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