Chapter 6

Mars Entry, Descent, and Landing

The seven minutes of terror—the time from atmospheric entry to surface touchdown—represent the most challenging and dangerous phase of any Mars mission. This chapter explores the technologies that make safe landing possible.

The EDL Challenge

Entry, Descent, and Landing (EDL) on Mars presents unique challenges found nowhere else in our solar system. Mars has enough atmosphere to require thermal protection during entry but not enough atmosphere for parachutes alone to slow a heavy lander to safe touchdown speeds. The result is what engineers call the "Mars EDL gap"—the problem of landing masses larger than about one metric ton.

Earth EDL benefits from a thick atmosphere that can slow spacecraft to low velocities using parachutes alone. Lunar landing is simpler in another way: with no atmosphere, purely propulsive descent is straightforward. Mars occupies an unfortunate middle ground where neither pure aerodynamic nor pure propulsive approaches work efficiently for heavy payloads.

Entry Phase

Spacecraft arrive at Mars traveling approximately 6 km/s relative to the planet. The entry phase begins at approximately 125 km altitude, where the spacecraft first encounters significant atmospheric resistance. Over the next few minutes, the spacecraft decelerates from interplanetary velocity to roughly Mach 2 (about 500 m/s) while experiencing intense aerodynamic heating.

Thermal Protection Systems

During entry, kinetic energy converts to thermal energy through atmospheric compression and friction. Peak surface temperatures can exceed 1,600°C—hot enough to melt steel. The Thermal Protection System (TPS) must absorb and radiate this heat while keeping the vehicle structure and interior at acceptable temperatures.

Mars landers have used ablative heat shields that sacrifice material to carry heat away from the vehicle. The Mars Science Laboratory (Curiosity) used PICA (Phenolic Impregnated Carbon Ablator), a material developed by NASA Ames that performs excellently at Mars entry conditions. SpaceX Starship uses a different approach with ceramic tiles that withstand heating through radiation rather than ablation.

TPS Type	Mechanism	Max Temp	Usage
PICA	Ablation	~2,000°C	MSL, Mars 2020
SLA-561V	Ablation	~1,500°C	Viking, Pathfinder
Ceramic Tiles	Radiation	~1,400°C	Space Shuttle, Starship
TUFROC	Ablation	~1,800°C	Stardust

Guidance and Control

Early Mars landers used ballistic entry—entering the atmosphere at a fixed angle and accepting the resulting landing footprint. Modern missions use guided entry, where the vehicle actively controls its trajectory by modulating lift. The MSL entry vehicle could adjust its target by banking left or right, reducing landing uncertainty from hundreds of kilometers to a few kilometers.

For human-scale landers, precision is even more critical. Landing must occur within reach of pre-positioned supplies, in safe terrain, and near resources. Guided entry combined with terrain-relative navigation will enable landing within tens of meters of designated targets.

Descent Phase

After peak heating and deceleration, the descent phase begins. At roughly Mach 2 and 10-15 km altitude, parachutes deploy to further slow the vehicle. The parachute phase has historically been the primary deceleration method, but its effectiveness is limited by Mars' thin atmosphere.

Supersonic Parachutes

Mars descent parachutes must deploy at supersonic speeds—a challenging regime where aerodynamic effects differ significantly from subsonic conditions. The Disk-Gap-Band (DGB) parachute design, used since Viking, has proven reliable at Mars conditions. The Mars 2020 parachute measured 21.5 meters in diameter and could slow the spacecraft from Mach 2 to about 100 m/s.

However, parachutes scale poorly for heavy landers. The largest parachute ever tested for Mars (Low-Density Supersonic Decelerator program) measured 30 meters but could only decelerate payloads of about 3 metric tons. For 50-100 ton human landers, parachutes are impractical or impossible.

The Supersonic Retropropulsion Solution

SpaceX pioneered supersonic retropropulsion (SRP) as a practical technique through their Falcon 9 landing program. SRP fires rocket engines while still traveling at supersonic speeds, using thrust to decelerate directly. This approach scales effectively to any mass—more mass simply requires more thrust.

For Mars missions, SpaceX plans to use Starship's engines for SRP beginning at approximately Mach 3 and continuing through landing. This eliminates the need for parachutes entirely, simplifying the EDL system and enabling landing of the massive payloads required for human missions.

Landing Phase

The final phase of EDL brings the vehicle from terminal descent velocity to a soft touchdown. This phase has seen multiple approaches over Mars exploration history, each with advantages and limitations.

Historical Landing Approaches

Viking (1976) used retrorockets to hover and land directly—a simple, reliable approach but propellant-intensive. Pathfinder (1997) and MER (2004) used airbags: after parachute descent, airbags inflated around the lander, which was then dropped to bounce and roll to a stop. This approach was lightweight but limited to small landers.

The Sky Crane, used by Curiosity (2012) and Perseverance (2021), represents a clever solution for medium-sized rovers. A descent stage with throttleable engines hovers above the surface while lowering the rover on cables. After touchdown, the descent stage flies away to crash at safe distance. This enables landing on wheels, ready to drive, without the mass penalty of landing legs on the rover.

Propulsive Landing for Human Missions

Human-scale landers will use fully propulsive landing, with engines providing precise control down to touchdown. SpaceX Starship exemplifies this approach: six Raptor engines provide redundant thrust, with sophisticated throttle and gimbal control enabling precision vertical landing.

The landing sequence for Starship involves a distinctive "belly flop" maneuver during descent, using the vehicle's broad flat surface for atmospheric braking. Near the surface, the vehicle performs a rapid flip to vertical orientation and lights engines for the final landing burn. This technique, while unusual, has been demonstrated successfully on Earth and is expected to work similarly on Mars.

Landing Method	Max Mass	Precision	Example
Airbags	~200 kg	~10 km	Pathfinder, MER
Sky Crane	~1,000 kg	~2 km	Curiosity, Perseverance
Direct Propulsive	~600 kg	~5 km	Phoenix, InSight
SRP + Propulsive	100+ tonnes	~100 m	Starship (planned)

Terrain-Relative Navigation

Precision landing requires knowing exactly where the spacecraft is relative to the terrain throughout descent. Terrain-relative navigation (TRN) systems use cameras and onboard computers to match observed terrain features against stored maps, determining position to within tens of meters.

The Mars 2020 lander successfully demonstrated TRN, enabling Perseverance to land in Jezero Crater—a location considered too hazardous for previous missions due to cliffs, sand dunes, and boulder fields. TRN identified safe landing zones in real-time and diverted the spacecraft as needed.

Hazard Detection and Avoidance

For human missions, landing site safety is paramount. Hazard detection systems use LIDAR (laser ranging), radar altimeters, and cameras to identify dangerous terrain features during descent. The landing system can then autonomously select the safest landing spot within reach, avoiding rocks, slopes, and craters.

Landing Site Candidates

Several Mars locations have been identified as particularly suitable for human missions. Selection balances safety, resource availability, and science objectives.

Arcadia Planitia

SpaceX has identified Arcadia Planitia as a likely landing zone for first Starship missions. This region in Mars' northern lowlands offers flat terrain, low elevation (providing more atmospheric braking), and evidence of subsurface ice within a few meters of the surface. The ice could provide water for life support and propellant production.

Other Candidates

NASA has studied numerous potential human landing sites including Jezero Crater (where Perseverance landed), Meridiani Planum (Opportunity's landing site), and various locations in the mid-latitudes. Each site has different advantages: some prioritize science return, others emphasize resource availability, and some offer particularly safe landing conditions.

Summary

Review Questions

1. Why is Mars EDL often called "seven minutes of terror"? What makes it particularly challenging compared to Earth or lunar landing?
2. Describe the three main phases of Mars EDL. What is the primary deceleration mechanism in each phase?
3. Why don't parachutes work for landing heavy payloads on Mars? What physical limitations do they face?
4. Explain supersonic retropropulsion. How does it solve the Mars heavy landing problem?
5. Compare ablative and radiative thermal protection approaches. What are the advantages and disadvantages of each?
6. What is terrain-relative navigation? Why is it important for human Mars missions?
7. What factors make Arcadia Planitia attractive as a human landing site? What other sites are being considered?
8. Describe the Starship landing sequence on Mars. How does the "belly flop" maneuver work?