,236.39 miles). Telecommunications satellites are commonly found in this orbit. Weather satellites are also found in this orbit for real-time imagery and data collection of the Earth’s surface and atmosphere for observation, oceanography, and atmospheric tracking. Moreover, navigation satellites in this orbit provide a known calibration point which serves to enhance <a href="https://spacesecuritylexicon.org/common-definition/global-navigation-satellite-system-gnss">GNSS</a> accuracy.</div></div>

Geostationary Orbit (GEO) 

<div><div>Geosynchronous orbit synchronizes with the rotation of the Earth and has an orbital period that matches one sidereal day (23 hours, 56 minutes, and 4 seconds). GSO <a href="https://spacesecuritylexicon.org/common-definition/satellite">satellites</a> are placed at an altitude of approximately 35,786 km (22,236.39 miles) and have a period of revolution that is equal to the period of rotation of the Earth around its axis. Telecommunications satellites —particularly broadcast TV and low-speed data communication— are commonly found in this orbit. Similarly, weather satellites can also be found in this orbit. <a href="https://spacesecuritylexicon.org/common-definition/geostationary-orbit-geo">GEO</a> orbit is a specific type of GSO that lies on the same plane as the equator. In other GSOs, satellites can have any inclination. </div></div>

Geosynchronous Orbit (GSO)

<div><div>Graveyard orbit, also called a junk orbit or disposal orbit, refers to orbits located above operational orbits (particularly beyond <a href="https://spacesecuritylexicon.org/common-definition/geostationary-orbit-geo">GEO</a>), where <a href="https://spacesecuritylexicon.org/common-definition/satellite">satellites</a> that are no longer operational are moved to reduce the probability of collisions with operational <a href="https://spacesecuritylexicon.org/common-definition/space-object">space objects</a> and avoid the creation of <a href="https://spacesecuritylexicon.org/common-definition/space-debris">space debris</a>.</div></div>

Graveyard Orbit 

<div><div>Low Earth orbit refers to the area situated closest to the Earth, below <a href="https://spacesecuritylexicon.org/common-definition/geostationary-orbit-geo">GEO</a> and <a href="https://spacesecuritylexicon.org/common-definition/medium-earth-orbit-meo">MEO</a>. <a href="https://spacesecuritylexicon.org/common-definition/satellite">Satellites</a> in LEO are located at an altitude of less than 2,000 km but could be as low as 80 km above Earth according to some experts (although there is some contention over whether a satellite can be considered to be in LEO at that low of an altitude). LEO satellites can be inserted into any plane bisecting the equator, meaning their orbit can be tilted relative to the rotational motion of the Earth. This is the orbit that is most commonly used for satellite imaging of Earth, due to its proximity to the surface of the Earth, which allows for higher resolution images. Certain communications satellites are also placed in this orbit; in fact, LEO is the orbit where very large constellations of satellites are being launched to provide Internet on Earth, and is the orbit where all of the kinetic <a href="https://spacesecuritylexicon.org/terminology/anti-satellite-asat">anti-satellite</a> (ASAT) tests have been held. It is also the orbit where the International Space Station (ISS) and the Tiangong Space Station are located. </div></div>

Low Earth Orbit (LEO) 

<div><div>Medium Earth orbit refers to the area situated between <a href="https://spacesecuritylexicon.org/common-definition/low-earth-orbit-leo">LEO</a> and <a href="https://spacesecuritylexicon.org/common-definition/geostationary-orbit-geo">GEO</a>. As with LEO, <a href="https://spacesecuritylexicon.org/common-definition/satellite">satellites</a> located in MEO do not have to have a specific inclination. Navigation satellites are commonly found in this orbit, generally at an altitude of around 20,000 km. MEO is also used for <a href="https://spacesecuritylexicon.org/common-definition/global-navigation-satellite-system-gnss">GNSS</a> and navigation applications. There are also certain constellations or satellite networks that can be found in MEO which deliver low-latency and highbandwidth (high-speed) data connectivity This is useful to provide optic fibre-like performance to remote areas, where laying fibre is not possible, such as cruise, commercial maritime, aero, offshore platforms, network backhaul in difficult terrain, and humanitarian relief operations.</div></div>

Medium Earth Orbit (MEO) 

<div><div>Molniya orbit is a highly elliptical orbit named after the Molniya communication <a href="https://spacesecuritylexicon.org/common-definition/satellite">satellites</a> used by the Soviet Union and, later, the Russian Federation. This orbit is used to provide communication and <a href="https://spacesecuritylexicon.org/common-definition/remote-sensing">remote sensing</a> services to high latitude areas in the northern hemisphere. The Molniya orbit has an inclination of 63.4 degrees, an argument of perigee —the angle between the point on the orbital path where an object crosses the equator and the point of its closest approach to the Earth— of 270 degrees, and an orbital period of approximately half a sidereal day.</div></div>

Molniya Orbit

<div><div>Polar orbit passes over the Earth’s polar regions, from north to south. Any orbit that passes within 20 to 30 degrees of the poles is considered to be a polar orbit. Polar orbits are used for reconnaissance and <a href="https://spacesecuritylexicon.org/common-definition/earth-observation-eo">Earth observation</a>.</div></div>

Polar Orbit

<div><div>Sun-synchronous orbit, also known as heliosynchronous orbit, it is a specific kind of <a href="https://spacesecuritylexicon.org/common-definition/polar-orbit">polar orbit</a>. <a href="https://spacesecuritylexicon.org/common-definition/satellite">Satellites</a> in this orbit are synchronous with the Sun, meaning that they pass over the same area of Earth at the same solar time, in a fixed position relative to the Sun. </div><div></div></div>

Sun-Synchronous Orbit (SSO)

<div><div>Earth observation is a form of <a href="https://spacesecuritylexicon.org/common-definition/remote-sensing">remote sensing</a> consisting in the gathering of information about Earth’s physical, chemical, and biological systems through different forms of <a href="https://spacesecuritylexicon.org/common-definition/satellite">satellite</a> imaging. Earth observation is used to monitor and assess the status of, and changes in, the natural and human-made environment, and has a growing number of applications including monitoring of infrastructure and the environment (for example, atmospheric gases, pollution, polar ice caps and sea level), urban planning, and damage assessment in conflict zones or after natural disasters, among others.</div></div>

Earth Observation (EO)

<div><div>Global Navigation Satellite System refers generically to a network of <a href="https://spacesecuritylexicon.org/common-definition/satellite">satellite</a> and ground stations used for navigation, through linking <a href="https://spacesecuritylexicon.org/common-definition/positioning-navigation-and-timing-pnt">positioning and timing</a> data, which can help to determine a location on the ground, in flight, or in space. Satellites transmit the positioning and timing data to GNSS receivers, which then use this data to determine location. There are several GNSS systems, including China’s BeiDou, Europe’s Galileo, the Russian Federation’s GLONASS (Global’naya Navigatsionnaya Sputnikovaya Sistema), and the United States’ GPS (Global Positioning System). GNSS can provide global coverage or service a particular region.</div></div>

Global Navigation Satellite System (GNSS)

<div><div>Three distinct capabilities enabled by <a href="https://spacesecuritylexicon.org/common-definition/space-system">space systems</a> which can be used separately or jointly. They are commonly used together to enable services such as <a href="https://spacesecuritylexicon.org/common-definition/global-navigation-satellite-system-gnss">GNSS</a>.</div><div><li><b>Positioning</b> refers to the ability to accurately and precisely determine location and orientation. It is mostly done in two-dimensions, but can be done three-dimensionally as well.</li><li><b>Navigation</b> refers to the ability to determine the current and desired position of an object or person and determine corrections to course, orientation, and speed to attain a desired position anywhere around the world, from sub-surface to surface and from surface to space.</li><li><b>Timing</b> refers to the ability to acquire and maintain accurate and precise time from a standard (Coordinated Universal Time, or UTC), anywhere in the world and within user-defined timeliness parameters. </li></div></div>

Positioning, Navigation And Timing (PNT)

<div><div>Remote sensing is the process of obtaining data about an area or object by detecting and monitoring its physical characteristics without making physical contact with it, but rather at a distance through the measurement of its reflected and emitted radiation (from a <a href="https://spacesecuritylexicon.org/common-definition/satellite">satellite</a>, although it can also be done from an aircraft). This can be achieved using sensor technologies and can be active or passive:</div><div><li>Active remote sensing wherein a signal is emitted by a satellite or aircraft to the object or area being monitored and its reflection is detected by the sensor.</li><li>Passive remote sensing measures energy that already exists, like sunlight, instead of emitting energy.</li><li><a href="https://spacesecuritylexicon.org/common-definition/earth-observation-eo">Earth observation</a> is one form of remote sensing.</li></div></div>

Remote Sensing

<div><div>Rendezvous and proximity operations are usually mentioned together, but they are two separate concepts. </div><div><li><b>Rendezvous operations</b> refer to the exercise two (or more) space objects carry out to manoeuvre in order to approach one another in a way that makes their orbital trajectory, plane, altitude, and phasing match. This places them very close to one another, usually to eventually join through docking —the joining of two free-flying objects— or berthing —the joining of two objects with the assistance of a robotic arm. </li><li><b>Proximity operations</b> refer to the manoeuvring of a <a href="https://spacesecuritylexicon.org/common-definition/space-object">space object</a> to place and maintain it in the vicinity of another space object on a relative planned path for a specific time duration to accomplish mission objectives.</li></div></div>

Rendezvous And Proximity Operations (RPO) 

<div><div>Satellite servicing refers to the act of performing technological upgrades, repairs, refuelling, and/or inspections of <a href="https://spacesecuritylexicon.org/common-definition/satellite">satellites</a> currently in orbit. Such activities require the ability to undertake a <a href="https://spacesecuritylexicon.org/common-definition/rendezvous-and-proximity-operations-rpo">rendezvous and proximity operation</a> (RPO). Acronyms used to refer to this practice include: ISAM (in-space servicing, assembly and manufacturing), OOS (on-orbit servicing), OSAM (on-orbit servicing, assembly and manufacturing), and IOS (in-orbit servicing).</div></div>

Satellite Servicing

<div><div>Space activities refers to operations and acts directly related to the exploration and use of outer space, including the Moon, other celestial bodies, and <a href="https://spacesecuritylexicon.org/common-definition/deep-space">deep space</a>, including, but not limited to scientific space research; the use of space technology for communications, television and radio broadcasting; Earth <a href="https://spacesecuritylexicon.org/common-definition/remote-sensing">remote sensing</a> from space, including State environmental sensing and meteorology; use of satellite navigation and surveying systems; crewed space flights; use of space equipment, space materials and space technologies in the interests of defence and security; observation of objects and phenomena in outer space; testing of equipment in space conditions; production of materials and other products in space; creation (including development, manufacturing and testing) and use (operation) of space equipment, space materials and space technologies and the provision of other services related to space activities; as well as the use of the results of space activities and international cooperation in the exploration and use of outer space.</div><div></div></div>

Space Activities

<div><div>Space domain awareness refers to the capability of tracking and characterizing <a href="https://spacesecuritylexicon.org/common-definition/space-object">space objects</a> within the space domain —in particular, Earth’s orbits— through the use of multiple <a href="https://spacesecuritylexicon.org/common-definition/space-situational-awareness-ssa">space situational awareness (SSA)</a> activities as well as consideration and assessment of the intent of actors, space policies, and strategies. Some use the term ‘space domain awareness’ (SDA) to refer to space situational awareness (SSA) when SSA is used in a military context. Others make a distinction between the two, and understand SSA as related to specific tasks, missions or objectives, whereas SDA takes a holistic approach and includes all the means available to the actor, including SSA’s technical data but also assessment of intent, and awareness of activities, space policies, and strategies, and other means of analysis and understanding the behaviour and intent of other actors.</div></div>

Space Domain Awareness (SDA)

<div><div>Space situational awareness refers to the capability or practice of tracking and characterizing specific <a href="https://spacesecuritylexicon.org/common-definition/space-object">space objects</a> and their operational environment in order to understand their current position, as well as to predict their future positions. SSA data can aid in identifying future conjunctions between objects and notifying space operators of potentially dangerous close approaches to enable them to carry out collision avoidance operations. SSA can be used for civil as well as military applications. Some use the term ‘<a href="https://spacesecuritylexicon.org/common-definition/space-domain-awareness-sda">space domain awareness</a> (SDA) to refer to space situational awareness (SSA) when SSA is used in a military context. There are others that make a distinction between the two, and understand SSA as related to specific tasks, missions or objectives, whereas SDA takes a holistic approach and includes all the means available to the actor, including SSA’s technical data but also assessment of intent, and awareness of activities, space policies, and strategies, and other means of analysis and understanding the behaviour and intent of other actors.</div></div>

Space Situational Awareness (SSA)

<div><div>Space surveillance and tracking refers to the use of sensor technology, including radars, telescopes, laser-ranging stations, and data centres, for the purposes of finding and tracking <a href="https://spacesecuritylexicon.org/common-definition/space-debris">space debris</a> and issuing alerts when evasive action may be necessary. A space surveillance and tracking system detects space debris, catalogues debris objects, and determines and predicts their orbits. It is considered to be a segment of <a href="https://spacesecuritylexicon.org/common-definition/space-situational-awareness-ssa">SSA</a> technology use specifically focused on debris identification and <a href="https://spacesecuritylexicon.org/terminology/monitoring">monitoring</a>.</div></div>

Space Surveillance And Tracking (SST)

<div><div>Space traffic management refers to a series of technical and regulatory provisions and also, according to some actors, the common or customary operational practices, for promoting safe access to outer space, the conduct of operations in outer space, and the return of <a href="https://spacesecuritylexicon.org/common-definition/space-object">space objects</a> from outer space in a manner that is safe, secure, and sustainable. It requires coordination among space actors as well as accurate <a href="https://spacesecuritylexicon.org/common-definition/space-situational-awareness-ssa">SSA</a> data.</div></div>

Space Traffic Management (STM)

<div><div>Data links refers to the connection that shares information between the space and <a href="https://spacesecuritylexicon.org/common-definition/ground-segment">ground segments</a> of a <a href="https://spacesecuritylexicon.org/common-definition/space-system">space system</a>. This includes the uplinks and downlinks, as well as services provided to the end users.</div></div>

Data Links / Link Segment

<div><div>Ground segment refers to the terrestrial part of a <a href="https://spacesecuritylexicon.org/common-definition/space-system">space system</a>, which includes all the facilities and elements needed to operate a <a href="https://spacesecuritylexicon.org/common-definition/space-object">space object</a> and deliver services to users. Examples of ground segment components include satellite dishes and receiving stations.</div></div>

Ground Segment

<div><div>Space segment refers to <a href="https://spacesecuritylexicon.org/common-definition/space-object">space objects</a>, which can be described as any object launched into orbit from the Earth, the Moon, or other celestial bodies to travel to, in or through outer space. The term ‘space segment’ includes component parts of a space object as well as its launch vehicle and parts thereof. Examples of space segment components are <a href="https://spacesecuritylexicon.org/common-definition/satellite">satellites</a> and <a href="https://spacesecuritylexicon.org/common-definition/space-launch-vehicle-slv">space launch vehicles</a>.</div></div>

Space Segment

<div><div>Space system refers to all the devices, components and infrastructure that work together to perform a task involving the space environment. This is an evolving concept, which has been used as a synonym of <a href="https://spacesecuritylexicon.org/common-definition/space-object">space objects</a>, and nowadays it is increasingly understood by most States and other stakeholders that not all of the components have to be located in space in order for them to be considered part of a space system. The different components of space systems are generally classified into three different groups, any of which can be interfered with and hamper <a href="https://spacesecuritylexicon.org/terminology/space-security">space security</a>: <a href="https://spacesecuritylexicon.org/common-definition/space-segment">space segment</a>, <a href="https://spacesecuritylexicon.org/common-definition/ground-segment">ground segment</a>, and <a href="https://spacesecuritylexicon.org/common-definition/data-links-link-segment">data links</a>.</div></div>

Common Definitions

Space Objects

Space Orbits and Locations

Space Services and Activities

Space System Components

A Lexicon for Outer Space Security