# Definition of Reentry Terms

**1st Deriv Mean Motion (/2)**- first derivative of the
*mean motion*divided by two, in units of revolutions per day**squared (rev/day²)** **2nd Deriv Mean Motion (/6)**- second derivative of the
*mean motion*divided by six, in units of revolutions per day cubed (rev/day³) **Apogee**- point on an orbit farthest from Earth’s center of mass
**Apogee Altitude at Epoch****distance above the Earth’s surface of***apogee*at the*elements epoch*, assuming a mean equatorial radius of 6378.14 km (3443.92 nmi)**Argument of Perigee**- angle from the
*ascending node*of an orbit to the*perigee*, measured in the orbital plane in the direction of motion of the satellite **Ascending Node****point where the satellite crosses the equatorial plane headed from south to north; the time at which this occurs is usually used as the***elements epoch***B***- one-half the product of
*ballistic coefficient*and sea-level atmospheric density, in units of inverse Earth radii **Ballistic Coefficient**- ratio of the product of drag coefficient (Cd) and reference area (A) to mass (m), giving CdA/m; also defined as the inverse of this quantity when used in other contexts; similarly, object weight (W) is sometimes used in place of object mass
**with the inverse definition, giving W/CdA** **Eccentricity**- for an elliptical orbit, one-half the ratio of the distance between foci and the semimajor axis; for a circular orbit,
*eccentricity*is zero **Elements Epoch**- the reference time for the element set; usually the time when the
*ascending node*is reached, in units of calendar year (first two digits), day of the year (next three digits), and day fraction (in decimal format – e.g., 11:30 UTC = (11 hours + 30 minutes/60 minutes/hour)/24 hours/day = 11.5/24 days = 0.47917 days) **Epoch**- reference time
**Inclination**- angle between the orbital plane and the equatorial plane
**Int’l Designation**- designation assigned by the ICSU World Data System by international agreement; indicates the year of the launch, the launch number for that year, and the object
**number for that launch** **Mean Anomaly**- product of the
*mean motion*and the time since*perigee* **Mean Motion**- average angular velocity required to complete one revolution on an orbit in the
*period*of the orbit, in units of revolutions per day **NORAD**- North American Aerospace Defense Command
**NORAD Name**- a non-unique descriptive identifier assigned by
*NORAD*for each Earth-orbiting artificial satellite in their satellite catalog **NORAD Number**- a unique five-digit number assigned by
*NORAD*for each Earth-orbiting artificial**satellite in their satellite catalog; assigned when first observed** **Observed Reentry Time****time at which eyewitnesses have reported seeing the reentering object, in units of***UTC***Orbit Number at Epoch**- total number of orbits completed at the
*elements epoch* **Perigee**- the point on an orbit closest to Earth’s center of mass
**Perigee Altitude at Epoch**- distance above the Earth’s surface of
*perigee*at the*elements epoch*, assuming a mean equatorial radius of 6378.14 km (3443.92 nmi) **Period**- time required to make one revolution around an orbit
**Predicted Reentry Time**- time at which a reentering object might first become visible due to aerodynamic heating, corresponding to an altitude of ~120 km (~65 nmi), in units of
*UTC*; for a**high-***eccentricity*orbit, the*perigee*may actually be much less than 120 km, which may lead to on-orbit fragmentation of the object **Prediction Epoch****the most recent***elements epoch*used to perform the reentry prediction, in units of*UTC***Prediction Ground Track****the location along the surface of the Earth where the reentering object may be visible, and where surviving debris may impact****RA of Ascending Node**- right ascension of the
*ascending node*; right ascension is the angular position measured eastward along the celestial equator from the*vernal equinox vector*to the hour circle of the object **UTC**- Coordinated Universal Time, the international time standard, corresponding to the time at the prime meridian; informally equivalent to Greenwich Mean Time (GMT) or Zulu time
**Vernal Equinox Vector**- the line pointing from the center of mass of the Earth to the center of mass of the Sun on the spring equinox

NOTICE: The materials about Upcoming Reentries are for informational purposes only and should not be used as a substitute for specific technical advice or opinions related to your particular facts and circumstances.