GLOSSARY

1. Current spacecraft state:

This is the time for which the status information applies. Time is given in HHMM format (HH=hours, MM=minutes) on a 24 hour clock in Greenwich Mean Time (GMT). For example, 1730 GMT 6/12/98, a time and date in this universal time format, is 5:50 PM the day of June 12, 1998 in Greenwich, England. GMT is the local Earth time at 0 deg longitude, 5 hours ahead of Eastern Standard Time and 8 hours ahead of Pacific Standard Time. GMT is a standard clock used for all space and aviation activities, since they involve participants around the globe (or even off the planet). Using LP as an example, since LP is tracked by the Deep Space Network (DSN), which has antennas in California, Spain, and Australia, using a well defined time is a requirement to ensure events are scheduled properly.

2. Orbit:

This number refers to the number of orbits completed about the Moon since lunar orbit insertion (LOI) on January 11th, 1998. LP was launched the evening of January 6th, 1998, arrived at the Moon on January 11th and began orbiting the Moon. The final mapping orbit is a 118 minute orbit, so the spacecraft now orbits the Moon roughly 365 times a month.

3. Downlink:

This number refers to the "bits per second" (bps) or the data rate of the telemetry (information) coming from LP's Command and Data Handling unit (C&DH). The C&DH is LP's "computer". Actually it's not a real computer in the sense of most modern spacecraft computers. It only stores information for a maximum of 53 minutes (so that data collected on the farside of the Moon can be transmitted after a 47 minute delay and not lost). Two transmit rates are possible: the nominal 3600 bits per second, and a slower rate of 300 bits per second.

4. Spin Rate:

The number of revolutions per minute at which Lunar Prospector is spinning. LP is a spin stabilized spacecraft. Like a Top, it spins to stay upright.

5. Spin Axis Attitude (ecliptic):

Lunar Prospector's orientation (or attitude) in space is given in terms of where the antenna is pointing (the +Z axis of the spacecraft). The coordinate system used to describe this pointing direction (a vector) is an ecliptic system where the ecliptic is the plane of the Earth's orbit around the Sun. The system is further defined as having an X axis which points along the vernal Equinox (the line formed by the intersection of the Earth's equator and the ecliptic plane at a reference date) and a Z axis which is perpendicular to the ecliptic plane. The date reference is required because of small permutations in the Earth's relationship to the plane over time. A pointing direction can be described relative to these axes (X and Z) in terms of two angles, sometimes referred to as Right Ascension (RA or Longitude) and Declination (DEC or latitude). If you you draw a celestial sphere with the X and Z axes as described, longitude is the angle from the X axis to the projection of the pointing vector onto the ecliptic plane. Latitude is the angle between the pointing vector and the ecliptic plane. PLEASE see the accompanying image to make things easier to understand.

The terms latitude and longitude are used because they are similar to how we identify the location of a point on the Earth in Spherical coordinates. LP is usually pointed within a few degrees ecliptic north, so the latitude of the spin axis direction is close to 90°. This means that it is spinning with the antenna almost perpendicular to the Earth direction (the best geometry for communications), and the solar panels are near perpendicular to the Sun direction (the best geometry for maximum power).

6. Periapsis Altitude:

Lunar Prospector orbits the Moon in an elliptical orbit. That means there is a "closest" (or lowest) point and a "farthest" (or highest) point (relative to the Moon's surface). Periapsis Altitude is the lowest point of Lunar Prospector's orbit relative to the surface of the moon. During the mapping phase of LP's primary mission (until the extended mission begins in January 1999), the targeted range of periapsis altitude is between 80 and 100 km.

7. Apoapsis Altitude:

Lunar Prospector orbits the Moon in an elliptical orbit. That means there is a "closest" (or lowest) point and a "farthest" (or highest) point (relative to the Moon's surface). Apoapsis Altitude is the highest point of Lunar Prospector's orbit relative to the surface of the moon. During the mapping phase of LP's primary mission (until the extended mission begins in January 1999), the targeted range of apoapsis altitude is between 100 and 120 km.

8. Inclination:

The angle that Lunar Prospector's orbit plane makes with the lunar equator. During the mapping phase, the targeted range of orbit inclination is between 89 and 91 degrees.

9. Orbit Period:

The time it takes for Lunar Prospector to travel one complete orbit.

10. Occultations:

The amount of time throughout each orbit that Lunar Prospector spends on the farside of the Moon, as viewed from the Earth. During this time, because LP is blocked from view by the Moon, controllers cannot communicate with the spacecraft.

11. Eclipses:

The amount of time throughout each orbit that Lunar Prospector spends in the shadow of the Moon. During this time, the spacecraft must draw energy from its battery to operate science instruments, heaters, and communications equipment, as well as respond top commands sent from Earth. When LP emerges from an eclipse, it uses its solar panels to generate electricity from the Sun's rays to operate equipment and recharge its battery.

12. DOY:

Day of the Year. These are standard days of the year, with January 1st being day 1. This section of the report often mentions anomalies. These are unusual things that happen. Even if there is a minor "anomaly". It must be noted as it could be significant later and could affect the accuracy of the science results.