Getting to Mars is a shooting gallery where all targets are moving
A fleet of robot spacecraft from Earth invaded Mars this month and just getting there is an astounding achievement. Many of the missions to the red planet have failed.
Three missions to Mars from three different countries all arrived at Mars this month, The United Arab Emirates’ Hope — an orbiter, China’s Tianwen-1, both an orbiter and rover/lander, and NASA’s Perseverance rover with the Ingenuity Mars Helicopter.
Travelling between Earth and Mars is a difficult task requiring incredible precision, vast distances, and targets that are moving. It is all too easy to miss the planet altogether, crash into it or have something go wrong along the way, which has been the fate of roughly half the missions that have attempted to get there in the past.
Flying between Earth and Mars is a shooting gallery where both the target and the shooter are moving, and the “bullet” does not fly in a straight line.
Escaping faster than a speeding bullet
When you think about it, Mars appears as a shining dot in the night sky. If you were to aim a gun at it and shoot, you would not hit it.
Even the most powerful guns do not propel their shells fast enough to escape the gravity of the Earth and reach interplanetary space.
That is more than 15 times the speed of a rifle bullet.
Like bullets, spacecraft are given all of their energy at the beginning of the flight.
Rockets fire for only a matter of minutes after liftoff, tripling its initial velocity in its first stage of flight, before the upper stage rockets fire a couple of times giving it the thrust it needs to escape Earth’s orbit, after which the craft coasts the rest of the way.
When shooting at any moving target you have to aim at a point ahead, and the same is true of Mars, except the Earth is also moving.
Both planets race around the sun with Mars on the outside lane. So the best time to take the shot is when the Earth catches up on the inside and the range is shortest, an opportunity that only comes up every two years.
A moving target
Once all these motions are taken into account, it takes the spacecraft seven months to make the trip.
Imagine waiting more than half a year for your bullet to reach the target. That means spacecraft have to be aimed at a point in space where Mars will be seven months after launch.
A trip from Earth to Mars is not a simple straight line because the gravity of the sun is always pulling back, causing a spacecraft to follow a curving path, which is really an elongated orbit around the sun that happens to cross the orbit of Mars known as a Hohmann transfer.
You can see why it is easier to miss Mars than to hit it. Spacecraft do have small thrusters so flight controllers can make fine adjustments along the way, but the general flight path has to be right from the beginning.
The final descent
Once you make it to the target, there are two choices: fire a rocket engine to slow down just enough to be captured by Mars gravity and go into orbit around the planet, which is what Tianwen-1 did, before releasing its rover/lander. Or you can take the straight-in approach of the Americans.
The Perseverance rover, encapsulated in a gumdrop-shaped capsule resembling a flying saucer (We send flying saucers to Mars!) hits the atmosphere at more than 20,000 km/h.
The angle of attack must be exactly right. Too steep and it will burn up, too shallow and it will skip off like a stone on water.
Then there are the famous “seven minutes of terror” where a complicated series of events must take place involving a heat shield, supersonic parachute, rocket engines and a sky crane to gently place the lander on the surface.
All of that takes place automatically because the travel time for a radio signal between Earth and Mars is more than 11 minutes, so controllers cannot interact with it in real-time.
The lander is entirely on its own, which is a real nail-biter for the people who programmed all those actions into the computer.
Many missions have been lost during those final few moments.
The landing spot is a single crater called Jezero that was chosen well in advance. So not only do they have to hit the moving target of Mars as a planet, that planet is rotating on its axis so the timing of the arrival must be just right so that the specific crater is below them when they touch down.
After travelling hundreds of millions of kilometres, the landing target is only 7.7 kilometres by 6.6 kilometres.
If everything goes right, the science can begin.
www.cbc.ca 2021-02-12 17:56:46