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.