Glossary

Arroyo Seco

A dry riverbed on the edge of Pasadena. Caltech students tested a rocket motor here on 31 October 1936, the event JPL counts as its beginning, and the laboratory still stands next to the site. The Rose Bowl stadium was built in the Arroyo in 1922. Read more →

Caltech California Institute of Technology

A small private research university in Pasadena with six academic divisions. It was founded in 1891 as Throop University and took its present name in 1920. Caltech has managed JPL for NASA since 1958 and co-manages LIGO with MIT. Read more →

Cruise

The phase of a mission between launch and arrival at the target, which may last a few months or many years. During cruise the team checks the spacecraft, calibrates its instruments, sends up new command sequences and fine-tunes its path. Read more →

Deep Space Network DSN

NASA’s network of large radio antennas for communicating with spacecraft beyond the Moon. Its three complexes, at Goldstone in California, near Madrid and near Canberra, stand roughly 120 degrees apart, so as the Earth turns at least one of them can always see a given spacecraft. Each has one 70-metre dish and several 34-metre dishes; the network is operated by JPL. Read more →

Delta-DOR delta differenced one-way ranging

A way of fixing a spacecraft’s direction in the sky very precisely. Two antennas on different continents listen to the spacecraft and then to a quasar whose position is known, and the difference between the two measurements gives the spacecraft’s angle. One measurement takes about an hour. Read more →

Discovery Program

NASA’s programme of competitively selected, cost-capped planetary missions, initiated in 1989 and legislated in 1993. Teams of scientists propose their own missions in answer to an Announcement of Opportunity. Its first mission was Mars Pathfinder, which carried the Sojourner rover in 1997. Read more →

Doppler tracking

Measuring a spacecraft’s speed towards or away from Earth from the slight shift in the frequency of its radio signal, like the change in pitch of a passing siren. In the two-way form a station sends a signal up, the spacecraft returns it and the two are compared; the Deep Space Network measures speed this way to fractions of a millimetre per second. Read more →

Entry, descent and landing EDL

The final phase of a Mars mission, from hitting the top of the atmosphere to standing on the ground. It takes about seven minutes, in which the spacecraft slows from roughly 21,000 km/h to zero, and because radio signals take several minutes to reach Earth it has to run entirely on its own. Engineers call it the “seven minutes of terror”. Read more →

Gravitational waves

Ripples in space and time, predicted by Einstein’s general theory of relativity, that travel at the speed of light. They are produced, for example, when two black holes or neutron stars orbit each other and merge. They were first detected directly by LIGO in September 2015. Read more →

Gravity assist

A close flyby of a planet planned so that the spacecraft takes a share of the planet’s orbital motion and leaves with a changed speed and direction; the planet loses a negligible amount in exchange. Voyager 2 used Jupiter, Saturn and Uranus in turn to reach Neptune. Read more →

Heliosphere

The protective bubble of particles and magnetic field blown out by the Sun. Beyond its edge lies interstellar space. Voyager 1 crossed it on 25 August 2012 and Voyager 2 on 5 November 2018; they are the only spacecraft ever to have operated outside it. Read more →

Hohmann transfer orbit

The path between two planetary orbits that uses the least propellant: an arc around the Sun that touches Earth’s orbit at one end and the target planet’s orbit at the other. The spacecraft must be launched so that the planet reaches the far end at the same time, which for Mars is possible about every 25 months. These recurring chances are called launch windows. Read more →

JPL Jet Propulsion Laboratory

NASA’s laboratory in Pasadena that builds and flies robotic missions to the planets and operates the Deep Space Network. It is NASA’s only federally funded research and development centre and is managed for the agency by Caltech. It began with a student rocket test in 1936 and joined NASA on 3 December 1958. Read more →

LIGO Laser Interferometer Gravitational-Wave Observatory

An observatory of two L-shaped laser detectors with 4-kilometre arms, in Hanford, Washington, and Livingston, Louisiana, built to detect gravitational waves directly. It is funded by the U.S. National Science Foundation and operated by Caltech and MIT. Its first detection, on 14 September 2015, led to the 2017 Nobel Prize in Physics. Read more →

Mars Relay Network

An international group of spacecraft orbiting Mars that pass rover data on to Earth: NASA’s Mars Odyssey and Mars Reconnaissance Orbiter and ESA’s ExoMars Trace Gas Orbiter and Mars Express. A rover sends its data by UHF radio to an orbiter flying overhead, which forwards it to the Deep Space Network. Every image from the surface of Mars since 2004 has come home this way. Read more →

Moment magnitude scale

The scale introduced by Hiroo Kanamori at Caltech in 1977 to measure the size of earthquakes. It gives more precise values for the energy of large earthquakes and replaced Charles Richter’s 1935 scale for earthquakes of magnitude 5 and above. Read more →

Mount Wilson Observatory

An observatory on a summit of the San Gabriel Mountains above Pasadena, about 1,740 metres high, founded in 1904 by George Ellery Hale with Carnegie money. Its 100-inch Hooker telescope was installed in 1917, and on the mountain Edwin Hubble showed in 1923 that there are galaxies beyond our own and in 1929 that the universe is expanding. Read more →

Optical navigation

Navigation in which a spacecraft photographs the planet or moon it is approaching against the background stars. The pictures tell navigators precisely where the spacecraft is relative to its target and complement the radio measurements. Read more →

Principal investigator PI

The scientist who proposes and leads a competitively selected mission and carries overall responsibility for its success. In programmes such as Discovery the principal investigator, not the agency, chooses the scientific goal. Read more →

Project life cycle Pre-Phase A to Phase F

NASA’s division of every project into seven phases: concept studies (Pre-Phase A), concept and technology development (A), preliminary design (B), final design and fabrication (C), assembly, integration, test and launch (D), operations (E) and closeout (F). Between the phases, formal reviews and key decision points decide whether the project may move on. Read more →

Quasar quasi-stellar radio source

A very distant and very bright source at the centre of a galaxy, linked to the region around a supermassive black hole. Through a telescope it looks like a star, hence the name. Because their positions are known, quasars serve as reference points in delta-DOR measurements of spacecraft. Read more →

Radioisotope thermoelectric generator RTG

A power source for spacecraft that turns the heat of naturally decaying plutonium-238 into electricity with thermocouples, without moving parts. It works far from the Sun, where solar panels would be useless. The two Voyagers are the longest-operating spacecraft powered this way; their output falls by about 4 watts a year. Read more →

Ranging

Measuring the distance to a spacecraft by sending it a coded radio signal that it returns at once. The time of the round trip, at the speed of light, gives the distance to about a metre. Read more →

Rose Parade Tournament of Roses

Pasadena’s flower parade, usually held on New Year’s Day. The first, on 1 January 1890, was a procession of flower-decked horses and carriages organised by the Valley Hunt Club. It is the city’s biggest yearly event, and the city fills with visitors around the New Year. Read more →

Safe mode

A protective state that a spacecraft enters on its own when its on-board fault protection detects a problem. The team on the ground then works out what happened and carefully returns it to normal operations. Read more →

Seismological Laboratory Caltech Seismo Lab

Caltech’s centre for earthquake science in Pasadena, founded in 1921 under the Carnegie Institution of Washington and part of Caltech since 1937. Richter’s and Kanamori’s magnitude scales were developed here, and with the U.S. Geological Survey it runs the Southern California Seismic Network. Read more →

Sky crane

The landing system used by Curiosity in 2012 and Perseverance in 2021. In the last moments of descent a descent stage with rocket engines lowers the rover on cables and flies away once the wheels touch down. Earlier rovers landed inside airbags. Read more →

Sol Martian day

One day on Mars, a little over 24½ hours long. Mission teams count a rover’s time on the surface in sols: Sojourner worked for 83 sols. A Martian year lasts 687 Earth days. Read more →

Telemetry

The data a spacecraft sends to Earth: engineering measurements of its own condition and the results of its science instruments. The word comes from the Greek for “far off” and “measure”. Signals sent from the ground to the spacecraft are called the uplink, those coming down the downlink. Read more →

Trajectory correction manoeuvre TCM

A short, planned firing of a spacecraft’s engines during cruise that adjusts its path, typically changing its speed by a few metres per second. The orbit determination team works out where the spacecraft is going, and the flight path control team decides what correction it needs. Read more →

Sources checked on: 1–3 November 2026