Frequently asked questions

Orbital mechanics is the branch of physics describing the motion of objects under gravitational forces. For satellites it covers how orbits are sized and shaped, how they evolve over time, and how to predict and control where a satellite will be. The foundations are Newton's laws of motion and Kepler's three empirical laws of planetary motion.

A GPS receiver listens to signals from at least 4 GPS satellites simultaneously. Each signal includes the satellite's precise position and the exact time it was sent. By measuring how long each signal took to arrive, the receiver computes its distance to each satellite. Four distances and four positions are enough to triangulate the receiver's position (latitude, longitude, altitude) and current time.

Starlink is a constellation of 5,000+ low-Earth-orbit satellites (operational target: ~12,000) that beam internet from a 550 km altitude. Each user terminal has a phased-array antenna that tracks the visible Starlink satellite overhead. Because LEO satellites move fast (7.6 km/s), the terminal hands off to a new satellite every few minutes. The result: ~20-30 ms latency vs ~600 ms for traditional geostationary satellite internet.

About 10,000 active satellites as of 2025, plus an estimated 30,000+ pieces of trackable debris (larger than a softball) and millions of smaller pieces. Starlink alone accounts for more than half of all active satellites.

About 7.66 km/s (17,150 mph). The ISS orbits at roughly 400 km altitude and completes one orbit every 92.5 minutes, giving 16 orbits per day. Astronauts on board see 16 sunrises and 16 sunsets every day.

Satellites and astronauts inside them are in free fall toward Earth — they're being pulled by gravity just like any falling object. But their high horizontal velocity (7.66 km/s at ISS altitude) means they keep missing Earth as they fall. The result is continuous "free fall" with no contact force, which feels like weightlessness.

Space debris is any non-functional object in orbit: dead satellites, spent rocket stages, fragments from collisions. At orbital velocities (~7.5 km/s in LEO), a 1 cm piece carries the kinetic energy of a hand grenade. Even small fragments can disable an operational satellite. The 2009 Iridium 33 / Kosmos 2251 collision and the 2007 Chinese ASAT test created massive new debris fields that are still tracked today.

That's the altitude where Kepler's third law gives an orbital period equal to one sidereal day (23 hours 56 minutes 4 seconds, not 24 hours). At that exact altitude in a circular equatorial orbit, the satellite's angular speed matches Earth's rotation, so it appears stationary in the sky.

Walker delta notation t/p/f encodes a constellation's geometry in three numbers. t = total satellites. p = number of orbital planes (must divide t evenly). f = phasing parameter, which controls how satellites in adjacent planes are offset. Together they uniquely specify the constellation: GPS is 24/6/1, Starlink Shell 1 is 1584/72/1, Iridium NEXT is 66/6/2.

Yes. Every lesson at every level (kids, novice, intermediate, advanced) is free with email signup. Anonymous users get Lesson 1 plus the entire kids mode. Free signed-in users get all 10 lessons, the AI Mission Architect (3 generations per day), and every simulator tool. The only paid surface is AI Pro ($5/month or $50/year) which lifts the daily AI cap. Voluntary support contributions are accepted but unlock nothing — they exist purely to fund the project.

Semi-major axis (a), eccentricity (e), inclination (i), right ascension of the ascending node (RAAN, Ω), argument of perigee (ω), and mean anomaly (M). Together they pin down a satellite's exact position and trajectory.

For an unperturbed two-body orbit, only mean anomaly (M) changes with time, at a steady rate set by the orbit size. In reality, perturbations like Earth's oblateness (J2) cause RAAN and argument of perigee to drift slowly too.

Semi-major axis is measured from Earth's center. Altitude is measured from Earth's surface. To go from one to the other, add or subtract Earth's radius (6,378 km at the equator). A satellite at 400 km altitude has a semi-major axis of about 6,778 km for a circular orbit.

Eccentricity (e) measures how stretched an orbit is. e = 0 is a perfect circle. 0 < e < 1 is an ellipse, with higher values meaning a longer, more stretched shape. The Earth's orbit around the Sun has e ≈ 0.017 (nearly circular). A Molniya orbit has e ≈ 0.74. A parabolic escape trajectory has e = 1.

Below 400 km, atmospheric drag pulls satellites down quickly. Above 500 km, the South Atlantic Anomaly (a region of intense radiation) increases health risks for crew. 400 km is the compromise: long enough orbital lifetime that the ISS only needs occasional reboost burns, low enough that Soyuz and crew Dragon can reach it economically.

Kepler's third law states that the square of an orbital period is proportional to the cube of the semi-major axis: T² ∝ a³. For Earth orbits, T² = (4π²/μ) × a³ where μ = 398,600.4418 km³/s² is Earth's gravitational parameter.

Because that's the altitude where Kepler's third law gives an orbital period equal to one sidereal day (23h 56m 4s). At that altitude in a circular equatorial orbit, the satellite's angular rate matches Earth's rotation rate, making it appear stationary in the sky.

About 92.5 minutes. At an average altitude of 400 km, the ISS completes 16 orbits per day. Astronauts on board see 16 sunrises and 16 sunsets every day.

GPS satellites are in roughly 12-hour orbits at 20,200 km altitude (more precisely, 11 hours 58 minutes, which is exactly half a sidereal day). Each satellite passes over the same ground track twice per day.

Slower. Counterintuitive but true: ISS at 400 km moves at 7.66 km/s; GEO at 35,786 km moves at 3.07 km/s. Higher means slower because gravity is weaker at higher altitudes and less centripetal acceleration is needed.

Kepler's second law states that the line connecting a planet (or satellite) to its central body sweeps out equal areas in equal times. The mathematical consequence: orbital speed varies inversely with distance to the central body.

Conservation of angular momentum. Lower altitude means less moment arm, so the satellite must move faster to keep angular momentum constant. Physically: as the satellite falls toward Earth, gravity accelerates it; as it climbs away, gravity decelerates it.

A Molniya orbit is a 12-hour highly elliptical orbit (e ≈ 0.74) inclined at 63.4° with apogee over the northern hemisphere. Its design uses Kepler's second law to keep the satellite nearly stationary over high-latitude regions like Russia for about 8 hours per orbit.

The vis-viva equation gives orbital speed at any point in an orbit: v² = μ × (2/r − 1/a), where μ is the gravitational parameter, r is current distance from the central body, and a is the semi-major axis. It works for any conic section orbit.

The higher the eccentricity, the bigger the speed difference between perigee (fastest) and apogee (slowest). The ratio is (1+e)/(1-e). A circular orbit (e=0) has constant speed; an e=0.5 orbit varies by a factor of 3; an e=0.74 Molniya orbit varies by a factor of about 7.

A ground track is the path on Earth's surface directly below a satellite's position over time. It's the projection of the orbit onto the rotating Earth.

Inclination sets the maximum latitude the ground track reaches. A 30° inclination orbit traces a track between 30°N and 30°S. A 90° (polar) orbit reaches every latitude. A retrograde orbit (inclination > 90°) appears to move westward across the ground.

At 98°, the orbital plane precesses (rotates) due to Earth's equatorial bulge at exactly the rate Earth orbits the Sun (one revolution per year). This keeps the orbit's sun angle constant year-round, which is essential for Earth-observation satellites that need consistent lighting.

At 55°, the constellation provides good coverage of populated regions (most of the world's population lives below 55° latitude) while requiring fewer satellites than a polar constellation would. The original 1970s GPS design balanced coverage, redundancy, and launch cost.

Two teardrop-shaped loops on a flat map, one over Russia/northern Europe and one over Canada/North America, connected by long arcs through the southern hemisphere. The teardrops are where the satellite hovers near apogee for ~8 hours per orbit.

J2 is the second zonal harmonic of Earth's gravity field, representing the equatorial bulge. Its value is approximately 0.001082. J2 is the dominant non-spherical perturbation on any Earth orbit and must be accounted for in operational satellite design.

J2, Earth's equatorial bulge. The bulge creates a gravitational torque that causes the orbital plane to precess. The drift rate depends on altitude and inclination. For a typical LEO at 500 km and 51.6° inclination, RAAN drifts westward at about 5°/day.

Because at that inclination and the right altitude (typically 600-800 km), J2 causes the orbital plane to precess at exactly +0.9856°/day, which matches Earth's motion around the Sun. The result is an orbit where every pass over a given point happens at the same local solar time.

63.4° (or its retrograde complement, 116.6°). At this inclination, J2-induced rotation of the line of apsides goes to zero, meaning argument of perigee stays fixed. Molniya orbits use 63.4° so their apogees stay locked over the northern hemisphere indefinitely.

The J2 perturbation's effect on RAAN drift depends on cos(inclination), which is zero at 90°. A perfectly polar orbit has no J2-induced RAAN drift. Other perturbations (atmospheric drag, solar radiation pressure, third-body gravity) still affect it, but J2 doesn't.

The angular extent of what the sensor can observe at any instant. A wide FOV means a larger footprint on the ground but lower spatial resolution per pixel. A narrow FOV means smaller footprint but higher resolution.

About 2,294 km to the horizon, assuming an unobstructed view. The visible footprint depends on the sensor FOV; a narrow sensor sees only directly below, a wide sensor sees out to the horizon.

Field of view (FOV) is the instantaneous angular coverage. Field of regard (FOR) is the total angle a sensor can be slewed across. A satellite with a small FOV but large FOR can image many different points by pointing the sensor at each, but only one at a time.

Because the horizon distance grows roughly as the square root of altitude. At 400 km, horizon is 2,294 km. At 20,200 km (GPS), horizon is 17,580 km. The footprint a sensor with the same FOV draws on Earth grows proportionally.

Pointing the sensor straight down, toward the point on Earth directly below the satellite. Nadir is the simplest pointing mode and gives the shortest slant range, highest resolution, and least atmospheric distortion. Most Earth-imaging satellites operate nadir-pointed by default.

LEO is low Earth orbit (200-2,000 km, 90-min periods, low latency, high spatial resolution, requires constellations). GEO is geostationary orbit (exactly 35,786 km altitude, 24-hour period, single-satellite continental coverage, high latency).

A 12-hour orbital period at MEO altitude lets a 24-satellite constellation provide global coverage with 4+ satellites always visible from any point on Earth (needed for triangulation). MEO also avoids the radiation-intensive Van Allen belts above and below it.

Exactly 35,786 km above Earth's surface, or 42,164 km from Earth's center. This is the only altitude where a circular equatorial orbit has a period equal to one sidereal day (23h 56m 4s), causing the satellite to appear stationary in the sky.

Highly elliptical orbits exploit Kepler's second law: the satellite moves slowly near apogee and quickly near perigee. By placing apogee over a region of interest (typically high latitudes that GEO can't serve well), a HEO satellite dwells over that region for hours per orbit. Molniya orbits dwell over Russia/northern Europe; Tundra orbits dwell over North America.

The two Van Allen radiation belts make sustained operation difficult between roughly 1,000-6,000 km (inner belt) and 13,000-60,000 km (outer belt, peaks around 25,000 km). Operational satellites cluster in LEO (below the inner belt), in the MEO "valley" between belts, and at GEO (above the outer belt).

Walker notation t/p/f describes a satellite constellation in three numbers: t = total satellites, p = number of orbital planes, f = phasing parameter (controls offset between satellites in adjacent planes). Example: GPS is 24/6/1, meaning 24 satellites in 6 planes with adjacent planes offset by 1 unit of phasing.

A Walker delta has all planes equally inclined (typically 50-65°) and spaced over 360° of RAAN, giving good mid-latitude coverage. A Walker star has near-polar inclinations and planes spaced over 180° of RAAN, giving true pole-to-pole coverage. GPS, Galileo, and Starlink use delta. Iridium and OneWeb use star.

To provide low-latency global internet, Starlink needs many satellites simultaneously visible from any point with minimal handoff delay. 72 planes × 22 satellites per plane gives dense, well-distributed coverage at 53° inclination. Higher plane counts mean smaller gaps between satellites and smoother handoffs.

The operational GPS constellation has 24 active satellites in a Walker delta 24/6/1 configuration. There are additional spare and backup satellites in orbit, bringing the total to about 31 active. Only 24 are needed for full global four-satellite-visibility coverage.

Iridium provides global voice communications, including to the poles. A Walker star constellation at 86.4° inclination gives true pole-to-pole coverage with planes that span all latitudes. A Walker delta at lower inclination would leave the polar regions uncovered.

Nadir is the direction directly below the satellite, toward Earth's center. Nadir-pointing means the sensor is rotated to always look straight down. This is the default mode for imaging and communications satellites because it minimizes slant range and atmospheric distortion.

Inertial stare means the sensor stays pointed at a fixed direction in inertial space (relative to the stars) rather than tracking Earth. The satellite body doesn't rotate relative to the stars as it orbits. Hubble, JWST, and GEO missile warning satellites all use this mode.

A target-tracking sensor uses predicted ephemerides of a moving target to slew the satellite or the sensor itself to keep the target in the sensor field of view. It requires accurate target predictions and fast actuation. Ground radars tracking LEO satellites and GSSAP satellites inspecting GEO objects both operate target-track.

Along-track means the sensor points in the direction the satellite is moving (its velocity vector). Some atmospheric sampling missions and limb-scanning satellites use this mode to take measurements through the atmosphere's edge as the satellite flies through it.

Hubble takes long-exposure images (sometimes hours) of distant astronomical objects. Inertial stare keeps the sensor pointed at the exact same celestial coordinate during the exposure, eliminating motion blur. Earth-pointing wouldn't work because the target would drift out of frame as the telescope moved through its orbit.

Space Domain Awareness (SDA, formerly SSA) is the practice of detecting, tracking, identifying, characterizing, and predicting the behavior of objects in orbit. It includes operational satellites, debris, and adversarial activity. SDA underpins orbital safety, collision avoidance, and space security.

About 30,000 objects larger than a softball are cataloged by the U.S. Space Surveillance Network. An estimated 500,000+ smaller pieces (1-10 cm) and millions of even smaller fragments exist but are too small to track. All of them are dangerous to satellites at orbital velocities.

DARC (Deep-space Advanced Radar Capability) is a network of three S-band radars in Australia, the UK, and Texas, designed to track objects in geostationary orbit 24/7. Unlike optical telescopes which only work at night, DARC operates in any weather and provides continuous coverage of the GEO belt.

GEODSS-style telescopes image the sky against the star background. Satellites appear as streaks (if moving relative to the stars during the exposure) or points (if tracking inertially). Image-processing algorithms compare to known star catalogs and known satellite predictions to identify each detected object.

Conjunction screening predicts close approaches between cataloged orbital objects. For each pair, the algorithm computes the time and miss distance of closest approach. When the probability of collision exceeds a threshold, operators are notified so they can plan an avoidance maneuver. The Space Force performs millions of screenings per day.