MARTIAN LANDERS ALWAYS LANDING DOWNRANGE
DO DOWNRANGE LANDINGS MEAN THINNER OR THICKER AIR? (Published 2/3/2010, updated 1/22/2015)
By David A. Roffman
Embry-Riddle Aeronautical University
My Mars Report centered on using dust devils on Mars as primary supporting evidence that Mars has a higher than advertised air pressure. I described problems with lander design and with the pressure sensitivity of Tavis and Vaisala pressure transducers that were sent to Mars.
The paper was well enough received at Embry-Riddle Aeronautical University (ERAU) for me to be invited to take a special topics course this semester to further explore my findings. In it, the professor has asked me to look for further, non dust devil evidence, to support my claim. This page will begin the search for such evidence by looking at a (NASA) paper published that challenges NASA's own assumptions about air pressure, although it goes in the opposite direction of what I think is true (however, only at high altitude). The article is entitled ALL RECENT MARS LANDERS HAVE LANDED DOWNRANGE - ARE MARS ATMOSPHERE MODELS MIS-PREDICTING DENSITY? It is by Prasun. N. Desai, NASA Research Center, MS 482, Hampton, VA 23681.
I sent a request to Dr. Desai on January 24, 2010, for details on Entry, Descent and Landing (EDL) required to further analyze his assertions. Hopefully, this page will reflect his input as it is received.
Briefly, Mars Pathfinder landed downrange 27 km, so did Spirit by 13.4 km, Opportunity 14.9 km, and Phoenix 21 km. If Desai is right about a lower density profile, we would have even less reason to understand how dust devils can develop on Mars. However, the answer might lie in the atmospheric pressure level profile. Perhaps there is less air at altitude (10 to 50 km), and higher pressure down low, under 10 km. Or, and here is where more info is needed, we might see a limited skip effect. If a spacecraft comes in too shallow, it could skip off the atmosphere and go back into space rather than land. But, if the angle is not too steep, the increased buoyancy felt from below might make it take a smaller skip, not causing it not to return to space, but resulting in it landing long. Desai seems to assume what I did before reading his article and later considering the buoyancy is issue - if the air is denser than expected, the friction will cause the probe to slow faster than expected, and land short of its target (not long, as was seen in Pathfinder, Spirit, Opportunity, and Phoenix).
Whatever the truth is, it seems clear that while NASA had the smarts to put Apollo 12 down next to Surveyor in the Moon (admittedly much closer than Mars), NASA has not had much success hitting the bulls-eye on Mars.
The pressure graphs in the Desai article are reproduced below with a mislabeled Figure 5 on his report corrected to read Figure 8 in accordance with his text). They show data beginning at 100 km (Spirit, which missed the mark by 13.4 km), 80 km (Opportunity, which missed the mark by 14.9 km) and 70 km (Phoenix, which missed the mark by 21 km). Missing in the Desai article was a graph for Pathfinder, which missed the mark by a full 27 km. That's almost 17 miles, no small quantity.
Figures 1, 2, 4, 5, 7, and 8 are taken from Prasun Desai's article, ALL RECENT MARS LANDERS HAVE LANDED DOWNRANGE - ARE MARS ATMOSPHERE MODELS MIS-PREDICTING DENSITY?
Desai is part of team that is responsible for entry and landing. He is clearly baffled by what his team has been seeing with results. To get a feel for this, look at the area where he claims that most deceleration occurs - 10 to 50 km high. Now look at the errors in landing sites - 13.4 to 27 km! These errors are huge! Imagine that a pilot on your flight begins a descent 10 to 50 km from an airport, but misses the runway by 13.4 to 27 km! Or consider a Space Shuttle that begins its descent 80 miles (without engines) from landing 5,000 miles out (8000 km) but it misses the runway by 13.4 to 27 km. Neither situation would be remotely acceptable.
Desai concludes that, "Although, the lower densities experienced by these recent missions were within the dispersions expected, does the fact that every one of these entries encountered a lower atmospheric density profile than predicted indicate a random occurrence or is there a systemic bias in current Mars atmospheric models? As such, a question is posed to the atmospheric community to consider if the Mars modeling assumptions are appropriate or is there underlying modeling issues that need to be reexamined or reevaluated. Additionally, although, the entire density profile is necessary for entry, descent, and landing design, nearly all deceleration during entry occurs between10-50 km. As such, prediction of density within this altitude band is most critical for entry flight dynamics and design.”
Desai's request for help with Mars modeling assumptions is well founded. The answers may introduce refute my beliefs about Martian atmospheric pressure, though I doubt it. Right answers are required to increase lander accuracy and to decrease the failure rate. A quick survey of attempted landings on Mars by non-U.S. nations shows no real successes. Of the probes that entered the Martian atmosphere: Soviet Mars 2 (November 27, 1971) crashed on surface; Mars 3 (December 2, 1971) landed softly (unknown if long or short) and ceased transmission within 15 seconds; Mars 6 (March 12, 1974) sent data during descent, but not after "landing" on Mars. British Beagle 2 lost contact (December 25, 2003) after separation from Mars Express - fate unknown. And, of course, the U.S. Mars Polar Lander crashed (December 3, 1999), supposedly due to improper hardware testing. The U.S. Mars Climate Orbiter supposedly crashed (September 23, 1999) due to metric-imperial unit mix-up. Update of January 22, 2015: Over 1 years after Beagle 2 was lost, it was found. There is, however, a question about how the original target coordinates were altered since 2003. This issue is discussed on this site here.
Desai informs us that for successful landers, the navigation errors upon Mars arrival were very small and that, as such, entry interface conditions (initial targeting on entry) was not responsible for downrange landings. But the moment aerodynamic issues are introduced for entry into an alien atmosphere, there are many places for errors to occur. Density is one such area, but again, not the only. In a 2004 textbook by Read and Lewis entitled THE MARTIAN CLIMATE REVISITED, Chapter 3, entitled Mars' Global-scale atmospheric structure, begins with a question "What determines the overall structure of the atmosphere, and what causes the atmosphere to move around?" The answer given by the Oxford University authors? "The simplest and most direct answer which a physicist would give to this last question is - because of the effects of buoyancy forces." These buoyancy forces undoubtedly combine with aerodynamic issues when it comes to getting a landing right. And, yes, increasing density of the fluid increases buoyancy forces, even before we consider parachute issues, which are not trivial.
SIZE OF PARACHUTES USED ON MARS
Obviously, a parachute could not be used to slow a rocket landing on the Moon. It is a vacuum there, and there would be no air for the parachute to catch. One would think that the low atmospheric pressures on Mars would require an enormous parachute to land there. The actual landers on Mars cut the parachutes loose before landing, going to retrorockets or air bags, but the parachutes used are actually not very large. In fact, the one used for Phoenix was reduced to 39 feet from the 42 feet used for Pathfinder. The Phoenix lander was 770 pounds on Earth (350 kg). As gravity on Mars is .38 of that in Earth, Phoenix would weigh about 293 pounds there. Any calculations about rate of fall should include an acceleration of about 3.7 meters/second2 rather than the 9.8 meters/second2 used on Earth. A photo was captured by the Mars Reconnaissance Orbiter's (MRO) HiRISE camera of the Phoenix lander with parachute deployed. The link just given only lists the parachute as being 30 feet wide.
Now, with respect to what would happen in terms of the parachute if the air was less dense or more dense than planned for by the NASA EDL team. If less dense, as Desai asserts, the parachute would be less effective, and the probe would come in faster. Impact would be at a higher speed, perhaps high enough to damage or destroy the probe. But if more dense than planned for, the parachute would work, but the probe would take longer to reach the surface, perhaps drifting for more time in the Martian winds. It would land long. That is what we see with Pathfinder, Spirit, Opportunity, and Phoenix. All of the data in this discussion of landing downrange and parachute considerations match what was discussed in my Report on Mars that focused on dust devils. Mars must have a denser atmosphere than advertised.
OTHER INDICATORS OF HIGHER PRESSURE
1. Excessive Deceleration by Mars Global Surveyor. In The Martian Climate Revisited, Atmosphere and Environment of a Desert Planet published by Peter L Read and Stephen R. Lewis in 2004, we are told of Mars Global Surveyor (MGS) launched in November 1996, that, “The spacecraft was intended eventually to reach 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 2h. To achieve this orbit at relatively low cost in fuel, MGS used the novel technique of aerobraking, in which the spacecraft was deliberately flown through the upper atmosphere of Mars during periapse to use the aerodynamic drag forces to modify its orbital parameters. In practice, this turned out to be less than straightforward, and the early maneuvers led to excessive decelerations.” While not discussed by the authors from Oxford, it is obvious that if Mars were to have a higher than expected atmospheric density, it would fully explain the unexpected excessive decelerations. As we will see in the graph and discussion below, the initial belief was that a dust storm produced the unexpected drag, but the effects at a normalized altitude of 121 km (75 miles) seem ludicrously high for a planet that is supposed have an average surface pressure of only about 7.5 mbar.
In a 1998 report entitled Mars Global Surveyor Aerobraking at Mars by M. D. Johnston, P. B. Esposito, V. Alwar, S. W. Demcak*, E. J. Graat, and R. A. Mase of the Jet Propulsion Laboratory, California Institute of Technology, we are told the following:
1. “On the onset of a dust storm, the atmospheric density could more than double in a 48 hour time period.”
2. “It should be noted that the dust storm threat to the MGS spacecraft is of particular concern since the aerobraking operations occur during the height of the dust storm season on Mars.”
3. “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.” Note: this happened.
4. “The 90% atmospheric density variation limit was derived from free stream heating rate requirements developed from the thermal limitations of the vehicle and were "mapped" into the dynamic pressure space to support the trajectory control process.”
5. “The free stream heating rate limits are not constant because they are dependent upon the amount of time the spacecraft spends in the atmosphere on a given orbit, called the drag pass duration, and other associated orbit geometry. Drag pass durations vary anywhere from 350 seconds in early main phase up to 1000 seconds during the walk-out.”
6. “November 23, 1997 (orbit number 50), the spacecraft encountered a dynamic pressure value of 0.32 N/m2 and atmospheric measurements indicated the presence of a large disturbance in the atmosphere. In response, the periapsis altitude of the spacecraft was raised to an altitude of 132 km. The resultant atmospheric disturbance evolved into a regional dust storm centered over the Noachis region of the planet (Latitude = 30 degrees South, Longitude = 20 degrees East). The signature of the storm is clearly evident when the dynamic pressure values are normalized to a common altitude. This signature is shown in Figure 15.”
Note the tremendous increase in dynamic pressure shown on the article’s Figure 15, reproduced below. At an altitude normalized to 121 km (about 75 miles), the above cited 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 first point just cited from the M.D. Johnson et al. 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. This is an enormous jump in pressure at such a great altitude.