In addition to X-rays, X-ray detectors also respond to energetic particles which produce a background signal. The van Allen radiation belts are a major source of these particles, and to be an effective X-ray mission we need to minimize the time the spacecraft spends in these belts. The original AXAF spacecraft was to be placed into low Earth orbit where it would fly below the belts for most of its orbit. When the program was restructured, the spacecraft was made smaller and lighter. This allows the spacecraft to be placed into a high orbit where it spends much of its time above the belts.

There are many considerations to selecting a set of orbital parameters. For a technical discussion of these issues will be discussed in the paper, "The Dynamics of the Proposed Orbit for the AXAF Satellite" by Larry D. Mullins and Steven W. Evans submitted to the Journal of the Astronautical Sciences. Here we only provide a brief overview of some of the issues. In order to minimize the time in the radiation belts, we require the apogee to be fairly high (>>60,000 km). If the apogee gets too high, perturbations from the Sun and Moon can make the orbit unstable. More importantly the currently designed on-board communications system can only transmit data at the full rate out to a distance of 140,000 km. Protons from the inner belts can, over time, damage one of the instruments. To prevent this from occurring we require that the perigee altitude stay above 10,000 km.

When these and other factors considered, the final proposed initial orbit has a perigee=10,000 km, apogee=140,000 km, inclination=28.5, argument of perigee of 270, and RA of ascending node of 200. The following figure shows how the orbital parameters change over a 10 year period.

(Click on figure to download larger image.)

The fact that the semimajor axis is constant over the 10 year period is a sign that the orbit is very stable. This is very good since it implies the the exact launch date and initial orbital parameters are not extremely critical.

It is also worthwhile to consider other parameters such as the fraction of time that the spacecraft spends above 60,000 km. This is a first order estimate for the time the spacecraft spends outside the radiation belts. Also of interest is the duration of the eclipses since the current design of AXAF allows for eclipses lasting at most 120 mins. For planning purposes the eclipse time is the time from preumbra entry to preumbra exit, and this is the duration that is plotted below. Finally we need to check perigee and apogee altitudes. All these are plotted below.

(Click on figure to download larger image.)

The current Shuttle manifest shows the launch date of AXAF to be 1998-Aug-27. The two stage IUS rocket will propel AXAF into an elliptical orbit with a 240 km perigee and a 59,000 km apogee. At the next perigee passage, the Integral Propulsion System (IPS) will fire to raise the apogee to 140,000 altitude. At apogee the IPS will again fire to raise the perigee to above 10,000 km. There is an alternate proposal being considered in which only two of the four IPS engines will be used. This would provide an extra margin of safety and if adopted would require an additional IPS burn. Once AXAF is in the final orbit, no additional IPS burns will be needed for the life of the mission.

Lunar swingby orbits were also considered. If you send the spacecraft on the right trajectory by the Moon, the Moon's gravity can be used to raise the spacecraft's perigee. The problem with this type of orbit is the apogee stays high (400,000 km). In such an orbit the orbital parameters tend to evolve chaotically, and therefore it is difficult to prove that all possible orbits generated by reasonable errors in launch times, burn vectors and burn durations will survive for a 10 year period. Eclipses are rarer but when they occur tend to last longer and in fact some are much longer than allowed by current design. Finally the apogee exceeds the 140,000 km allowed by the current communications systems.

Go to AXAF Project Science Page or to X-ray Astronomy at MSFC

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