This page serves as a guide to help interpret the History of Orbital Parameters page. One of the more interesting parameters to track is the argument of periapsis, which can be use to determine the latitude of the point of periapsis passage. The argument of periapsis is measured in degrees from the line of the ascending node on the equatorial plane to the point of periapsis passage. Referring to the History of Orbital Parameters, we see that at the start of aerobraking the argument of periapsis was about 148.44 degrees. This corresponds to a North latitude of 180 degrees - 148.44 degrees, or 31.56 degrees. Due the oblateness of Mars (Mars is not perfectly spherical) the orbit has undergone precession, and therefore the argument of pariapsis has decreased over the last 6 months.

We can also see that the eccentricity has been decreasing as the orbit becomes more circular. If the orbit is perfectly circular, the radius of apogee equals the radius of perigee and therefore the eccentricity is zero. When we reach the circular mapping orbit in March of 1999 the eccentricity will be close to zero. Referring to the History of Orbital Parameters we can see that the semimajor axis (SMA) has also been decreasing as the orbit has become more circular over the aerobraking period.