NASA's InSight spacecraft has completed its first major course correction, setting it on its way to Mars. On May 22, the cruise stage with its robotic lander made a 40-second burn using four of its eight thrusters to not only alter the probe's trajectory, but to protect the Red Planet from biocontamination by the mission's Centaur rocket booster.

Many people with more than a passing interest in space exploration would tell you that there are currently five man-made objects leaving the Solar System: Pioneer 10, Pioneer 11, Voyager 1, Voyager 2, and New Horizons. That, however, is a bit off. The actual number is nine plus a couple of bits of jettisoned gear.

What are these other four objects? Classified payloads on some top secret mission? Some obscure Soviet probes that shut down soon after lift off? No, they're the upper stage boosters used to launch the interstellar probes and are now on their own one-way journeys into infinity.

In order to put the five deep space probes on a trajectory out of the Solar System, they needed to be accelerated to a high enough velocity to break free of the Sun's gravitational field. To do this, they needed a rocket to speed them up before they could separate and go their own way. However, what many people overlook is that by the time of separation, the booster was moving just as fast as the probe, so if the probe was heading out of the System, never to return, so was the booster.

And if you're wondering why there are five probes and four boosters, that's because the Pioneer 11 upper stage made a close encounter with Jupiter and was thrown into a solar orbit after an unintended slingshot maneuver.

What's true for interstellar probes is also true for interplanetary ones. For almost every satellite that's escaped the Earth's gravity well, there's also a booster stage now orbiting the Sun. In the case of the Interior Exploration using Seismic Investigations, Geodesy and Heat Transport (InSight) mission, there's the Centaur upper stage of the Atlas V 401 rocket that launched the spacecraft on May 5 from Vandenberg Air Force Base in California.

According to NASA, this Centaur rocket following InSIght poses a potential problem. Under international agreement, the InSight spacecraft has been carefully sterilized to prevent it from contaminating Mars with terrestrial microbes, but those rules don't apply to the Centaur. So, to avoid the chance of the Centaur colliding with Mars, the initial launch trajectory was deliberately set a bit off target.

This meant that after InSight made its Tuesday course correction, the probe was now aimed at Mars, but the Centaur was following the original trajectory where it will miss the Red Planet by a wide margin.

The space agency says that this will not be the only course correction that will be made on InSight's journey, which will require up to six thruster burns before it reaches Mars orbit on November 26.

This is because space navigation is not a precise science. True, anyone who understands basic mathematics can calculate a simple trajectory from Earth to Mars, but things soon get very complicated. There are many different bodies in the Solar System that are pulling at the spacecraft, plus the push of the solar winds. These can all alter its orbit in significant ways.

InSight must also fire its four attitude thrusters on a daily basis to keep its radio antenna pointed at Earth. Theoretically, these burns should cancel one another out, but in practice they, too, can change the trajectory. To help InSight and other missions reach their destinations, NASA tracks them using its Deep Space Network (DSN) and work out what their actual trajectories are and how to correct them.

"Navigation is all about statistics, probability and uncertainty," says Fernando Abilleira of JPL, InSight's Deputy Mission Design and Navigation Manager. "As we gather more information on the forces acting on the spacecraft, we can better predict how it's moving and how future maneuvers will affect its path."

Source: NASA