Why Planetary Landing Plume Effects Are Important

Rocket exhaust effects are different on the Moon vs. asteroids vs. Mars. Below is a brief description of what can happen on each.

Plume Effects on the Moon

On the Moon, the rocket exhaust blows soil and dust in a horizontal sheet along the surface away from the lander. This sheet can be so opaque that it blocks the view of boulders or craters, creating a hazardous condition for landing. For example, during the crew debrief from Apollo 12, commander Pete Conrad reported,

…at 300 feet, we picked up a tremendous amount of dust; much more so than I expected…the dust went as far as I could see in any direction and completely obliterated craters and anything else. All I knew was there was ground underneath that dust…I couldn’t telll what was underneath me…I was just going to have to bite the bullet and land, because I couldn’t tell whether there was a crater down there or not.

The view looking down in Apollo 15 at three different times during landing.

In Apollo 15, the crew did not see that they were descending onto the rim of a broad crater. After shutting off the engine, the Lunar Module rocked backwards about 11 degrees into the crater until the rear footpad contacted the ground. The front footpad was then off the ground, bearing no weight. This 11 degree tilt was near the design limits for the Lunar Module. This illustrates the dangers of not being able to see what you are landing on.

The Apollo 15 Lunar Module tilted backward into a crater that was not visible during landing due to the thick sheet of blowing regolith.

The spraying sand and dust will also sandblast any surrounding hardware. This is especially bad for a lunar outpost, because repeatedly landing nearby will cumulatively degrade all the sandblasted surfaces. An example was in Apollo 12, when the Lunar Module landed 16o m from the Surveyor 3 spacecraft, which had sat on the Moon for 2 1/2 years. The Apollo 12 mission was visiting it to cut pieces off and return them to Earth for analysis, to see what 2 1/2 years exposure to the lunar environment did to the various materials. When the hardware was examined on Earth, it was found to have permanent shadows etched into its surfaces from the sandblasting. Members of our team analyzed the materials and found lunar soil particles embedded in the cracked and damaged surfaces.

Piece of Surveyor 3, cut from the spacecraft by Apollo 12 astronauts and returned to Earth. It shows evidence of the sandblasting by the Apollo 12 Lunar Module landing.

Scanning Electron Microscope image of a piece of Surveyor 3, comparing an area that was not sandblasted with an area that was sandblasted.

Other experiments sandblasted various materials with the amount of lunar soil simulant that would produce the same amount of damage as a single Lunar Module landing from 200 m away in the direct aim of the spraying sheet of soil. An example of the sandblasted glass is shown below.

Glass that was sandblasted with the amount of lunar soil simulant that would produce the same damage as one Lunar Module landing 200 m away. Source: Luke Roberson (NASA/KSC), Ryan Clegg (FIT), Philip Metzger (NASA/KSC)

Even worse damage would have occurred if a rock had struck the Surveyor. In the Apollo landing videos, our team has identified and measured the size and velocity of rocks blown by the rocket exhaust, and these measurements confirmed the predictions of computer simulations.

Images of a rock blowing at high speed during an Apollo landing.

On the Moon, the plume does not generally dig a crater or deep scour hole. this is because there is no significant atmosphere on the Moon to focus the rocket exhaust onto a narrow patch. Instead, it is spread out with very gradual pressure gradients on the surface. The main effect is to sweep away the loose material. The plume also injects gas into the pore spaces between the sand grains, causing chemical contamination that could disturb science measurements. It also causes a change in the surface texture of the lunar soil over a wide area about 100 m diameter (order of magnitude). The exact mechanism that causes this texture change over that size area is not yet known.

Loose dusty regolith has been swept away beneath and around the Apollo 11 Lunar Module descent engine nozzle.

Plume Effects on Asteroids

For asteroids we have very little data how a rocket thruster will disrupt the surface. We know that asteroids are covered with regolith (rocks, gravel, sand and dust), and that collisions between asteroids knock dust clouds into space. These dust clouds get swept by the sun’s radiation pressure to form a long tail of dust pointing away from the asteroid.

A view of regolith on asteroid Itokawa (Credit: U. of Tokyo, JAXA)

Asteroids have very little gravity. Often they are held together more by cohesion than by gravity. Our work suggests the following processes will probably occur:

A rocket thruster’s high speed gas can scour material completely off the surface into space, but it will hang around the asteroid creating a risk for spacecraft operating near the asteroid.
If the spacecraft is close enough to the asteroid, the thruster can inject significant amounts of gas into the pore spaces between the grains of regolith. This “bulb” of subsurface gas pressure will then blow material off the asteroid back toward the spacecraft.

Plume Effects on Mars

For large, human-class missions on Mars, all the research concurs: there will be very deep, violent cratering of the regolith that can pose serious hazards to the life of the crew. This is very different than what happened during the lunar landings. It is also very different than what happened on the smaller, robotic landings on Mars. The reason it is so different? (1) Mars has larger gravity and requires larger landers for the long human-class missions, compared to the Moon. (2) Mars has an atmosphere that focuses the rocket exhaust into a jet, which creates sharp pressure gradients on the soil causing it to fail by shearing as well as by driving pressure into the subsurface, which we call Bearing Capacity Failure (BCF) and Diffusion-Driving Shearing (DDS), neither of which can happen on typical lunar soil. (3) Martian soil is much weaker and more porous than lunar soil. Lunar soil has been gardened by micrometeoroids and thermal cycled by direct sunlight for billions of years in vacuum. Martian soil has experienced geological sorting and has not endured the degree of micrometeoroid bombardment or thermal cycling because its atmosphere protects the surface. These three factors mean that human-class landings on Mars will be in a different regime of behavior than all prior experience. It will be a lot like landing a rocket on desert sand on Earth, blasting a deep and wide hole, and shooting rocks back up at the lander at high speed.

The Viking spacecraft landings on Mars minimized its disturbance of the soil by using a “showerhead” rocket nozzle that broke the plume into 18 tiny jets. This would cause the rocket exhaust to mix with the surrounding atmosphere better, causing the jets to be much shorter so they would not dig such deep holes in the soil. Still, the soil disturbance could be seen in the images around the lander. The jets probably dug holes in the soil that collapsed after the thrusters were shut off, so all that was seen after landing were the residual, collapse craters.

Viking Spacecraft. The yellow circle marks one of the showerhead rocket nozzles.

For the Mars Phoenix lander, the much larger-diameter rocket nozzles had greater potential to dig holes, but there was a solid sheet of ice jus a few centimeters beneath the top of the soil and that minimized the amount of disturbance. The rocket exhaust blew off this thin layer of soil and uncovered the ice. This lander used pulsing rocket exhaust, so the shock effects of the periodically diffusing gas fluidized the soil and it was easily swept away. This mechanism was dubbed “Diffused Gas Explosive Erosion” by its discoverers, Manish Mehta (NASA/MSFC, part of the CLASS Planetary Landing Team) and Anita Sengupta (NASA/JPL). The presence of the ice table shows the effectiveness of landing pads for planetary landings. The ice was effectively the landing pad covered by a thin layer of soil.

Ice can be seen where the soil was blown off by the Phoenix lander’s rocket exhaust.

For the Curiosity landing, a Sky Crane was used to lower the rover to the surface while keeping the rocket thruster nozzles higher above the surface to reduce plume effects. Still, simulations by Anita Sengupta showed that the supersonic cores of the jets from these nozzles would still reach all the way to the soil (they are much longer on Mars than in the thicker atmosphere of Earth), and they would blow holes in the soil that would redirect ejecta back toward the Curiosity rover. Indeed, after landing there was gravel found on the top of the rover, and one of the wind sensors was broken, believed to be the result of gravel striking it at high speed. Despite the obvious damage, it would have been much worse except for the fact that the regolith was shallow at this landing site, so the bedrock just below the surface minimized the depth of holes dug by the jets and minimized the amount of material blown back at the rover.

Four of the eight plume craters caused by thruster on the Skycrane while hovering to set Curiosity on the Martian surface. Source: Jeffrey Vizcaino and Manish Mehta, NASA/MSFC, published in AIAA 2015-1649)

For human-class landers (20 to 60 tons mass), the effects will be far worse and will pose a hazard to the spacecraft. This will be explained on the next page about the research our team members have accomplished for Mars plume effects.

Next: The Science of Plume Effects >>
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