MARS CORRECT BASIC REPORT - SECTIONS 10 TO 11
Mars Global Surveyor Excessive Aerobraking and Mars Pathfinder Issues (Updated 5/17/2017)
10. 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 38 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.
Figure 38 above – Actual Dynamic Pressure – Normalized to an Altitude of 121 km (reproduced from Johnson, et al, 1998)
Note the tremendous increase in dynamic pressure shown on Figure 38. 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.
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."11.
11. MARS PATHFINDER PRESSURES
For Pathfinder (with an air access tube just 2 mm in diameter), the upper range of the transducer was only 12 mbar during descent, but only 10 mbar on the surface.83
A 10 mbar limit seems very strange given the Viking-2 10.72 mbar pressure seen. Note that the terrestrial dust storm which hit Luke Air Force Base and Phoenix, Arizona on July 5, 2011 increased air pressure by at least 6.6 mbar, and given that both terrestrial and Martian dust storms can turn day to night, the decision to reduce pressure sensitivities of Pathfinder, Phoenix and MSL landers seems highly ill-advised. There remains the question of what happened to the second Pathfinder sensor ordered that could measure up to 1,034 mbar (15 psia) shown on Figure 10B. Perhaps NASA is not as dumb as they seem to be, and they flew that sensor with a program inserted to cut reported pressures to 1% of what it actually measured. We really need to know the final disposition of this transducer, corresponding to Tavis Dash No. 1 on Tavis CAD Diagram 10484.
What were the Pathfinder pressures made public? Lower than expected. MPF landed on July 4, 1997 at an elevation of -3.682 km, most similar to Viking 1 which sat at -3.627 km. For MPF it was late northern summer at Ls 142.7. As noted earlier in Section 7, Schofield et al. (1997)67 indicate that Pathfinder had no pressure data for the most crucial sol – its first operational day on Mars (JPL wiped out all pressure data for the first 9 days of MSL). The reason given by the above reference is there were “various spacecraft software reset and downlink problems.” MPF pressures are shown on Figure 39A.
Figure 39A: Adapted from Science. Pressures reported by MPF. None is given for the critical landing day.
Two sols worth of MPF hourly pressures are shown on Figure 39B where they are compared to the only sol of published hourly pressure data for MSL.
At first it seemed a bit surprising that MSL and Pathfinder displayed a similar diurnal pressure cycle on Figure 38B. Pathfinder had no RTG heater on board. However, the Pathfinder battery was used to heat the probe's electronics to slightly above the expected nighttime temperatures on Mars.95 So again, at local midnight, measured pressures went up because the heater was operating at that time. What was being measured was not ambient pressure. It was just the pressure behind the (likely) clogged dust filter.
Figure 39B: Adapted from Science. Diurnal pressure cycles for MSL and Mars Pathfinder.
With Phoenix, there was a requirement for the lander to wait 15 minutes after the landing before deploying solar panels. This was to allow dust to settle (http://www.jpl.nasa.gov/news/press_kits/phoenix-launch-presskit.pdf). But it is unclear as to whether there was any way to prevent dust from being sucked into the pressure transducer and intermediate dust filter before powering up after the solar panels deployment. Since the dust filter was much smaller on the Phoenix than what was found in the ¼ inch diameter Viking air access tubes, the rate of ingestion of dust up front here is particularly important.