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One of the most significant obstacles for landing on Mars will continue to present problems for our heroic helicopter now that it is safely on the surface. The

One of the most significant obstacles for landing on Mars will continue to present problems for our heroic helicopter now that it is safely on the surface. The atmospheric pressure on the surface of Mars is only about 1% that of Earth. Does this also limit the use of parachutes on Mars, or require bigger canopies? I know that Perseverance had a parachute for part of the descent, but not the final landing sequence. IIRC one of the earlier probes was designed to bounce rather than gently floating down.

dogma1138 6y ago
Both, but the main limit of a parachute is that our probes don’t go into orbit first but rather crash on Mars. So you enter the atmosphere at higher than orbital speeds. From orbit a parachute might be doable despite the relatively thin atmosphere. Stationary probes can use retro rockets to land because you don’t care about their stationary mass that much. For rovers it’s more tricky so you have to either use airbags like the smaller rovers did or a propulsive landing like Percy/Curiosity. The sky crane was chosen as the method for those because they are too massive to do a cushioned crash landing and they won’t be likely able to move with the weight of the propulsive landing system so we have to ditch it. The sky crane is basically the worlds most expensive bungee jump. The rockets on the sky crane bring the rover to hover and then it drops to the ground on a set of arresting cables.
- grecy 6y ago
  Thanks for the extremely interesting info >our probes don’t go into orbit first but rather crash on Mars. I'm wondering why they don't go into orbit, and what extra complexities that introduces? In the future are we anticipating Starship to go into orbit, or will it also directly "crash" ?
  - mdorazio 6y ago
    Going into orbit first would require a significant amount of extra fuel to decelerate and circularize. Heading straight into the atmosphere means more mass can go into the rovers instead of into fuel. It's a trade-off.
- lmm 6y ago
  > Both, but the main limit of a parachute is that our probes don’t go into orbit first but rather crash on Mars. So you enter the atmosphere at higher than orbital speeds. From orbit a parachute might be doable despite the relatively thin atmosphere. > So you enter the atmosphere at higher than orbital speeds. From orbit a parachute might be doable despite the relatively thin atmosphere. Nonsense. We send orbiters to Mars all the time, often on the same missions as landers. We could absolutely brake to orbit first (which is almost free via aerobraking) before dropping landers, but it wouldn't make any difference to being able to use parachutes, the velocity difference is insignificant.
  - dogma1138 6y ago
    Getting into orbit for a lander means you need a much higher transit mass to include fuel and rockets capable of slowing you down. The Martian atmosphere isn’t thick enough for this kind of aerobraking especially if want to get there within a relatively short period during the transit window. Yes we do send orbiters and these have to be smaller and so are their landers due to you having to slow down to get into an orbit first. The vast majority of landers do not get into orbit.
    - lmm 6y ago
      > The Martian atmosphere isn’t thick enough for this kind of aerobraking especially if want to get there within a relatively short period during the transit window. Nonsense, once you've made your capture transit is over, you can take as long as you like aerobraking to circularise the orbit. Even if you don't aerobrake at all, delta-V from a Mars intercept trajectory to even a low Mars orbit is about 1.4 km/s. Delta-V from that low orbit to the surface is 3.8 km/s. So the idea that direct descent vs descent from orbit is what makes the difference between parachutes being usable or not is total bollocks.
NortySpock 6y ago
Mars atmosphere is sometimes characterized as "enough so that you have to deal with it, but not enough to finish the job [with parachutes]" I expect NASA just builds the biggest single parachute they think they can safely use, and then works downstream of that on a landing system. Spirit and Opportunity used heatshield+parachute+rockets + bouncy airbags. https://en.wikipedia.org/wiki/Mars_Exploration_Rover https://en.wikipedia.org/wiki/Mars_Exploration_Rover
Daniel_sk 6y ago
Yes it does. The parachutes are unable to slow down the probes to a soft landing - they would have to be extremely large, which would make it even more heavy for landing. The large parachute on Perseverance slowed down the entry module to about 200 mph (321 km/h) which is not enough. Perseverance has a weight of 2,260 lbs (1,025 kilograms). The Mars Exploration Rover (Spirit) used parachute + retrorockets + inflatable airbags, but it weights only 408 lb (185 kilograms).
bryananderson 6y ago
Yes indeed. Mars entry, descent, and landing (EDL) is hard. On Earth it’s easy: we can use the thick atmosphere to decelerate to a gentle drop into the ocean. On the Moon it’s easy: the minimal gravity makes powered descent feasible. On Mars it’s hard: too much gravity for an easy powered descent, too little atmosphere for an easy aerobraked descent. Larger spacecraft such as Curiosity and Perseverance (sky crane) and Spirit and Opportunity (bouncing air bags) have to get creative.
- ilamont 6y ago
  Thanks for your reply. I never thought a NASA engineer would answer me, but one of the things that blows me away about this community is truly knowledgeable people are participants in discussions and often weigh in.

dogma1138 6y ago

Both, but the main limit of a parachute is that our probes don’t go into orbit first but rather crash on Mars. So you enter the atmosphere at higher than orbital speeds. From orbit a parachute might be doable despite the relatively thin atmosphere. Stationary probes can use retro rockets to land because you don’t care about their stationary mass that much. For rovers it’s more tricky so you have to either use airbags like the smaller rovers did or a propulsive landing like Percy/Curiosity. The sky crane was chosen as the method for those because they are too massive to do a cushioned crash landing and they won’t be likely able to move with the weight of the propulsive landing system so we have to ditch it. The sky crane is basically the worlds most expensive bungee jump. The rockets on the sky crane bring the rover to hover and then it drops to the ground on a set of arresting cables.

grecy 6y ago
Thanks for the extremely interesting info >our probes don’t go into orbit first but rather crash on Mars. I'm wondering why they don't go into orbit, and what extra complexities that introduces? In the future are we anticipating Starship to go into orbit, or will it also directly "crash" ?
- mdorazio 6y ago
  Going into orbit first would require a significant amount of extra fuel to decelerate and circularize. Heading straight into the atmosphere means more mass can go into the rovers instead of into fuel. It's a trade-off.
lmm 6y ago
> Both, but the main limit of a parachute is that our probes don’t go into orbit first but rather crash on Mars. So you enter the atmosphere at higher than orbital speeds. From orbit a parachute might be doable despite the relatively thin atmosphere. > So you enter the atmosphere at higher than orbital speeds. From orbit a parachute might be doable despite the relatively thin atmosphere. Nonsense. We send orbiters to Mars all the time, often on the same missions as landers. We could absolutely brake to orbit first (which is almost free via aerobraking) before dropping landers, but it wouldn't make any difference to being able to use parachutes, the velocity difference is insignificant.
- dogma1138 6y ago
  Getting into orbit for a lander means you need a much higher transit mass to include fuel and rockets capable of slowing you down. The Martian atmosphere isn’t thick enough for this kind of aerobraking especially if want to get there within a relatively short period during the transit window. Yes we do send orbiters and these have to be smaller and so are their landers due to you having to slow down to get into an orbit first. The vast majority of landers do not get into orbit.
  - lmm 6y ago
    > The Martian atmosphere isn’t thick enough for this kind of aerobraking especially if want to get there within a relatively short period during the transit window. Nonsense, once you've made your capture transit is over, you can take as long as you like aerobraking to circularise the orbit. Even if you don't aerobrake at all, delta-V from a Mars intercept trajectory to even a low Mars orbit is about 1.4 km/s. Delta-V from that low orbit to the surface is 3.8 km/s. So the idea that direct descent vs descent from orbit is what makes the difference between parachutes being usable or not is total bollocks.

NortySpock 6y ago

Mars atmosphere is sometimes characterized as "enough so that you have to deal with it, but not enough to finish the job [with parachutes]" I expect NASA just builds the biggest single parachute they think they can safely use, and then works downstream of that on a landing system. Spirit and Opportunity used heatshield+parachute+rockets + bouncy airbags. https://en.wikipedia.org/wiki/Mars_Exploration_Rover https://en.wikipedia.org/wiki/Mars_Exploration_Rover

Daniel_sk 6y ago

Yes it does. The parachutes are unable to slow down the probes to a soft landing - they would have to be extremely large, which would make it even more heavy for landing. The large parachute on Perseverance slowed down the entry module to about 200 mph (321 km/h) which is not enough. Perseverance has a weight of 2,260 lbs (1,025 kilograms). The Mars Exploration Rover (Spirit) used parachute + retrorockets + inflatable airbags, but it weights only 408 lb (185 kilograms).

bryananderson 6y ago

Yes indeed. Mars entry, descent, and landing (EDL) is hard. On Earth it’s easy: we can use the thick atmosphere to decelerate to a gentle drop into the ocean. On the Moon it’s easy: the minimal gravity makes powered descent feasible. On Mars it’s hard: too much gravity for an easy powered descent, too little atmosphere for an easy aerobraked descent. Larger spacecraft such as Curiosity and Perseverance (sky crane) and Spirit and Opportunity (bouncing air bags) have to get creative.

ilamont 6y ago
Thanks for your reply. I never thought a NASA engineer would answer me, but one of the things that blows me away about this community is truly knowledgeable people are participants in discussions and often weigh in.