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Soft landing
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A soft landing is any type of aircraft, rocket or spacecraft landing that does not result in significant damage to or destruction of the vehicle or its payload, as opposed to a hard landing. The average vertical speed of a soft landing (on Earth) should be about 2 meters (6.6 ft) per second or less.[1] On other astronomical bodies with weaker gravity, the safe speed could potentially be higher.
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A soft landing can be achieved by
- Parachute—often this is into water.
- Vertical rocket power using retrorockets, often referred to as VTVL (vertical landing referred to as VTOL, is usually for aircraft landing in a level attitude, rather than rockets) — first achieved on a suborbital trajectory by Bell Rocket Belt and on an orbital trajectory by the Surveyor 1.
- Horizontal landing, most aircraft and some spacecraft, such as the Space Shuttle, land this way accompanied with a parachute.
- Being caught in midair, as done with Corona spy satellites and followed by some other form of landing.
- Reducing landing speed by impact with the body's surface, known as lithobraking.
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See also
[edit]References
[edit]- ^ Sreedhar, Vidya (2023-08-23). "Chandrayaan-3 Effect! These 7 space-related stocks scale 52-week highs". The Economic Times. ISSN 0013-0389. Retrieved 2023-08-27.
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Soft landing
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Definition and Principles
Core Definition
A soft landing refers to a controlled descent and touchdown of aircraft, rockets, or spacecraft onto a planetary surface or other landing zone, where the vertical velocity is typically limited to below 5-6 m/s for Earth land-based systems using parachutes or decelerators, and around 2-3 m/s for powered vertical landings on airless bodies like the Moon, with adjustments for local gravity to prevent structural damage to the vehicle or destruction of the payload.[9][10] This process ensures the vehicle's integrity and enables mission success, often allowing for reusability in subsequent operations. In contrast to a crash or hard landing, which involves excessive impact velocity exceeding safe thresholds—typically resulting in g-forces above 5g and potential vehicle loss—a soft landing maintains peak accelerations below this limit, prioritizing controlled energy dissipation through deceleration systems.[9] The fundamental physics governing a soft landing revolve around balancing gravitational acceleration with opposing forces to manage descent speed. Gravity imparts a downward force that accelerates the vehicle toward the surface, while an atmosphere, if present, generates drag that can limit the descent to a terminal velocity determined by the vehicle's mass, shape, and atmospheric density.[11] Without sufficient deceleration, terminal velocity would exceed safe limits, leading to destructive impacts; thus, soft landings require engineered interventions to reduce this velocity to survivable levels, adapting to environments with or without atmospheres.[9] Soft landings occur across varying scales of mission profiles. In suborbital flights, such as those using vertical takeoff and landing vehicles, the focus is on brief powered descents to achieve low-impact touchdowns. Orbital missions involve spacecraft returning from higher altitudes, often requiring precise guidance to counteract orbital velocities during reentry and final approach. Atmospheric reentry landings, common for Earth-return vehicles, combine hypersonic deceleration with terminal phase control to transition from high-speed entry to gentle surface contact.[9]Key Physical Requirements
A soft landing in planetary missions demands precise control of impact dynamics to prevent damage to the spacecraft and its payloads. The primary measurable criterion is the vertical touchdown velocity, typically targeted below 3 m/s, with design specifications allowing up to approximately 3 m/s to qualify as soft depending on the mission and vehicle, ensuring minimal structural stress and serving as a standard in lunar and other airless body descent designs. On Earth, where atmospheric deceleration aids the process, vertical velocity limits vary by system but are often below 5 m/s for land-based systems to achieve controlled contact. Horizontal touchdown velocity is also constrained, typically to below 1-2 m/s, to ensure stability and prevent lateral sliding or tipping on uneven surfaces.[12] For airless bodies like the Moon, deceleration during the landing gear's compression phase is constrained to ≤10 m/s², limiting peak g-forces to approximately 3–6 times local gravity (about 4.9–9.7 m/s² on the lunar surface) to safeguard crew or instrumentation. These requirements derive from fundamental kinematic principles. The stopping distance required for energy absorption is given by the equation
where is the touchdown velocity (e.g., 2 m/s) and is the maximum allowable deceleration (e.g., 10 m/s²), yielding m—a critical parameter for landing gear stroke length. The kinetic energy to be dissipated, , must be managed through propulsive thrust, drag forces (in atmospheres), or mechanical damping, with kept low to reduce proportionally.
Several factors influence adherence to these thresholds. Surface composition, such as the compressibility of lunar regolith (which can sink up to 10–30 cm under load), affects energy dissipation and stability but introduces risks like footpad burial. Payload sensitivity is paramount; delicate scientific instruments demand shock attenuation below 1g (≈9.8 m/s² on Earth or 1.6 m/s² on the Moon) to avoid misalignment or failure. Environmental variables, including gravitational field perturbations (e.g., mascons on the Moon causing ±0.01 m/s² variations) and atmospheric density (which modulates drag on Earth or Mars), necessitate adaptive control to maintain velocity profiles.
Real-time verification relies on onboard sensors: accelerometers measure instantaneous deceleration and g-forces, while radar altimeters provide velocity and altitude data for closed-loop guidance, enabling adjustments within the final descent phase. Early mission attempts in the 1960s, including several Soviet Luna probes, exceeded these velocity limits due to propulsion anomalies, resulting in hard impacts and mission losses.
Historical Development
Early Concepts and Attempts (1950s–1960s)
The development of soft landing technology in the 1950s and 1960s was deeply rooted in post-World War II rocketry advancements, particularly the German V-2 missile program. Captured V-2 rockets and personnel, including Wernher von Braun, provided the foundational liquid-fueled propulsion and guidance systems that influenced early U.S. and Soviet efforts. In the United States, V-2 derivatives like the Redstone missile enabled initial suborbital tests, while in the Soviet Union, direct copies such as the R-1 rocket laid the groundwork for more ambitious lunar probes. These technologies shifted focus from ballistic trajectories to controlled descents, though early concepts emphasized retro-rockets for velocity reduction in vacuum environments.[13] In the early 1950s, von Braun, now working for the U.S. Army, outlined visionary proposals for lunar exploration in Collier's magazine articles, advocating multi-stage rockets and winged landers equipped with retro-rockets to enable soft touchdowns on the Moon. These ideas, part of a broader spaceflight blueprint published between 1952 and 1954, envisioned a 50-person expedition using rocket braking for landing and ascent, highlighting the need for precise thrust control to counter gravitational forces without atmospheric drag. A key milestone in testing recovery concepts came in 1958, when the U.S. Army's Jupiter-C rocket conducted suborbital flights to evaluate ablative reentry vehicles, successfully deploying parachutes for post-descent recovery of nose cones after reaching altitudes of up to 600 miles. These tests validated deceleration systems essential for future soft landings, influencing NASA's Project Mercury.[14][15] The Soviet Union pursued aggressive lunar objectives through its Luna program, starting with hard-impact attempts that underscored the challenges of transition to soft landings. Luna 2, launched on September 12, 1959, achieved the first human-made impact on another celestial body, crashing into the Moon's Mare Imbrium region at over 6,000 mph after a 34-hour journey, confirming the absence of a lunar magnetic field. Earlier efforts, including Luna 1 in January 1959, missed the Moon by about 4,000 miles due to upper-stage burn anomalies, while three 1958 launches failed outright from R-7 rocket issues. Precursors to the first soft landing, such as Luna 4 through Luna 8 (1963–1965), suffered repeated setbacks from engine malfunctions; for instance, Luna 5 crashed after an early retrorocket shutdown, Luna 7 exploded due to improper thrust timing, and Luna 8 impacted hard following a premature engine cutoff during descent. These failures highlighted propulsion reliability issues in vacuum conditions.[16] Parallel U.S. attempts via the Ranger program encountered similar hurdles, with navigation and guidance errors plaguing early Block II missions. Ranger 3, launched January 26, 1962, missed the Moon entirely due to trajectory inaccuracies from launch vehicle performance, passing approximately 23,000 miles (37,000 km) away.[17] Ranger 4 reached the lunar far side on April 23, 1962, but failed to deploy solar panels, rendering it inert before impact. Ranger 5, on October 18, 1962, suffered power loss and missed the Moon by 460 miles, while Ranger 6 impacted successfully on February 2, 1964, but transmitted no images due to a camera malfunction. These crashes, attributed to rudimentary onboard computing incapable of real-time corrections, prompted a shift to the Surveyor program in 1961 for dedicated soft-landing tests.[18] Throughout the era, key challenges included the absence of advanced computers for autonomous guidance, forcing reliance on ground-based tracking prone to errors, and an overall high failure rate exceeding 70% for early lunar probes—such as seven out of eight U.S. attempts from 1958 to 1960 ending in total loss. These setbacks, driven by propulsion unreliability and imprecise velocity control, underscored the experimental nature of soft landing development during the Space Race.[19]Pioneering Successes (1960s–1970s)
The Soviet Union's Luna 9 mission achieved the first successful unmanned soft landing on the Moon on February 3, 1966, in Oceanus Procellarum, utilizing solid-fuel retro-rockets that fired to reduce descent velocity to approximately 22 km/h before shutdown at about 5 meters altitude, followed by an airbag system for impact cushioning that allowed the spherical lander to bounce several times before stabilizing.[20][4] After four petals deployed to upright the craft, it transmitted the first panoramic photographs from the lunar surface over three days (February 3–6, 1966), confirming the viability of soft landing technology for future missions.[21] Later that year, Luna 13 landed successfully on December 24, 1966, and conducted soil mechanics experiments using a groundmeter-penetrometer device that measured penetration depths up to 5 cm, revealing a weakly coherent, light granular layer with a volumetric weight of about 0.77 g/cm³ and porosity indicative of porous sand or clay-like material at least 5 cm thick.[22][4] In response, NASA's Surveyor program demonstrated American soft landing capabilities, with Surveyor 1 achieving the first U.S. success on June 2, 1966, in the Ocean of Storms, where retro-rockets and vernier thrusters slowed descent, resulting in an impact velocity of approximately 7 mph (3.5 m/s) after free-falling the final about 14 feet (4.3 m), allowing the three-legged craft to transmit over 11,000 images of the firm, cohesive lunar surface that resolved details down to 1 mm.[2][23][24] Of the program's seven missions launched between 1966 and 1968, five succeeded in soft landings (Surveyor 1, 3, 5, 6, and 7), providing extensive imaging and surface data across mare and highland sites.[25] Notably, Surveyor 3, which landed on April 20, 1967, equipped with a television camera for surface imaging, had components including that camera retrieved by Apollo 12 astronauts in November 1969 for analysis of lunar environmental effects, such as microbial survival and material degradation after 2.5 years exposure.[26][27] These unmanned successes paved the way for manned landings, culminating in the Apollo 11 mission's historic soft touchdown on July 20, 1969, in the Sea of Tranquility, where the Eagle lunar module's descent propulsion system—powered by hypergolic propellants Aerozine 50 fuel and nitrogen tetroxide oxidizer, throttled from 10,000 to 3,000 pounds of thrust—encountered a guidance issue prompting Neil Armstrong to manually override the automatic system at about 500 feet altitude, piloting the craft to a safer site four miles from the planned location.[28][29] Subsequent Apollo missions 12 through 17 (excluding the aborted Apollo 13) completed soft landings between 1969 and 1972, totaling six manned successes that gathered 382 kg of samples and conducted extensive surface operations.[30] The pioneering efforts verified key lunar soil properties, such as cohesion ranging from 0.007 to 0.17 N/cm², bearing strength up to 5.5 N/cm² at shallow depths, and particle sizes predominantly under 1 mm with high porosity near the surface, confirming the regolith's stability for lander footpads and enabling safe manned exploration without sinking into deep dust.[25] These data directly informed Apollo landing gear design and site selection, while insights into descent propulsion, including reliable hypergolic ignition without complex systems, ensured throttlable control for precise touchdowns across varied terrains.[25][29]Landing Techniques
Retropropulsion Methods
Retropropulsion methods involve the use of rocket engines or thrusters fired in the direction opposite to the spacecraft's velocity to decelerate and counteract gravitational forces during the terminal phase of descent, enabling a controlled soft landing.[31] These techniques encompass various engine types, including single-stage solid-fuel retro-rockets that provide a fixed-thrust impulse for initial braking, as demonstrated in the Surveyor program's vernier and retromotor system.[32] Throttleable liquid-propellant engines allow variable thrust levels for precise velocity adjustments, such as the Apollo Lunar Module's Descent Propulsion System, which used hypergolic propellants to modulate thrust from 10% to 100% during lunar approach.[33] In modern applications, aerodynamic control surfaces like grid fins complement retropropulsion by enabling steering during atmospheric reentry and descent, as seen on SpaceX's Falcon Heavy boosters where they orient the vehicle for targeted landings.[34] The underlying physics requires the engine thrust $ F $ to satisfy $ F = m(a + g) $, where $ m $ is the spacecraft mass, $ a $ is the desired deceleration (positive upward), and $ g $ is local gravity, ensuring net force opposes descent.[35] For a stationary hover ($ a = 0 $), the thrust-to-weight ratio $ T/W = F/(mg) $ must equal 1; ratios greater than 1 allow for ascent or powered maneuvers beyond mere hovering.[35] Representative examples include the Soviet Luna 9 mission's main braking engine, which fired retropropulsively starting at high altitude to significantly reduce descent velocity before shutting off at low altitude, with small solid-propellant contact motors firing just before touchdown to complete the landing sequence.[36] Key advantages of retropropulsion include high precision in vacuum environments where aerodynamic aids are unavailable, allowing fine control over descent trajectory and touchdown velocity.[31] Additionally, these methods facilitate reusability by enabling powered recoveries that minimize structural wear compared to ablative or impact-based alternatives.[37]Aerodynamic and Passive Systems
Aerodynamic and passive systems for soft landings rely on atmospheric drag and mechanical energy absorption to decelerate spacecraft without sustained propulsion, making them ideal for bodies with sufficient atmospheres like Mars or Earth. These methods exploit the drag generated by parachutes or other decelerators to reduce velocity from hypersonic entry speeds to terminal descent rates, followed by impact attenuation structures that dissipate remaining kinetic energy through deformation or inflation. Such approaches minimize fuel requirements and complexity compared to fully propulsive systems, though they are constrained by atmospheric density. Parachutes serve as the primary aerodynamic decelerators in these systems, deploying sequentially to manage descent phases. A smaller drogue parachute typically deploys first to stabilize and slow the entry vehicle from supersonic speeds, followed by a larger main parachute that further reduces velocity to near-terminal rates suitable for final landing. For instance, the Viking Mars landers employed a 16.15-meter diameter disk-gap-band main parachute, mortar-deployed at approximately 6 km altitude and 250 m/s, which slowed the descent to enable subsequent touchdown.[38] The physics of parachute drag is governed by the aerodynamic drag force equation:
where is atmospheric density, is velocity, is the drag coefficient, and is the parachute area; this force balances gravitational weight at terminal velocity , where is mass and is gravitational acceleration.[39] These parameters ensure controlled deceleration, with for disk-gap-band designs typically around 0.6–0.8 in Martian conditions.
Passive impact attenuation complements aerodynamic braking by absorbing residual energy upon surface contact. Crushable structures, such as aluminum honeycomb elements in landing legs, deform irreversibly to convert kinetic energy into plastic deformation. The Viking landers utilized inverted-tripod legs with stacked honeycomb cartridges that limited peak accelerations to approximately 14 g during touchdown at a vertical velocity of about 2.5 m/s.[40][41] Similarly, inflatable airbags provide cushioning through gas compression and rebound, allowing the lander to bounce and roll to a stop on uneven terrain. The Mars Pathfinder mission deployed airbags that attenuated impacts from 14 m/s velocities, constraining decelerations to under 40 g.[42][43]
Hybrid configurations integrate heat shields for initial aerobraking with parachutes and minimal retro-rockets for final velocity reduction before passive absorption. In the Mars Pathfinder entry, descent, and landing sequence, a heat shield protected against peak heating, followed by parachute deployment and brief retro-rocket firing to 14 m/s, after which airbags handled the impact.[44] These systems prioritize drag and mechanical dissipation over active thrust.
Limitations of aerodynamic and passive systems arise in environments with thin or absent atmospheres, where insufficient drag prevents effective velocity reduction. On airless bodies like the Moon, parachutes provide negligible deceleration, necessitating alternative methods such as propulsion or tethered systems.[39] Even on Mars, with its tenuous atmosphere (surface density ~0.02 kg/m³), parachute performance is sensitive to deployment conditions and dust loading, restricting applicability to lower-mass payloads.
Notable Missions and Applications
Lunar Soft Landings
The Soviet Union's Luna program marked the beginning of successful lunar soft landings, with Luna 9 achieving the first controlled soft touchdown on February 3, 1966, in Oceanus Procellarum, transmitting the initial panoramic images from the lunar surface.[4] Subsequent missions in the series, including Luna 13 in 1966, Luna 16 in 1970 (the first automated sample return), Luna 17 in 1970 (deploying the Lunokhod 1 rover), Luna 20 in 1972 (another sample return), Luna 21 in 1973 (Lunokhod 2 rover), and Luna 24 in 1976 (final sample return), demonstrated advancing capabilities in propulsion and surface operations.[45] Overall, the program recorded eight soft landing successes out of 10 attempts between Luna 9 and Luna 24 from 1966 to 1976, despite setbacks like the Luna 15 crash during the Apollo 11 era.[46] The United States followed with the uncrewed Surveyor program, launching seven missions from 1966 to 1968 to test soft landing technologies ahead of manned flights; five achieved successful touchdowns, including Surveyor 1 (June 2, 1966, the first U.S. soft landing), Surveyor 3 (April 1967, which survived a micrometeoroid impact), Surveyor 5 (September 1967, performing the first alpha particle X-ray spectrometer analysis), Surveyor 6 (November 1967, the first lunar liftoff and repositioning), and Surveyor 7 (January 1968, landing near Tycho crater to study highland terrain).[25] These robotic precursors paved the way for NASA's Apollo program, which conducted six manned soft landings from 1969 to 1972 using the Lunar Module's descent propulsion system for retropropulsion-controlled touchdowns. Apollo 11 (July 1969) achieved the first human lunar landing in the Sea of Tranquility, followed by Apollo 12 (November 1969, precision landing near Surveyor 3), Apollo 14 (February 1971), Apollo 15 (July 1971, introducing the lunar rover), Apollo 16 (April 1972), and Apollo 17 (December 1972, the longest surface stay at three days).[47]| Mission Series | Agency | Period | Key Successes | Notable Features |
|---|---|---|---|---|
| Luna 9–24 | Soviet Union | 1966–1976 | 8 soft landings (e.g., Luna 9, 16, 17, 24) | First soft landing, sample returns, rover deployments |
| Surveyor 1–7 | NASA | 1966–1968 | 5 soft landings (1, 3, 5, 6, 7) | Surface imaging, soil mechanics tests, chemical analysis |
| Apollo 11–17 | NASA | 1969–1972 | 6 manned landings (11, 12, 14–17) | Human exploration, EVAs, scientific experiments, rover use |