More specifically, after the main launch adjustments have been made using the primary boosters and temporary adjusting propellants how might even the slightest trajectory and orientation corrections be made? I realize that there are mono propellants, however that surely could not be used efficiently for major corrections (not including orientation, which is what the propellants are used for).

Take, for example, the New Horizons spacecraft. The probe was launched in 2006, and on the eve of the year 2019 it experienced one and only flyby over Ultima Thule. Over it's 12 year journey it had to make corrections to maintain it's orbital intercept course with the asteroid. So how? I'm sure I am missing something obvious here...


2 Answers 2


New Horizons used its monopropellant attitude control thrusters for course correction. Because it couldn't make large course changes with the limited fuel available, the choice of targets was severely constrained:

Mission planners searched for one or more additional Kuiper belt objects (KBOs) of the order of 50–100 km (31–62 mi) in diameter as targets for flybys similar to the spacecraft's Plutonian encounter. However, despite the large population of KBOs, many factors limited the number of possible targets. Because the flight path was determined by the Pluto flyby, and the probe only had 33 kilograms of hydrazine propellant remaining, the object to be visited needed to be within a cone of less than a degree's width extending from Pluto. The target also needed to be within 55 AU, because beyond 55 AU, the communications link will become too weak, and the RTG power output will have decayed significantly enough to hinder observations.

It's also not making "many flybys" of Ultima Thule; it got one pass at Pluto in 2015 and one pass of Ultima Thule in 2018, and it is unclear if it will be able to steer toward a third target in the future.

This is typical of deep space probes. Flyby probes will use small attitude control thrusters, typically, since there's plenty of time to make course changes. For interplanetary orbiters, somewhat larger engines are generally needed for orbital insertion, and that usually makes it economical to use bipropellant thrusters (typically MMH/NTO or similar hypergolics) with better specific impulse than monoprops.

In contrast, a few interplanetary probes have used ion thrusters, which provide about 10x the specific impulse (fuel efficiency) of chemical rockets, albeit with extremely low thrust. The Dawn mission, for instance, was able to orbit two different dwarf planets in the asteroid belt, which is an unprecedented achievement.

  • $\begingroup$ Thank you, I guess I underestimated monopropellant efficiency. $\endgroup$ Jan 4, 2019 at 0:03
  • $\begingroup$ Small monoprop thrusters are typically only 70% as fuel efficient as their biprop brethren, but the course corrections needed are generally very small. $\endgroup$ Jan 4, 2019 at 0:26
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    $\begingroup$ "will not be able to steer to any other targets in the future" - The same article you linked says "2020s: The probe may be able to fly by a third KBO. The probe approached 2014 MU69 along its rotational axis, which simplified trajectory correction maneuvers, saving fuel that could be used to target another KBO. After the flyby, the spacecraft is left with 11 kg (24 lb) of fuel." $\endgroup$
    – molnarm
    Jan 4, 2019 at 8:54

With the exception of ion propulsion, all major maneuvers from spacecraft are done with either monopropellants, with a few exceptions, most notably the bipropellant dinitrogen tetroxide, used by the Apollo program, and a few other similar missions. The exceptions are all crewed vehicles, where masses are typically much larger. Monopropellants are easier to design and work well enough, so they aren't worth the extra difficulty to get slightly more performance. Hydrazine is by far the most commonly used monopropellant, and has been used to land on Mars, orbit spacecraft throughout the solar system, and more.

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    $\begingroup$ Hydrazine is not only used as a monopropellant but also as a bipropellant component. $\endgroup$
    – Uwe
    Jan 4, 2019 at 17:32
  • $\begingroup$ That is true, but the monopropellant version of it is the more commonly used one. $\endgroup$
    – PearsonArtPhoto
    Jan 4, 2019 at 17:36

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