The Kronos Orbit

One of the unique features of the Kronos mission is the orbit, which is chosen to give long on-target observation times with minimal interruptions, to avoid both the Earth's radiation belts and rapid day/night cycles (which cause thermal/mechanical stresses), and still allow large the mass-to-orbit and keep communication costs low. No previous satellite has utilized this particular type of orbit, although the Microwave Anisotropy Probe used a similar phasing-loop/lunar swing-by trajectory to reach the L2 Lagrangian point orbit.

The Delta 2925 launch vehicle launch Kronos into a series of  “phasing loops,” in which the apogee of the orbit is gradually increased through repeated engine burns at perigee, to ascend to a close encounter with the Moon. The final phasing loop will take Kronos to within about 6000 km of the lunar surface. The satellite will perform a lunar swing-by, thereby picking up the Moon's orbital speed (about 1 km/s), and thus increasing the satellite's perigee to around 7-18 RE and apogee to 60-70 RE, and rotating the orbital plane to an inclination of 25 - 70°. It also introduces a 90° phase shift between the major axis of the satellite orbit and the Earth-Moon line. A propulsion burn at first perigee establishes the final orbit, which, for stability, needs to have a period nearly equal to half the lunar orbital period. The final orbit is an “open” (i.e., non-repeating) orbit with perigee, apogee, and inclination varying over the mission lifetime. Analysis has shown that this orbit remains stable for at least six years. Eventually Kronos will collide with the Earth (where it will burn up on re-entry through the atmosphere) or the Moon, or will be ejected into a solar orbit.  


The Kronos orbit. The Earth is at the center. The yellow lines show the initial ascent and phasing loops that lead to the lunar swing-by, shown in blue. The lunar swing-by increases the inclination of the final orbit, shown in purple. The phasing loops allow for adjustments that increase the launch windows (which are several days in duration and about twice per month) and correct for launch errors.

Advantages of the Kronos orbit:

(1)  Relative to other high-Earth orbits, low cost to orbit, low communications costs

(2)  Excellent power/thermal and ambient radiation environment

(3)  Outside Earth’s geosphere and radiation belts

(4)  Excellent observational duty-cycle (continuous sampling with only short interruptions due to perigee passage)

Go to:[Kronos Home Page] [OSU Astronomy Home Page]

Updated 29 October 2001