What if you used a parachute on Mars?

Drop a toy with a little parachute. It floats down slowly.

The parachute catches lots of air. But Mars has hardly any air. What would happen there?

Open a parachute and air pushes up hard against its wide canopy, so whatever hangs below it drifts down gently. That push is called air resistance, or drag.

Mars is a good place to test the idea. Its gravity is weaker than Earth's, which sounds helpful. But NASA describes its atmosphere as ultra-thin. Which matters more? In this lab you can drop the same parachute on Earth and on Mars in a model and compare the landings.

A parachute works by maximizing drag, F = ½ρC_dAv², until drag balances weight at a terminal speed v_t = √(2mg/ρC_dA). Both g and the air density ρ appear, and on Mars both are smaller.

This lab drops a 10 kg load under a 2 m² canopy (drag coefficient 1.3) from 50 m, first with Earth's g = 9.81 m/s² and sea-level air (1.225 kg/m³), then with the model's Mars values, g = 3.71 m/s² and ρ = 0.020 kg/m³. Which effect wins, and by how much?

On Earth, the parachute opens wide. Air pushes up on it.

The box floats down slowly. It lands softly.

In the model, a box of supplies weighing 10 kilograms hangs under a parachute 2 square meters across. Dropped from 50 meters, it speeds up for a moment, but the air soon pushes back as hard as gravity pulls. From then on it falls at a steady 7.8 meters per second and lands after about 7 seconds.

Without air, the same drop would take only about 3 seconds and end at 31 meters per second.

With m = 10 kg, C_d = 1.3, A = 2 m², ρ = 1.225 kg/m³ and g = 9.81 m/s², the model's terminal speed is √(196.2/3.185) = 7.85 m/s. The 50 m drop takes 6.93 s and the box lands at 7.85 m/s, having reached 95% of terminal speed after about 1.5 s and 7 m of fall.

With air switched off, the same drop takes √(2h/g) = 3.19 s and ends at √(2gh) = 31.3 m/s. On Earth, the parachute cuts the landing speed by a factor of four.

Now Olly goes to Mars. Mars pulls more gently, but its air is very, very thin.

The parachute catches hardly any air. The box falls fast and lands with a thump!

Choose Mars: gravity is about 38 percent of Earth's, and in the model the air is about 60 times thinner. In the model, the box now lands after about 5.3 seconds at about 18 meters per second, more than twice as fast as on Earth.

Compare it with no air at all: on Mars that gives 19.3 meters per second. So the parachute barely helps. To land at Earth's 8 meters per second in this model, the box would have to be about 23 times lighter, or the parachute about 23 times bigger.

With g = 3.71 m/s² and ρ = 0.020 kg/m³ the terminal speed becomes √(74.2/0.052) = 37.8 m/s, far above anything the box reaches in 50 m. It lands after 5.30 s at 18.1 m/s, against 19.3 m/s in a vacuum: drag removes only about 6% of the speed.

Matching Earth's 7.85 m/s needs m·g/ρ to match too: m = 10 × (9.81 × 0.020)/(3.71 × 1.225) ≈ 0.43 kg, or an area of about 46 m² for the 10 kg box. The model confirms 7.83 m/s at 0.43 kg. Lower gravity alone, with Earth's air, would give a gentle 4.8 m/s; the thin air is what spoils it.

A parachute needs lots of air to push on.

Thin air pushes only a little. So the parachute can't slow the box much.

Air resistance comes from the parachute bumping into air as it falls. Thick air means lots of air to push against and a big upward push. Mars air is so thin that the push is tiny, so the box has to fall much faster before the push grows big enough to matter.

Mars's weaker gravity does help: it pulls the box down more gently. But the air is about 60 times thinner while gravity is only about two and a half times weaker, so the thin air wins.

Since v_t² = 2mg/(ρC_dA), the Mars-to-Earth ratio is (3.71/9.81)/(0.020/1.225) = 0.378/0.0163 ≈ 23, so v_t grows by √23 ≈ 4.8. A drop of only 50 m on Mars never comes close to that terminal speed, so the box behaves almost as in free fall.

Model limits: constant air density with height (the real Martian atmosphere thins with altitude and varies by season), a fixed drag coefficient, and a canopy that is always fully open. NASA's Mars fact sheet gives a surface density of about 0.016 ± 0.006 kg/m³; the model's 0.020 sits inside that range, and the model's equations at 0.016 give a landing at 18.3 m/s (the lab's density slider moves in steps of 0.005). The same sheet gives a mean surface gravity of 3.73 m/s², a little above the model's 3.71; at 3.73 the box still lands at 18.1 m/s.

Real Mars rovers use giant parachutes. Perseverance had one 21 and a half meters across.

Even so, it was still going too fast. So rockets slowed it the rest of the way.

NASA's Perseverance rover landed on Mars in February 2021. NASA reports that its parachute was 21.5 meters across. It opened about 11 kilometers up, while the spacecraft was moving about 1,512 kilometers per hour, and slowed it to about 320 kilometers per hour.

That is still far too fast to land. So, as NASA explains, a descent stage with rocket engines took over and, about 20 meters above the ground, gently lowered the rover on cables: a move called the sky crane. Earlier Mars landers also used parachutes plus rockets, and some used airbags.

NASA/JPL-Caltech state that Perseverance's 21.5 m (70.5 ft) parachute deployed about 240 s after atmospheric entry, at about 11 km altitude and about 1,512 km/h, slowing the vehicle to about 320 km/h. Entry had begun at nearly 20,000 km/h.

NASA's landing overview calls the Martian atmosphere ultra-thin and describes a layered toolkit: Viking's landers in 1976 used parachutes and then liquid-fueled rocket engines; Pathfinder used a parachute, rockets and airbags. Curiosity and Perseverance used the sky crane: Curiosity's descent stage controlled its descent rate with eight rocket engines, and Perseverance's descent stage began lowering the rover on cables about 20 m above the surface. A parachute is necessary on Mars, but nowhere near sufficient.