By Betty Robinson, bettyrrobinson.ca
NASA’s newest rover—Perseverance—is scheduled to land on Mars on Feb. 18, 2021, at about 3:30 p.m. EST. Perseverance will be looking for evidence of past microbial life, as well as studying Mars’s geology. An ingenious passenger onboard Perseverance is a helicopter. That’s right. A helicopter. The first aircraft on another planet! It has the appropriate name Ingenuity Mars Helicopter.
Perseverance will be the fifth rover to land on Mars. The others are Pathfinder, Spirit, Opportunity, and Curiosity. Credit: NASA
Aircraft on Earth are designed, naturally, to work in Earth’s gravity and atmosphere. None of these are similar to Mars conditions. Mars has a little over one-third of Earth’s gravity, and its atmosphere is about 1% the density of Earth’s atmosphere (and a completely different gaseous composition). Consequently, the atmospheric pressure on Mars is next to nothing. And it’s cold on Mars. Much, much colder than on Earth. Nights can be as cold as –90°C at Jezero Crater, where Perseverance is going to land.
The purpose of Ingenuity is to test the waters for future flight missions on Mars and other bodies in the solar system. Ingenuity doesn’t carry any science instruments so it won’t be doing any experiments. It does have two cameras, though: one black and white, and one colour. However, Ingenuity’s first challenge may be to survive one night on Mars. Fortunately, it has a heater.
Artist’s illustration of the Ingenuity Mars Helicopter. Ingenuity is about 0.5 metres high. Credit: NASA/JPL-Caltech
So how does an Earth-designed helicopter work in Mars conditions? For starters, Ingenuity has to operate on its own. It cannot be directly controlled from Earth like we control drones, for instance, because of the length of time it takes to get signals to and from Mars. It will, however, get the initial command to start, and other commands, from Earth via Perseverance.
Ingenuity’s blades are 1.2 metres long and made of carbon fibres. They rotate at about 2,400 rpm, which is eight times faster than the average helicopter rpms on Earth. The longer blades and faster rotation speed are required because of the almost non-existent atmospheric pressure on Mars. If the air is thin, as in higher altitudes on Earth, the aircraft needs to go faster because there is less air under and above the wings. So with such a thin atmosphere on Mars, the higher rotation speed and longer “wingspan” are needed.
For an aircraft to take off, it needs to be able to overcome gravity/weight. To achieve this, the aircraft needs more lift force than weight. Air moving under and over the aircraft’s wings provides the lift. This force acts upwards to counteract the downward force of gravity/weight. Ingenuity is quite light, only 1.8 kg. The light weight is helpful with overcoming Mars’s gravity.
Gravity pulls everything downward, toward Earth’s centre. When talking about weight in science, weight is a force and refers to the mass of an object times the force of gravity. For Earth, the force of gravity, or g, is 9.8 m/s2. For Mars, the number is 3.7 m/s2. So someone with a mass of 50 kg has a weight of 50 kg x 9.8 m/s2 = 490 N (newtons) on Earth and 50 kg × 3.7 m/s2 = 185 N on Mars. A newton is the unit of force. On Earth, Ingenuity’s weight is 1.8 kg × 9.8 m/s2 = 17.6 N. On Mars, its weight is 1.8 kg × 3.7 m/s2 = 6.7 N. Considerably less.
For an aircraft to move forward, it needs thrust. An increase in thrust allows the aircraft to accelerate and overcome drag (see below). An aircraft’s engine provides the thrust. The thrust on Ingenuity is provided by rechargeable solar-powered batteries. With its reduced mass and Mars’s reduced atmosphere, thrust will be less challenging than thrust on Earth.
The force working against thrust is called drag. It is caused by air resistance and acts in the opposite direction to the motion. The amount of drag depends on the object’s shape, the atmospheric density, and the object’s speed. With such a thin atmosphere on Mars, there is little drag. That can be a good thing. Thrust must be greater than drag for the helicopter to take off, so with little drag there can easily be more thrust. And when landing, thrust is easy to reduce so that the speed decreases and thrust becomes less than drag.
Watch landing online: https://mars.nasa.gov/mars2020/timeline/landing/watch-online/
News briefings and launch commentary will be streamed on https://mars.nasa.gov/mars2020/timeline/launch/watch-in-person/