08Glossary

Audience: everyone. Plain-language first, then the precise meaning. Use Ctrl/⌘-F to jump to a term. Alphabetical.

Access interval (pass)#

Plain: the window of time a ground station can "see" a satellite above the horizon and talk to it. Analogy: a lighthouse can only signal a ship while it's above the horizon. In Delta V: the Access panel lists each pass with its max elevation, range, and duration. (AOS = start, LOS = end.)

Altitude#

Height above Earth's surface (not from Earth's center). Earth's radius is ~6,371 km, so a satellite "at 400 km" is ~6,771 km from the center.

Aphelion / Perihelion#

The farthest / closest points of an orbit around the Sun. (Around Earth it's apogee / perigee.)

Apoapsis / Periapsis#

The general words for the farthest / closest points of any orbit, around any body. Apogee/perigee (Earth) and aphelion/perihelion (Sun) are the named versions.

Apogee / Perigee#

The high point (apogee, farthest) and low point (perigee, closest) of an orbit around Earth. Analogy: the top and bottom of a skateboard half-pipe — slowest at the top, fastest at the bottom.

AOS / LOS#

Acquisition Of Signal / Loss Of Signal — the moment a satellite rises into / sets out of a ground station's view. Delta V marks these as green (AOS) and red (LOS) pins on the orbit.

BTJD#

Barycentric TESS Julian Date — the day-counting system the TESS telescope community uses for timestamps. Appears only in the Deep Field room.

Δv (delta-v, "delta-vee")#

Plain: how much a spacecraft can change its speed/direction over its whole life — the currency of spaceflight. Analogy: a fuel gauge, but measured in speed (m/s). Every maneuver has a Δv "price"; run out and the mission ends. The company is named after it.

Differential corrector#

Plain: a solver that automatically finds the burn that achieves a goal — "I want apogee at 1,422 km; find me the Δv." Analogy: adjusting a shower's hot tap, feeling the temperature, adjusting again, until it's right. In Delta V: the Target panel animates each guess converging on the answer.

Eccentricity (e)#

How squashed an orbit is. e = 0 is a perfect circle; closer to 1 is a long, stretched ellipse. Analogy: how oval a racetrack is.

ECI (Earth-Centered Inertial)#

The reference frame Delta V uses for orbits — origin at Earth's center, axes fixed relative to the stars (they don't spin with the Earth). Analogy: a camera bolted to the sky, not to the ground.

Ellipse#

A squashed circle — the shape of almost every orbit. The body being orbited sits at one focus, not the center.

Ephemeris#

A table/model of where a body (Sun, Moon, planet) is at a given time. Delta V uses ephemerides to place the Moon and Sun correctly.

Epoch#

A specific reference time. Delta V's default is J2000 (noon, Jan 1, 2000) — a standard "time zero" for astronomy. The Flight Deck clock shows the current epoch.

Finite burn#

A real engine burn that takes minutes (vs an idealized instant "impulsive" burn). Because the engine pushes while gravity keeps acting, some thrust is "wasted" fighting gravity — the gravity loss. Delta V's Finite burn panel shows this.

Fidelity#

How faithfully a simulation matches reality. Higher fidelity = more physical effects = more accurate but slower. Delta V labels its fidelity level (L0–L3, plus the L2+ server) so you always know how much to trust a number. See chapter 7.

Fork#

Plain: make your own copy of a mission/vehicle/discovery that remembers where it came from, so you can improve it and credit the original. Analogy: forking a repository on GitHub. The lineage is the fork chain.

GEO (Geostationary Orbit)#

An orbit ~35,786 km up where a satellite circles Earth in exactly one day, so it appears to hang over one spot — where TV/weather satellites live.

Golden bits#

The test fixtures that lock the engine's output to known correct values bit-for-bit. If any computation changed by a single bit, the test fails. This is how determinism is guaranteed.

Graticule#

The grid of latitude/longitude lines drawn on a map (e.g. the ground-track and coverage maps).

Ground station#

A dish/antenna on Earth that talks to satellites. Delta V ships a catalog of 8.

Ground track#

The path a satellite traces over the Earth's surface — a wavy sinusoid on a world map, because the Earth rotates underneath the orbit.

GTO (Geostationary Transfer Orbit)#

A stretched ellipse used to climb from low orbit up to GEO — low at one end, GEO-height at the other.

Hohmann transfer#

Plain: the fuel-efficient two-burn way to move between two circular orbits — burn to stretch your orbit up, coast to the top, burn again to circularize. Analogy: merging onto a higher highway — accelerate up the on-ramp, coast, accelerate to match traffic. In Delta V: the Maneuver panel.

Inclination (i)#

How tilted an orbit is relative to the equator (degrees). 0° hugs the equator, 90° goes over the poles, the ISS is ~51.6°. Analogy: does the racetrack lie flat or tip up toward the poles?

Isp (specific impulse)#

Plain: an engine's fuel efficiency, measured in seconds — higher is better (more push per kilo of propellant). Analogy: miles-per-gallon for rocket engines.

J2#

The largest correction for the fact that Earth isn't a perfect sphere — it bulges at the equator. J2 makes orbit planes slowly rotate, which is exactly how Sun-synchronous orbits are engineered. Delta V's L1 fidelity adds it.

J2000#

The standard astronomy epoch: 12:00 on January 1, 2000. Delta V's default time zero.

Lambert problem#

Plain: "given where I am, where I want to be, and how long I'll take, what exact path connects them and what does it cost?" Solving it many times over a grid of dates produces a porkchop plot. The foundation of intercepts and rendezvous.

LEO (Low Earth Orbit)#

The low band (~160–2,000 km) where the ISS and most satellites fly. Orbital speed there is ~7.7 km/s.

Light curve#

A graph of a star's brightness over time. A planet transiting causes periodic dips in it. The raw material of Deep Field.

Link budget#

Plain: an accounting of whether a radio signal is strong enough to get through between a satellite and a ground station, and how fast data can flow. Analogy: checking whether a flashlight is bright enough to be seen across a field — and how clearly. Delta V labels its model "first-order."

Molniya orbit#

A highly elliptical, ~63.4°-inclined orbit that lingers for hours over the far north — designed by the USSR for high-latitude coverage.

NORAD catalog number (catnr)#

The unique ID assigned to every tracked object in orbit. You can type one in the Track panel to fly that satellite.

Orbital elements#

The six numbers that fully describe an orbit (size, shape, tilt, orientation, and current position). An orbit's "address." See chapter 2 §3.

Orbital period#

The time for one full orbit. The ISS's is ~92.9 minutes; GEO's is exactly one day.

Orekit#

The mature, open-source astrodynamics library Delta V runs server-side for operational, high-fidelity (L2+) computations. Used widely in industry.

Periodogram / BLS#

A search that scans many candidate orbital periods to find the one that best explains a star's repeating dips. BLS (Box Least Squares) is the standard algorithm for boxy transit shapes. The Deep Field Periodogram station.

Phase fold#

Plain: stacking every orbit's worth of data on top of each other so the faint, repeating transit dip adds up into a clear shape. Analogy: overlaying many laps of a racetrack to find the one bump that's on every lap.

Porkchop plot#

Plain: the "fare calendar" for an interplanetary trip — cost colored by departure date and trip length; the cheap valley is the launch window. Named for its shape. In Delta V: the Porkchop tool in the Solar System view (Venus, Mars, Jupiter).

Propellant mass fraction#

The fraction of a vehicle's mass that must be fuel to achieve a given Δv. Big Δv ⇒ big fraction ⇒ why rockets are mostly tank.

Provenance#

The recorded trail of where a result came from — the data source, the pipeline steps, the document citations. A first-class part of every Delta V document.

RAAN (Ω)#

Right Ascension of the Ascending Node — which compass direction a tilted orbit's plane is turned toward. One of the six orbital elements.

Rocket equation (Tsiolkovsky)#

The relationship linking Δv, engine efficiency (Isp), and the fuel-to-empty mass ratio. The intuition: efficiency and shedding dead weight (staging) are what buy you Δv. You never compute it by hand — Delta V does.

SGP4 / TLE#

A TLE (Two-Line Element set) is the standard text format describing a satellite's current orbit, published daily (e.g. by CelesTrak). SGP4 is the matching propagation model. Delta V ingests TLEs in the Track panel (a quick-look parse today; full SGP4 precision is a future "Precision Desk" feature).

SI units#

The scientist's metric system — metres, seconds, kilograms, radians. Delta V uses SI everywhere except the Deep Field astronomy units.

Slant range#

The straight-line distance from a ground station to a satellite (longer when the satellite is low on the horizon, shortest when overhead). Drives the link budget.

SRP (Solar Radiation Pressure)#

The tiny but real push of sunlight on a spacecraft. Included in the L2+ (Orekit) server model.

SSO (Sun-Synchronous Orbit)#

A near-polar orbit, engineered using J2, that crosses each latitude at the same local time every day — so imaging satellites always see the ground in consistent lighting.

Staging#

Dropping spent fuel tanks/engines mid-flight so you stop accelerating dead weight. Analogy: dropping empty water bottles while climbing a mountain. Why big rockets have multiple stages.

Terminator#

The day/night line on a planet. Delta V draws Earth's real terminator from the Sun's true position.

TESS / Kepler#

NASA space telescopes that measured the brightness of millions of stars to find transiting planets. Their public data powers Deep Field.

TLI (Trans-Lunar Injection)#

The burn that sends a spacecraft from Earth orbit toward the Moon — ~3,100 m/s from low orbit. Apollo's famous "go for TLI."

TOI (TESS Object of Interest)#

A star flagged by the TESS pipeline as possibly hosting a planet — a candidate to investigate. THE FIELD plots thousands of them.

Transit#

Plain: a planet crossing in front of its star, dimming it slightly. The repeating dip is how most exoplanets are found. Analogy: a moth flickering across a distant streetlight. See chapter 2 §9.

True anomaly (ν)#

Where the spacecraft is right now along its orbit. The "current position on the racetrack." One of the six orbital elements.

Two-body problem#

The simplest orbit model: one body (Earth) as a point mass, one orbiting spacecraft, nothing else. Delta V's L0 fidelity. Exact and instant, but ignores the bulge, drag, Sun, and Moon.

Walker constellation#

A standard pattern for arranging many satellites into evenly spaced planes (how GPS, Galileo, etc. are laid out), specified as i: T/P/F (inclination, total sats, planes, phasing). Built in the Coverage panel.

WASM (WebAssembly)#

A fast, portable compiled format that lets the Rust physics engine run inside your browser at near-native speed — the same binary the server runs.

WebGPU#

The modern browser graphics API (successor to WebGL) that renders Delta V's 3D scene, with automatic fallback to WebGL2 on older setups.

Missing a term? It's likely defined in context in Space without the math — or ask at the Visitor Desk.