A spacecraft often needs to orbit the target celestial object to conduct its science mission. So, how does an orbit insertion work? Let’s find out.
The Launch Window
Well, getting into an orbit of another planet requires extreme precision. The first thing to get right is the launch window, which is the time that makes traveling to a destination outside Earth possible. In short, if the spacecraft has to meet something other than our own planet, it needs to launch at the right time, or it will not reach the destination on time.
Figuring out the launch window is even more complicated when the mission requires gravity assists from multiple planets or has multiple flybys. If the spacecraft misses the launch window, it has to wait for a long time or even repurpose the mission from the drawing board entirely. An example of this is the Rosetta mission, which changed its destination because it missed a launch window.
Undoubtedly, mistakes are always made on launch. Therefore, we need to correct its trajectory to make sure the spacecraft is precisely in the right heliocentric orbit. That’s essential to make sure that it will rendezvous with its target object sometime later.
Therefore, trajectory correction maneuvers (TCMs) have to be made. For instance, the Perseverance rover has 7 opportunities to fire its cruise stage thrusters to aim at the red planet more precisely.
Moreover, gravity assists are sometimes necessary to adjust the spacecraft’s orbit, and the spacecraft may need to fire its thrusters to match the orbit of its target. That way, it can meet the target at a lower relative velocity, conserving fuel for the orbit insertion.
The Orbit Insertion
When the spacecraft arrives at the destination, the orbit insertion happens. Let’s find out how to enter the orbit of another planet, asteroid, or comet.
You have to slow down your spacecraft (adequately) to achieve orbit. To get the velocity, you need to understand the orbital velocity. Basically, the slower you go relative to the object, the smaller your orbit will be (or even fall onto the object). To plan a spacecraft’s orbit, you need to get to the optimal speed that requires the least amount of fuel. Then, you have to calculate how long the spacecraft will burn the engines based on the necessary deceleration.
If it’s a planet, a relative velocity of a few kilometers per second is enough for it to achieve orbit. However, for tiny objects such as asteroids and comets, it might need to slow down to less than a meter per second relative to it. This requires the orbits of the spacecraft and the asteroid or comet to almost perfectly match.
At that time, the spacecraft will hopefully be in orbit around the target. If it’s incorrect, it can adjust the orbit with its thrusters, the target’s atmosphere, or natural satellites.
If it did not enter orbit, it just flies by the planet while changing its trajectory. In that case, the mission team needs to adjust its heliocentric orbit (probably significantly) to reach its target again, which could use a lot of fuel. In some cases, it might be better to abandon the spacecraft or repurpose the entire mission.
After entering an orbit of another planet, the spacecraft needs to adjust its trajectory for science operations.
In some orbit insertions, the orbit ends up being too elliptical for science operations. Therefore, the space probe can aerobrake. After orbit insertion, the spacecraft will lower its apoapsis using atmospheric drag on its periapsis. After aerobraking is complete, the periapsis must be raised in order to achieve its science orbit and prevent it from breaking apart in the atmosphere as the orbit decays.
Another way of optimizing the spacecraft’s orbit is using gravity assists if the planet has natural satellites of significant sizes. For example, a space probe heading to Jupiter can use Jupiter’s moons to shrink or expand its orbit, allowing as much useful scientific data as possible.
In any case, the spacecraft can change its trajectory using its onboard thrusters. After aerobraking, it needs to do this to raise its periapsis, while the space probe might need some TCMs to meet the right orbit around the target.
So, we explained the essential steps to get into the desired orbit of another planet. Keep in mind that it’s a complicated procedure that requires a lot of time and some decent creativity. If you’re curious, you can visit the sites in the references to learn more.
References and Credits
- (n.d.). ESA – What is a ‘launch window’? Retrieved April 7, 2021, from http://www.esa.int/Science_Exploration/Space_Science/What_is_a_launch_window
- (2015, April 30). Rosetta: The whole story . Retrieved April 7, 2021, from https://www.bbc.com/future/bespoke/story/20150430-rosetta-the-whole-story/
- (n.d.). vehicle’s cruise to Mars – NASA Mars. Retrieved April 7, 2021, from https://mars.nasa.gov/mars2020/timeline/cruise/
- Daniel Scuka. (2016, October 17). The technology of the Mars Orbit Insertion burn. Retrieved April 7, 2021, from https://blogs.esa.int/rocketscience/2016/10/17/burn-baby-burn-the-technology-of-the-mars-orbit-insertion-burn/
- (n.d.). Asteroid Operations – OSIRIS-REx Mission. Retrieved April 7, 2021, from https://www.asteroidmission.org/asteroid-operations/
- (n.d.). Basics of Space Flight – Solar System Exploration: NASA Science. Retrieved April 7, 2021, from https://solarsystem.nasa.gov/basics/chapter3-4/
- (n.d.). Aerobraking – NASA Mars. Retrieved April 7, 2021, from https://mars.nasa.gov/mro/mission/timeline/mtaerobraking/