Space Exploration Stack Exchange is a question and answer site for spacecraft operators, scientists, engineers, and enthusiasts. It only takes a minute to sign up.
Sign up to join this community
I was wondering why the (relatively) recent Perseverance (USA), Chandrayaan-2,3 (India) and Luna-25 (Russia) don't leverage the advantage of the cushion based approach that Spirit, Opportunity, and Sojourner leveraged?
Is it not the case that they improve the chances of a successful landing in case of any miscalculation or malfunction or some local environmental factors (particularly Mars)?
References :
According to NASA Facts; Mars Exploration Rover:
With the heat-shield portion of the aeroshell pointed forward, the spacecraft slammed into the atmosphere at about 5.4 kilometers per second (12,000 miles per hour). Atmospheric friction in the next four minutes cut that speed by 90 percent, then a parachute fastened to the backshell portion of the aeroshell opened about two minutes before landing. About 20 seconds later, the spacecraft jettisoned the heat shield. The lander descended on a bridle that unspooled from the backshell. A downward-pointing camera on the lander took three pictures during the final half-minute of the flight. An onboard computer instantly analyzed the pictures to estimate horizontal motion. In the final eight seconds before impact, gas generators inflated the lander’s airbags, retro rockets on the backshell fired to halt descent speed, and transverse rockets fired (on Spirit’s lander) to reduce horizontal speed. The bridle was cut to release the lander from the backshell and parachute. Then the airbagencased lander dropped in free fall.
@Hobbes' answer mentiones that the masses of Curiosity, Perseverance, Luna 25 and Chandrayaan-2 are in the 900 to 600 kg range. And while the answer mentions the lander's masses at about 530 kg, Wikipedia's Spirit (rover) cites the lander mass even lower as 348 kg.
So as @Hobbes points out, these are much heavier. The linked source is a popular science page (rather than a scientific source with data, measurements, calculations, etc.) so "too heavy" is probably short for a much more in-depth analysis.
Assuming they were to hit the surface at the same speed, there'd be three times the kinetic energy to dissipate in the several bounces. That means tougher material, tougher stitching, and a lot of recalculation and optimization.
But I don't think that it can't be done. Instead it's much more likely that propulsive landing is simply safer i.e. more reliable and gentler and much more accurate.
The block quote continues:
Spirit landed on Jan. 4, Universal Time (at 8:35 p.m. Jan. 3, Pacific Standard Time). It bounced about 8.4 meters (27.6 feet) high. After 27 more bounces and then rolling, it came to a stop about 250 to 300 meters (270 to 330 yards) from its first impact. Spirit had journeyed 487 million kilometers (303 million miles). JPL navigators and engineers successfully put it only about 10 kilometers (6 miles) from the center of its target area...
and later
Opportunity landed on Jan. 25, Universal Time (at 9:05 p.m. Jan. 24, Pacific Standard Time). It traveled about 200 meters (220 yards) while bouncing 26 times and rolling after the impact, with a 90-degree turn northward during that period. It came to rest inside a small crater. One scientist called the landing an “interplanetary hole in one.” Opportunity had flown 456 million kilometers (283 million miles) from Earth and landed only about 25 kilometers (16 miles) from the center of the target area.
Curiosity landed within about 2.4 km of its target, and Wikipedia's Perseverance (rover); Landing says:
Having come within sixteen feet (~5 meters) of its target, the landing was more accurate than any previous Mars landing; a feat enabled by the experience gained from Curiosity's landing and the use of new steering technology.
Propulsive landing (on both Earth and Mars) is a proven technology to land within meters of target. It uses live cameras and live image processing to do what Neil Armostrong did on the first Moon landing, continuously evaluate the landscape, avoid the boulders, keep a keen eye on remaining fuel, update "acceptable site" criteria based on that, find a suitale flat area and land on it gently.
The 2007 paper The challenges of landing on Mars gives a decent introduction to the subject, and the evolution of Mars landing methods.
The key challenges with a legged system revolve around the fact that in order to make the lander safe for landing in regions with large rocks the legs must either be made long or the belly of the lander must be made very strong. Neither solution is attractive, resulting in either a top heavy lander incapable of landing on sloped terrain, or a significant amount of belly reinforcement carried along for the remote chance of incurring a direct rock strike. A second major challenge of the legged landing architecture is that of safe engine cut-off.
Advantage: reduced cost (landing systems are simpler), improved landing robustness.
Disadvantage:
The Sojourner rover and the Spirit and Opportunity rovers all experienced their most difficult and threatening mobility condition as they made their way off of their landers.
The third generation landing system being deveIoped for the MSL mission is being designed to directly address all of the main challenges present in the first and second generation landing system while completely eliminating the challenge of egress.
The Skycrane Landing System (SLS) eliminates the use of a dedicated touchdown system by directly placing the rover onto the surface of Mars, wheels first.
At 900 kg, Curiosity and Perseverance are too large to use air bags. Luna 25 is in the same weight bracket, the Chandrayaan-2 lander is about 600 kg, which is slightly more than the ~530 kg of the MER landers.
Airbags were considered for MSL:
As you can see, that's a lot of air bags.
another source that confirms air bags were not viable for MSL:
But the Mars Science Lab (MSL) team quickly determined that airbags would not be a viable solution for something as big as Curiosity. An airbag system designed to accommodate the size and mass of Curiosity would be very large and heavy and significantly different from the Pathfinder and MER designs. In addition, egress from the top of the deflated airbags and lander platform is a complex and tricky maneuver; it would be even more complex with an airbag design large enough for Curiosity.
Edit: added more information on the tradeoffs, and my initial claim about airbags being more complex was incorrect.
The airbag system was used for Pathfinder and MER because it was projected to be less expensive than a traditional propulsive landing:
They basically looked at the two options and said, Well, propulsion...that's the old way of doing business. You guys will never get this job done if you do it that way. It's too expensive."
The landing software can be simpler with an airbag system:
The Mars Pathfinder airbag system was designed to protect the lander regardless of its orientation upon impact with the surface of the planet. The system also was designed to handle lateral movement as well as vertical descent.
The Pathfinder and MSR missions had no way to scan the terrain and avoid problems. All they had was an altimeter that could fire the retrorockets and separate the lander from the retrorocket package.
Past Mars Mission utilized radio-based approach navigation, no control of the lift vector during entry, a single stage parachute, and hazard tolerance only to the extent that either airbags or landing struts are utilized. There was no hazard detection/avoidance capability.
The landing rockets can be simple solid rockets, without much accuracy. No vernier thrusters are needed.
The brief firing of the solid rocket motors at an altitude of 80-100 meters is intended to essentially bring the downward movement of the lander to a halt some 12 meters (±10 m) above the surface. The bridle separating the lander and heatshield is then cut in the lander, resulting in the backshell driving up and into the parachute under the residual impulse of the rockets, while the lander, encased in airbags, falls to the surface.
Because it is possible that the backshell could be at a small angle at the moment that the rockets fire, the rocket impulse may impart a large lateral velocity to the lander/airbag combination. In fact the impact could be as high as 25 m/sec (56 mph) at a 30 deg grazing angle with the terrain.
The lander was designed to right itself after a landing in any orientation (i.e. you don't have to care about that during the landing, but it requires extra hardware to rectify the orientation after landing.
The choice of landing system is a matter of tradeoffs:
airbags have disadvantages. The bags and their supporting hardware are themselves heavy, which reduces the amount of scientific equipment a lander can carry. And for precision landings, legs generally do a better job. Spirit bounced 28 times before finally settling to a stop 300 yards from its point of first impact.
On the other hand, airbags aren’t bothered by large rocks that might tip a legged lander. On the other other hand, spherical airbags can roll into a deep crater from which a rover would never escape.
Spirit and Opportunity hit the ground hard enough that they experienced a deceleration of 19g. Curiosity (and propulsive landings in general) made a much softer landing.
For the next generation, they wanted to make a more precise landing:
Scientists also expect to set it down with pinpoint precision to maximize their chances of finding interesting geology.
