TES News

August/September 1993, Volume 2, No. 2

Thermal Emission Spectrometer Project Mars Observer Space Flight Facility Department of Geology, Box 871404 Arizona State University Tempe, Arizona 85287-1404, U.S.A.

Return to the Red Planet: Mars Observer Orbit Insertion

Thirty years ago, the surface of Mars was known only from the view through a telescope. In 1965, Mariner 4 became the first spacecraft to see the planet up close. In 1976, two Vikings landed on Mars and tested its soil for evidence of life.

Now in 1993, a new era of Mars exploration is beginning. Mars Observer was launched from the Kennedy Space Center, Florida, on September 25, 1992. After an 11-month journey, it is the first U.S. spacecraft to reach the fourth planet since 1976.

Mars Observer will examine the climate and geology of the Red Planet for at least one martian year (687 Earth days) from an altitude of about 380 km (236 mi). From this vantage point, Mars Observer will monitor the climate, polar ice caps, and surface features as they change with the seasons.

After its arrival on August 24, 1993, the spacecraft will be carefully moved into a nearly circular, polar orbit over a period of about three months. From its final mapping orbit, Mars Observer will pass near both the north and south poles 12 times a day, and it will cross the equator at about 2 PM local time on the day side and 2 AM on the night side of the planet.

The period from August 24, 1993, to mid-November 1993, is called the orbit insertion phase. The figures shown here (below) illustrate the main orbital changes that are planned to occur during this time frame. These spacecraft maneuvers are under the direction of the Mars Observer navigation team at the Jet Propulsion Laboratory in Pasadena, California. This newsletter was written prior to Mars orbit insertion on August 24th, thus the dates that are given for events illustrated in these figures may be subject to change.

<--- Figure 1- Mars Orbit Insertion (MOI)

Fig. 1. Mars Orbit Insertion (MOI) occurs on occurs on August 24, 1993. This diagram shows the transition from Mars Observer's interplanetary cruise to the initial three day capture orbit around Mars. While in this orbit, Mars Observer takes three days to go around the planet. By comparison, Mars Observer will circle Mars once every 2 hours during the mapping phase which begins in mid-November (see Fig. 5). The three day capture orbit crosses the daytime martian equator at about 5 PM, whereas the final mapping orbit in November will cross it around 2 PM. The orbits of the martian moons, Phobos and Deimos, are also shown here. The tick marks along the spacecraft orbit path occur every 1 hour.

<--- Figure 2- Ellipse Change Maneuver #1 (ECM-1)

Fig. 2. Ellipse Change Maneuver #1 (ECM-1). Around September 4th, Mars Observer fires its rockets for its first change in orbit. The new configuration, called the three day drift orbit, is very similar to the previous three day capture orbit except now the spacecraft will begin to change its position until it crosses the daytime martian equator at about 4 PM. The tick marks along the spacecraft orbit occur every 1 hour.

<--- Figure 3- Ellipse Change Maneuver #2 (ECM-2)

Fig. 3. Ellipse Change Maneuver #2 (ECM-2). Around September 15th, the rockets are fired again to slow Mars Observer so that it can drop to a lower average altitude (see smaller ellipse). Now it is in a one day drift orbit, meaning that it circles the planet once a day. The term "drift" refers to the fact that the orbit will now slowly change from one that crosses the equator at 4 PM local time to one that crosses at 2 PM. Recall that crossing the equator at 2 PM local time is desired for the final mapping orbit which will begin in November. A minor orbit adjustment, ECM-3, may occur around October 7th (not illustrated here). Some of Mars Observer's instruments will obtain limited amounts of data during the one day drift period illustrated here, however most data collection will not begin until late November. The tick marks along the spacecraft orbit occur every 1 hour.

<--- Figure 4- Tranfer to Low Orbit #1 (TLO-1)

Fig. 4. Transfer to Low Orbit (TLO-1). Now that Mars Observer is approximately in the 2 PM equator-crossing position, it must lower itself toward the desired 380-km altitude. The TLO-1 maneuver will occur around October 17th, and will put Mars Observer in a lower and less elliptical orbit that will carry it over each polar region every 4.2 hours. The 4.2 hour orbit is the smaller ellipse in the diagram. The tick marks along the orbit occur every 1 hour.

<--- Figure 5- Transfer to Low Orbit #2 (TLO-2)

Fig. 5. Transfer to Low Orbit #2 (TLO-2). Around October 28th, Mars Observer's rockets will again be fired to place it in the nearly circular polar orbit known as the mapping orbit. In this figure, the 2 hour mapping orbit (inner ellipse) is nearly circular, while the previous 4.2 hour orbit is shown as the larger, more elliptical path. A final rocket firing (the orbital change maneuver, OCM-1) is planned for November 8th (not illustrated). This final maneuver will simply fine-tune the mapping orbit to correct for any errors. Mapping from this orbit will begin around November 24, 1993. The tick marks along the orbit path occur every 10 minutes.

Text and Figures by: D.E. Melendrez and K.S. Edgett

Original (hardcopy) Typesetting by: J. Goldblatt

Original Text:  August 1993

Hypertext:  21 June 1994