Spacecraft retirement

Most notable examples of spacecraft retirement is the retirement of the Cassini-Huygens probe in 2017.

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.

History

The first spacecraft to be retired was the Soviet Union's Sputnik 1 probe. Sputnik was originally launched in 1957, and eventually completed its mission by naturally deorbiting due to gravitational forces. It spent roughly 3 months orbiting Earth, completing 1,440 orbits in the duration.

In 1958, NASA launched the Vanguard 1 probe with the aim of winning in the Space Race. Six years later, NASA officially concluded the mission as the probe had fulfilled all its experimental and objective requirements. NASA made sure the probe would survive for up to 2,000 years. However, due to solar radiation pressure and atmospheric drag, the hypothesized reentry date has been reduced to approximately 240 years from now, where Vanguard 1 is expected to re-enter the Earth's atmosphere and burn up.

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 plunging into Saturn's atmosphere. The decision to end the mission was prompted by the minimal power supply from the probe's radioisotope thermoelectric generators (RTGs). The maneuver started on November 29, 2016, where Cassini performed a flyby of Saturn's moon Titan, which positioned it into the orbital plane of Saturn's F Ring for its grand finale. Cassini performed another subsequent flyby of Titan on April 22, 2017, during which positioned the probe to pass within a precise 3,100 km (1,900 mi) of Saturn's clouds. The process of flybys was repeated 22 times until September 15, 2017, at 11:55:46 UTC, when communication with Cassini's antenna was lost during the 22nd flyby of Saturn. Cassini's grand finale represented the inaugural intentional entry of a spacecraft into the atmosphere of a gas giant, and marks the first major spacecraft to burn up in the atmosphere of a gas giant (excluding the Galileo Atmospheric Probe). The mission's success has significantly influenced the planning and execution of both present and future missions focused on gas and ice giants.

Reasons

Radioisotope thermoelectric generator decrement

Main articles: Voyager program and Radioisotope thermoelectric generator

Both Voyager 1 and Voyager 2 were launched in 1977 and are equipped with radioisotope thermoelectric generators (RTGs) to power their instruments and communication systems.

Many modern spacecraft utilize radioisotope thermoelectric generators (RTGs) as their power source. Over time, the power output of RTGs diminishes due to the gradual depletion of the radioactive isotopes they rely on for electricity generation.

These RTGs 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 takes roughly 50 years to happen. As a result, spacecraft equipped with RTGs will eventually run out of power sufficient enough to power their instruments.

Deorbitation

Main article: Atmospheric entry

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.

Mission completion

Main article: Planetary protection

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.

Loss of contact

Main articles: Communications blackout, Pioneer 10, and Pioneer 11

Artists depiction of Pioneer 10 or Pioneer 11 in the interstellar medium

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.