SLA-561V

When a spacecraft slams into a planet’s atmosphere, the air in front of it heats up to thousands of degrees. SLA-561V is a lightweight, cork-filled material that protected most of NASA’s Mars landers from that fire for nearly four decades.

Quick facts

• Maker: Lockheed Martin Space Systems (originally built by Martin Marietta for the Viking landers in the 1970s).
• What it is: A “Super Lightweight Ablator” (SLA) — a thermal protection system, meaning the shield that keeps the rest of a spacecraft from cooking during atmospheric entry.
• Type: A low-density elastomeric ablator, also called a syntactic foam (a foam made by mixing tiny hollow spheres into a flexible resin).
• Density: Roughly 0.25 grams per cubic centimeter (about 256 kg/m³) — under 0.3 g/cm³, which is why it’s called “super lightweight.”
• Thermal conductivity: Less than about 0.5 W/m·K (a low number, meaning it resists letting heat pass through).
• Starts ablating at: A heat flux of roughly 110 watts per square centimeter.

What it is and how it works

SLA-561V is an ablator — a material that protects against extreme heat by sacrificing itself rather than simply blocking the heat. Picture a candle whose surface slowly melts and burns away: the burning carries energy off instead of letting it soak inward. SLA-561V works on the same idea, but on purpose and at far higher temperatures.

The recipe is unusual. A silicone resin binder (about 25% by mass) is mixed with silica microballoons and chopped silica fibers (about 40%), plus ground cork and tiny hollow phenolic spheres. (“Phenolic” is a heat-resistant plastic resin.) This paste is hand-packed into the cells of a honeycomb core — a stiff, lightweight grid that holds the soft foam in place, much like the walls of an ice-cube tray hold each cube.

As the shock-heated gas in front of the spacecraft drives the surface to very high temperatures, the silicone and phenolic break down in a process called pyrolysis (thermal decomposition — the material chemically falling apart from heat). This forms a black, carbon-rich crust called a char layer. Three effects then carry heat away together: the chemical reactions and phase changes soak up energy; the char layer radiates heat back outward and insulates the cool structure beneath; and the gases released by pyrolysis seep out through the surface (an effect called “blowing”), pushing the searing shock-layer gases away from the shield wall and cutting the heating it feels. Because the surface erodes as it works, the shield is built thick enough that the aeroshell — the protective outer body — stays cool for the whole entry.

Why it matters

For decades, SLA-561V was NASA’s workhorse heat-shield material for Mars and sample-return missions. Its very low weight and low cost let mission designers keep the aeroshell light and the budget down while reliably surviving entry. That long flight record made it the default “Mars-heritage” choice, so each new Mars lander tended to start by assuming SLA-561V would do the job.

Its limit is just as instructive. When the Mars Science Laboratory needed a much larger, heavier aeroshell entering faster, SLA-561V failed late-stage qualification testing — receding far faster than the models predicted, with catastrophic failure under the combined turbulent flow and high shear of a roughly 250 W/cm² environment, more than double the heating where it normally begins to ablate. That forced a rapid switch to Phenolic Impregnated Carbon Ablator (PICA). The episode became a landmark “lessons-learned” case in heat-shield engineering: flight heritage does not guarantee performance in a new, harsher environment, and it nudged heavier Mars missions toward carbon-based ablators.

Where it’s been used

• Viking 1 and 2 (1976): The original SLA-561V heat shields, built by Martin Marietta for the Viking aeroshells.
• Mars Pathfinder (1997): Used an SLA-561V forebody heat shield; NASA Ames ran dedicated shear tests of the material for it (NASA TM-110402).
• Mars Exploration Rovers, Spirit and Opportunity (2004): Both used SLA-561V heat shields.
• Stardust (comet sample return): SLA-561V covered the capsule’s afterbody (its backshell), while the main forebody heat shield used PICA — a common split where the milder backshell heating suits SLA-561V.
• Mars Science Laboratory / Curiosity (2012): SLA-561V was the baseline and even passed early reviews, but failed shear and turbulence qualification for the heat shield and was replaced by tiled PICA. It was still hand-packed into Curiosity’s backshell, where the heating is milder. (Mars 2020’s Perseverance followed a similar split — PICA on the heat shield, SLA-561V on the backshell.)