MARS CORRECT BASIC REPORT - SECTION 10
Mars Global Surveyor Excessive Aeroraking (Updated 9/14/2018)
EXCESSIVE DECELERATION DURING AEROBRAKING OPERATIONS.
It is cost efficient to slow a spacecraft approaching a planet like Mars by aerobraking – dipping the probe into the atmosphere to use free drag rather than expensive fuel. This was done with Mars Global Surveyor (MGS) and Mars Reconnaissance Orbiter (MRO). In both cases, more air was encountered than expected.
10.1 Mars Global Surveyor (MGS).
When MGS was launched in 1996, the intent was to achieve a circular pole-to-pole, Sun-synchronous orbit around Mars with an altitude of approximately 300 km above the surface and an orbital period of just under 2 hours. In an attempt to accomplish this orbit using minimal fuel, MGS used aerobraking. It was deliberately flown through the upper atmosphere of Mars during periapse to use the aerodynamic drag forces to modify its orbital parameters. The effort did not go as planned and the early maneuvers led to excessive decelerations (Read & Lewis 2004, 11).78
If Mars has a higher than expected atmospheric density, it would explain unexpected excessive decelerations. As shown in Figure 40 and discussion below, it is believed that a dust storm produced the unexpected drag, but the effects at a normalized altitude of 121 km (75 miles) seem quite high for a planet that is supposed to have an average surface pressure of only about 6.1 mbar.
Johnston et al. (1998)81 reported that (1) “On the onset of a dust storm, the atmospheric density could more than double in a 48 hour time period,” and (2) “If during aerobraking, the spacecraft experiences dynamic pressure values greater than this limit line, the periapsis altitude of the orbit must be raised immediately in order to re-establish the 90% atmospheric density capability.” Both happened.
Note the tremendous increase in dynamic pressure shown on Figure 39. At an altitude normalized to 121 km, the dust storm caused dynamic pressure to rise from about 0.15 N/m2 on November 9th, 1997 to 0.84 N/m2 on December 7, 1997. While the Johnson et al. (1998) article referred to atmospheric density more than doubling during a dust storm, the increase in dynamic pressure felt at 121 km over four weeks was 5.6 times the pre-storm values.
Figure 39 – Actual Dynamic Pressure – Normalized to an Altitude of 121 km (reproduced from Johnson, et al, 1998)
10.2 Mars Reconnaissance Orbiter (MRO).
MRO also employed an aerobraking process. Its navigation team relied on an atmospheric model called the Mars-GRAM (Global Reference Atmospheric Model). Mars-GRAM is a computer database of information from what previous missions have encountered. It provided a prediction of the atmospheric density, giving the navigators an estimate of how far down into the atmosphere the spacecraft should go.
The atmospheric density that MRO actually experienced was much different than what was predicted by the Mars GRAM (Atkinson, 2006).82 Two quotes are most notable in the Atkinson article:
(1) “At some points in the atmosphere, we saw a difference in the atmospheric density by a factor of 1.3, which means it was 30% higher than the model,” said Han You, Navigation Team Chief for MRO. “That’s quite a bit, but around the South Pole we saw an even larger scale factor of up to 4.5, so that means it was 350% off of the Mars GRAM model.”
(2) “When we first started out at a somewhat higher altitude, the Mars GRAM model was doing pretty well,” said Richard Zurek, Project Scientist for MRO. “When we got to the lower altitude the scale factor to which it was off was larger and it became even larger as periapsis moved toward the South Pole.”
This article is continued with Section 11 HERE.