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The retirement of a spacecraft refers to the discontinuation of a spacecraft from active service. This can involve deorbiting the spacecraft, discontinuing of the probes operations, passivating it, or loss of contact with it. One notable example of spacecraft retirement is Cassini's retirement in 2017. [1] [2]
The first spacecraft to be retired was the Soviet Union’s Sputnik 1 probe. Launched in 1957, Sputnik 1 completed its mission when it naturally deorbited due to atmospheric drag after approximately three months in orbit, completing a total of 1,440 orbits around Earth. [3]
In 1958, NASA launched the Vanguard 1 probe as part of its efforts in the Space Race. Six years later, NASA officially concluded the mission as the probe had fulfilled all its experimental and objective requirements. Initially, NASA estimated that Vanguard 1 would remain in low Earth orbit for up to 2,000 years. However, due to solar radiation pressure and atmospheric drag, this estimate has been revised. Vanguard 1 is now expected to re-enter Earth’s atmosphere and burn up in approximately 240 years. [4] [5]
Following the discontinuation of the Vanguard 1 probe, many space agencies and subsequent spacecraft have ceased operations through various methods.
In 2017, the Cassini–Huygens probe concluded its 19-year exploration mission by descending into Saturn’s atmosphere. The decision to end the mission was prompted by the minimal power supply remaining in the probe’s radioisotope thermoelectric generators (RTGs). The maneuver began on November 29, 2016, when Cassini performed a flyby of Saturn’s moon Titan, positioning it into the orbital plane of Saturn’s F Ring for its grand finale. [6] [7] Cassini conducted another flyby of Titan on April 22, 2017, positioning the probe to pass within a precise 3,100 km (1,900 mi) of Saturn’s clouds. This maneuver was repeated 22 times until September 15, 2017, at 11:55:46 UTC, when communication with Cassini’s high-gain antenna was lost during the final flyby of Saturn. Cassini’s grand finale represented the first intentional entry of a spacecraft into the atmosphere of a gas giant and marked the first major spacecraft to burn up in such an atmosphere. The mission’s success has significantly influenced the planning and execution of both current and future missions focused on gas and ice giants. [8]
In the beginning of the 21st century retirement is not yet a routine end-of-mission process for the geostationary satellites. [9]
The satellites expend their propellant for the orbital station-keeping and eventually run out of it. For the high-value satellites in the geostationary orbit, the amount of the onboard propellant is usually the limiting factor of their service life (12-15 years at the beginning of the 21st century). [10]
Many modern spacecraft utilize radioisotope thermoelectric generators (RTGs), and spacecraft electric propulsion as their power source. Over time, the power output of RTGs or SEPs diminishes due to the gradual depletion of the radioactive isotopes they rely on for electricity generation. [11]
Radioisotope thermoelectric generators convert the heat generated by the decay of radioactive isotopes into electricity, providing a reliable power source for spacecraft equipped with RTGs. As the radioactive isotopes decay over time, the power output of the RTGs gradually decreases. It’s estimated that the power output from the RTGs will fall below the threshold required to operate the spacecraft’s systems effectively, including communication with Earth and scientific instruments. As a result, spacecraft equipped with RTGs will eventually have insufficient power to run their instruments. [12] [13]
Spacecraft electric propulsion systems utilize electric fields to accelerate charged ions to high velocities, generating thrust for propulsion. Unlike chemical propulsion systems, which rely on the combustion of propellant, electric propulsion systems ionize propellant using electrical energy and then accelerate the resulting ions to produce thrust. As the ions are expelled from the spacecraft at high speeds, they create thrust in the opposite direction, enabling spacecraft maneuverability. Electric propulsion systems offer high efficiency and long-duration capabilities, making them suitable for precise trajectory adjustments, extended deep space missions, and potential future crewed missions. These systems rely on electrical energy for ionization and acceleration of propellant, typically supplied by onboard power sources such as solar panels or batteries. However, over time, these energy sources may become depleted, particularly in missions with extended durations or in environments with limited sunlight. Additionally, the components of SEP systems, including power electronics and ionization chambers, may degrade due to exposure to harsh space conditions, leading to decreased efficiency and power output. Furthermore, some SEP systems have a limited supply of propellant, such as xenon gas, which can be exhausted over the course of a mission, rendering the system unable to generate thrust. Ultimately, the end of a spacecraft's mission or operational life may also result in the cessation of power generation by the SEP system, marking the conclusion of its propulsion capabilities. [14] [15]
Spacecraft instruments are susceptible to various external factors, including solar wind and other space phenomena. Solar wind, consisting of charged particles emitted by the Sun, can disrupt sensitive instruments like magnetometers and particle detectors, potentially reducing sensitivity or causing complete loss of functionality. Other space phenomena, such as cosmic rays and micrometeoroid impacts, also pose risks, potentially damaging spacecraft components and degrading instrument performance over time. Additionally, temperature extremes and outgassing of materials in space can further impact instrument functionality.
Communication loss between spacecraft and Earth is a known occurrence, particularly as spacecraft travel deeper into space. Factors like distance, technical malfunctions, or power source depletion can all contribute to this loss. Efforts may be made to re-establish communication, but success isn’t guaranteed due to technological limitations or the spacecraft’s condition.
Pioneer 10 and Pioneer 11 serve as prominent examples of spacecraft encountering communication failures with Earth. As these probes traveled deeper into space, the distance between them and Earth grew, posing challenges for maintaining contact. Ultimately, their antennas dropped below Earth’s radio detection threshold, leading to the loss of communication.
When spacecraft lose funding or complete their scientific objectives, they may alter their trajectories in various ways. One common approach is to redirect the spacecraft towards a path where it will burn up in the atmosphere of the celestial body they were orbiting. This intentional reentry minimizes the risk of the spacecraft becoming space debris, or helps the contamination of planets or moons.
Some spacecraft are retired upon completing their scientific objectives. Depending on the mission’s requirements and circumstances, various retirement strategies may be employed. For example, spacecraft can be intentionally deorbited, Alternatively, they may be left in orbit around a planet or moon, however, leaving a spacecraft in orbit around a planet or moon carries the risk of contamination or the creation of space debris. Contamination could occur if the spacecraft carries microbes or organic material from Earth, potentially affecting the planetary or lunar environments. Additionally, if the spacecraft were to malfunction or fragment in orbit, it could contribute to the accumulation of space debris, posing hazards to other spacecraft and future missions. Therefore, decisions regarding the fate of retired spacecraft must carefully weigh these risks and implement measures to mitigate them.
For a satellite in the geostationary orbit (GEO) a more economically feasible way of disposal is raising the orbit by a few hundred of kilometers and shutting down its transmitter. This will prevent the retired satellite from interfering with other GEO satellites. [16] United Nations and the United States Federal Communications Commission independently have policies in place requiring an increase in altitude of at least 300 kilometers. This requires the reserve of propellant that otherwise could be used for station-keeping during about 6 months of additional service life. High economic value of the GEO satellites prompts researchers to evaluate the use of a space tug for the final change of orbit. [10]
Under certain circumstances, scientists can adjust a spacecraft's orbit to ensure it remains at a safe distance from the celestial body it's orbiting. This adjustment can prolong the spacecraft's orbital stability for hundreds of thousands to millions of years, mitigating the effects of planetary gravity, atmospheric drag, or solar radiation pressure. One notable example is NASA's Laser Geodynamics Satellite (LAGEOS), launched in May 1976. LAGEOS carries a time capsule, created by Carl Sagan, intended to burn up in Earth's atmosphere approximately 8.4 million years from now. [17]
Pioneer 11 is a NASA robotic space probe launched on April 5, 1973, to study the asteroid belt, the environment around Jupiter and Saturn, the solar wind, and cosmic rays. It was the first probe to encounter Saturn, the second to fly through the asteroid belt, and the second to fly by Jupiter. Later, Pioneer 11 became the second of five artificial objects to achieve an escape velocity allowing it to leave the Solar System. Due to power constraints and the vast distance to the probe, the last routine contact with the spacecraft was on September 30, 1995, and the last good engineering data was received on November 24, 1995.
Spacecraft propulsion is any method used to accelerate spacecraft and artificial satellites. In-space propulsion exclusively deals with propulsion systems used in the vacuum of space and should not be confused with space launch or atmospheric entry.
Voyager 1 is a space probe launched by NASA on September 5, 1977, as part of the Voyager program to study the outer Solar System and the interstellar space beyond the Sun's heliosphere. It was launched 16 days after its twin, Voyager 2. It communicates through the NASA Deep Space Network (DSN) to receive routine commands and to transmit data to Earth. Real-time distance and velocity data are provided by NASA and JPL. At a distance of 165.2 AU from Earth as of October 2024, it is the most distant human-made object from Earth. The probe made flybys of Jupiter, Saturn, and Saturn's largest moon, Titan. NASA had a choice of either doing a Pluto or Titan flyby; exploration of the moon took priority because it was known to have a substantial atmosphere. Voyager 1 studied the weather, magnetic fields, and rings of the two gas giants and was the first probe to provide detailed images of their moons.
Voyager 2 is a space probe launched by NASA on August 20, 1977, as a part of the Voyager program. It was launched on a trajectory towards the gas giants Jupiter and Saturn and enabled further encounters with the ice giants Uranus and Neptune. It remains the only spacecraft to have visited either of the ice giant planets, and was the third of five spacecraft to achieve Solar escape velocity, which allowed it to leave the Solar System. Launched 16 days before its twin Voyager 1, the primary mission of the spacecraft was to study the outer planets and its extended mission is to study interstellar space beyond the Sun's heliosphere.
A spacecraft is a vehicle that is designed to fly and operate in outer space. Spacecraft are used for a variety of purposes, including communications, Earth observation, meteorology, navigation, space colonization, planetary exploration, and transportation of humans and cargo. All spacecraft except single-stage-to-orbit vehicles cannot get into space on their own, and require a launch vehicle.
Pioneer 10 is a NASA space probe launched in 1972 that completed the first mission to the planet Jupiter. Pioneer 10 became the first of five planetary probes and 11 artificial objects to achieve the escape velocity needed to leave the Solar System. This space exploration project was conducted by the NASA Ames Research Center in California. The space probe was manufactured by TRW Inc.
The Voyager program is an American scientific program that employs two interstellar probes, Voyager 1 and Voyager 2. They were launched in 1977 to take advantage of a favorable planetary alignment to explore the two gas giants Jupiter and Saturn and potentially also the ice giants, Uranus and Neptune - to fly near them while collecting data for transmission back to Earth. After Voyager 1 successfully completed its flyby of Saturn and its moon Titan, it was decided to send Voyager 2 on flybys of Uranus and Neptune.
Ulysses was a robotic space probe whose primary mission was to orbit the Sun and study it at all latitudes. It was launched in 1990 and made three "fast latitude scans" of the Sun in 1994/1995, 2000/2001, and 2007/2008. In addition, the probe studied several comets. Ulysses was a joint venture of the European Space Agency (ESA) and the United States' National Aeronautics and Space Administration (NASA), under leadership of ESA with participation from Canada's National Research Council. The last day for mission operations on Ulysses was 30 June 2009.
Cassini–Huygens, commonly called Cassini, was a space-research mission by NASA, the European Space Agency (ESA), and the Italian Space Agency (ASI) to send a space probe to study the planet Saturn and its system, including its rings and natural satellites. The Flagship-class robotic spacecraft comprised both NASA's Cassini space probe and ESA's Huygens lander, which landed on Saturn's largest moon, Titan. Cassini was the fourth space probe to visit Saturn and the first to enter its orbit, where it stayed from 2004 to 2017. The two craft took their names from the astronomers Giovanni Cassini and Christiaan Huygens.
A gravity assist, gravity assist maneuver, swing-by, or generally a gravitational slingshot in orbital mechanics, is a type of spaceflight flyby which makes use of the relative movement and gravity of a planet or other astronomical object to alter the path and speed of a spacecraft, typically to save propellant and reduce expense.
Uncrewed spacecraft or robotic spacecraft are spacecraft without people on board. Uncrewed spacecraft may have varying levels of autonomy from human input, such as remote control, or remote guidance. They may also be autonomous, in which they have a pre-programmed list of operations that will be executed unless otherwise instructed. A robotic spacecraft for scientific measurements is often called a space probe or space observatory.
The Jupiter Icy Moons Orbiter (JIMO) was a proposed NASA spacecraft designed to explore the icy moons of Jupiter. The main target was Europa, where an ocean of liquid water may harbor alien life. Ganymede and Callisto, which are now thought to also have liquid, salty oceans beneath their icy surfaces, were also targets of interest for the probe.
The Van Allen Probes, formerly known as the Radiation Belt Storm Probes (RBSP), were two robotic spacecraft that were used to study the Van Allen radiation belts that surround Earth. NASA conducted the Van Allen Probes mission as part of the Living With a Star program. Understanding the radiation belt environment and its variability has practical applications in the areas of spacecraft operations, spacecraft system design, mission planning and astronaut safety. The probes were launched on 30 August 2012 and operated for seven years. Both spacecraft were deactivated in 2019 when they ran out of fuel. They are expected to deorbit during the 2030s.
The exploration of Saturn has been solely performed by crewless probes. Three missions were flybys, which formed an extended foundation of knowledge about the system. The Cassini–Huygens spacecraft, launched in 1997, was in orbit from 2004 to 2017.
Titan Saturn System Mission (TSSM) was a joint NASA–ESA proposal for an exploration of Saturn and its moons Titan and Enceladus, where many complex phenomena were revealed by Cassini. TSSM was proposed to launch in 2020, get gravity assists from Earth and Venus, and arrive at the Saturn system in 2029. The 4-year prime mission would include a two-year Saturn tour, a 2-month Titan aero-sampling phase, and a 20-month Titan orbit phase.
Nuclear power in space is the use of nuclear power in outer space, typically either small fission systems or radioactive decay for electricity or heat. Another use is for scientific observation, as in a Mössbauer spectrometer. The most common type is a radioisotope thermoelectric generator, which has been used on many space probes and on crewed lunar missions. Small fission reactors for Earth observation satellites, such as the TOPAZ nuclear reactor, have also been flown. A radioisotope heater unit is powered by radioactive decay and can keep components from becoming too cold to function, potentially over a span of decades.
The Cassini space probe was deliberately disposed of via a controlled fall into Saturn's atmosphere on September 15, 2017, ending its nearly two-decade-long mission. This method was chosen to prevent biological contamination of any of the moons of Saturn now thought to offer potentially habitable environments. Factors that influenced the mission end method included the amount of rocket fuel left, the health of the spacecraft, and funding for operations on Earth.
OCEANUS is a mission concept conceived in 2016 and presented in 2017 as a potential future contestant as a New Frontiers program mission to the planet Uranus. The concept was developed in a different form by the astronautical engineering students of Purdue University during the 2017 NASA/JPL Planetary Science Summer School. OCEANUS is an orbiter, which would enable a detailed study of the structure of the planet's magnetosphere and interior structure that would not be possible with a flyby mission.
Galileo was an American robotic space probe that studied the planet Jupiter and its moons, as well as the asteroids Gaspra and Ida. Named after the Italian astronomer Galileo Galilei, it consisted of an orbiter and an entry probe. It was delivered into Earth orbit on October 18, 1989, by Space ShuttleAtlantis, during STS-34. Galileo arrived at Jupiter on December 7, 1995, after gravitational assist flybys of Venus and Earth, and became the first spacecraft to orbit an outer planet.
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