1.3 Spacecraft
1.3.1 Classification of Spacecraft
1.3.1.3 Lander Spacecraft
Lander spacecraft are designed to land on the surface of another planet or other celestial body. The landing may be a “soft” landing, where the spacecraft survives and is able to perform other functions on the surface, or it may be a high velocity impact, where the spacecraft obtains data during its descent, but is not designed to survive the impact. At the time of writing, lander spacecraft have successfully impacted or soft landed on the Moon, Mercury, Mars, Venus, Saturn’s moon Titan, and a few asteroids and comets.
The first type of lander spacecraft that was flown into space was the impactor lander. This is, perhaps, the simplest type of lander spacecraft, since it does not have the design complexity to soft land on a surface. Data is collected about the planet or celestial body, including the atmosphere, if present, as the impactor descends to the surface. The Russian impactor spacecraft, Luna 2, was the first manmade object to “land” on another celestial body, impacting the Moon on 14 September 1959. With an estimated impact speed of 3.3 km/s (7382 mph), the vehicle was certainly destroyed upon impact. Luna 2 was the second in a series of lunar explorer spacecraft sent to the Moon by Russia. The first intended Russian impactor lander, Luna 1, was unsuccessful, due to an error in its trajectory, causing it to miss hitting the Moon by about 6000 km (3700 miles).
As shown in Figure 1.46, the 390 kg (860 lb) Luna 2 spacecraft was spherical in shape with pro-truding antennas. Sensors and instrumentation on board the spacecraft included radiation detectors, micrometeorite detectors, and a magnetometer to detect the Moon’s magnetic field. Data from these sensors confirmed that the Moon does not have a radiation belt or any significant magnetic field.
On its way to the Moon, Luna 2 released an amount of sodium gas into space, which created a
Figure 1.46 An impactor spacecraft, Soviet Luna 2, the first manmade object to land on another celestial body, the Moon, 1959. (Source: NASA.)
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to study the behavior of a gas in outer space.
A variation of the impactor lander is the penetrator spacecraft, which is designed to survive the tremendous forces of the impact and penetrate into the surface. It then makes measurements that are telemetered back to Earth, typically by relaying the data to a “mothership” spacecraft in orbit.
The stationary lander and the surface rover make soft landings on a planet or other body. The sta-tionary lander remains at its landing spot, while the surface rover is able to move about the surface.
The rover has the advantage of being able to move about the surface, allowing it to explore a larger area than the stationary lander, but this comes at a higher risk of damage in navigating the terrain and surface obstacles. Both the stationary lander and the surface rover may have semi-autonomous functions, such as unfolding solar arrays or antennas, but they are often sent commands by con-trollers on Earth. The movement of a surface rover on another planet is precisely choreographed by controllers on Earth to ensure the safety and success of the vehicle’s movements. We discuss the Curiosity Mars surface rover in a later section.
If there is sufficient atmosphere to produce significant frictional heating, shielding may be required to protect the lander spacecraft from high heating during its entry and descent. The landing must be soft enough so that the spacecraft is undamaged and able to perform its mission on the surface, typically scientific data collection. Parachutes, rockets, or both, may be used to decelerate the lander during its descent. Rockets may be fired, right before touchdown, to reduce the landing impact velocity. Touchdown of the spacecraft may be on mechanical landing gear or inflatable cushions or bags, to absorb the final landing loads. For landing on bodies with very low gravity, a harpoon-type device may fire an anchor cable into the surface to hold the spacecraft onto the surface.
This was the scheme used for the European Space Agency (ESA) Philae, a small robotic lander that made the first soft landing on the surface of a comet, 67P/Churyumov–Gerasimenko, on 12 November 2014 (Figure 1.47). The Philae lander had anchoring harpoons on its belly that would fire downward into the comet, when the spacecraft touched down on the gravity-less comet. The
Figure 1.47 Depiction of Philae landing on Comet 67P Churyumov–Gerasimenko. (Source: Adapted from DLR German Aerospace Center, “Rosetta’s Philae Touchdown” https://en.wikipedia.org/wiki/File:
Rosetta%27s_Philae_touchdown.jpg, CC-BY-SA-3.0. License at https://creativecommons.org/licenses/by-sa/
3.0/legalcode.)
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lander would then fire a thruster, on top of the spacecraft, gently pushing it onto the surface, while ice screws were drilled into the surface from its three landing footpads. Unfortunately, the landing impact was softer than planned, so that the anchoring harpoons did not fire. Without the anchors in place, the Philae lander bounced off the comet surface a few times, but luckily settled down to a permanent landing without being damaged.
The first spacecraft to soft land on another celestial body was the Russian Luna 9, which landed on the Moon on 3 February 1966. This was followed by the United States Surveyor 1, which soft landed on the Moon four months later, on 2 June 1966. Both landers answered a question that was in debate prior to a spacecraft actually landing on the Moon: would a spacecraft sink deeply into the dust on the surface of the Moon, perhaps even burying the lander? This was a question that was of some concern for future plans to land people on the Moon. The lunar surface supported the weight of the landers, definitively putting this issue to rest. Both Luna 9 and Surveyor 1 were stationary landers, and both used retrorockets to reduce their rate of descent for a soft landing. One of the major objectives achieved by Surveyor 1 was the validation of the technologies required to soft land on the moon, paving the way for manned landers.
The Lunar Excursion Module (LEM) and the Lunar Roving Vehicle (LRV) were manned lander spacecraft that landed on the moon during the Apollo program (Figure 1.48). The LEM was a stationary lander that carried two astronauts from Moon orbit to the lunar surface. It was a two-stage spacecraft, with a descent stage and an ascent stage. The ascent stage returned the astronauts to rendezvous in lunar orbit with the Apollo Command Module spacecraft. There were six successful landings of the LEM on the moon between 1969 and 1972. The LRV was a surface rover that was carried to the lunar surface by the LEM. It was an open-frame, electrically powered, four-wheeled, car-like vehicle with side-by-side seating for two astronauts. The LRV was used on the moon for the last three Apollo missions.
Figure 1.48 Apollo 16 Lunar Excursion Module and Lunar Roving Vehicle on the moon, 1972. (Source:
NASA.)
k k 1.3.1.4 Atmospheric Probe
Some spacecraft carry smaller, specially instrumented atmospheric probe spacecraft that are released to enter the atmosphere of another planet. These atmospheric probes are used to collect scientific data about the planet and its atmosphere as they descend. Entering the atmosphere at hypersonic speeds, they typically encounter large aerodynamic forces and high heating.
Deceleration and thermal protection may be required using an aeroshell, a rigid shell structure that detaches from the beneath the probe. High drag devices, such as parachutes, may also be deployed to decelerate the probe and allow more time for data collection during its descent. Data from the probe is typically telemetered to the orbiter “mothership” spacecraft, where it is relayed back to Earth. Often, atmospheric probes are not designed to survive to landing on the planet surface, burning up in their fiery descent.
The Pioneer 13 spacecraft, also known as the Pioneer Venus Multiprobe, carried four atmo-spheric probes to the planet Venus in the late 1970s. The probes were atmo-spherically shaped, pressure vessels with aeroshells. One probe was 1.5 m (4.9 ft) in diameter and the other three were smaller, at 0.8 m (2.6 ft) in diameter (Figure 1.49). The aeroshell of the large probe detached to provide ther-mal protection, and a parachute was used to decelerate the probe. The sther-maller probes did not have parachutes and their aeroshells remained attached. The probes were not designed to survive to land-ing on Venus, but amazland-ingly, one of the small probes continued to transmit signals for over an hour after impacting the surface. The main spacecraft body or bus (to be discussed in the next section) was also used as an atmospheric probe, even though it did not have a heat shield or parachute.
Net flux radiometer
Atmosphere structure door
Atmosphere structure temperature
sensor Atmosphere structure pressure
inlet and spin control vane
Yo-yo cable cutter
Yo-yo despin weight Yo-yo despin cable
Carbon phenolic heat shield Antenna
housing Ground coolant access cover
Nephelometer door (shown closed)
Probe/bus interface ring for separation
clamp
Deceleration module
Figure 1.49 Diagram of Pioneer 13 small probe. (Source: NASA.)
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When the 2.5 m (8.2 ft) diameter, 290 kg (639 lb) cylindrical spacecraft was no longer able to stay in orbit around Venus, it entered the atmosphere and transmitted scientific data until it burned up at an altitude of about 110 km (68.4 miles).