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Spacecraft slow down near an asteroid to reduce their speed relative to the asteroid—not to stop moving through space. A rendezvous requires the spacecraft to arrive with nearly the same velocity and direction as its target. Otherwise, it will rush past instead of remaining nearby for observations, mapping, or later operations.
What “slowing down” means in space
An asteroid and a spacecraft both travel around the Sun. For a rendezvous, the key measure is the difference between their velocity vectors: how fast each is moving and in what direction. NASA’s Trajectory Browser describes small-body rendezvous delta-v in terms of this vector difference. The spacecraft must change its trajectory enough to reduce that difference.
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It does not become stationary in an absolute sense. It continues moving around the Sun while matching the asteroid’s motion closely enough to stay in its vicinity. The required change in velocity is called delta-v.
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Why not just fly straight to the asteroid?
A direct-looking path does not guarantee a gentle arrival. The asteroid is moving, and the spacecraft must reach the same place at the same time with a suitably similar velocity. If the spacecraft crosses the asteroid’s location while moving much faster relative to it, the encounter is a flyby: the spacecraft passes by rather than remaining near the body.
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A flyby can be appropriate when a mission only needs to observe a target during a brief encounter. Rendezvous has a different goal: reducing target-relative motion so the spacecraft can continue operating nearby. NASA’s trajectory material distinguishes interception from the velocity reduction needed for orbit capture and sustained operations.
How OSIRIS-REx slowed down at Bennu
NASA reported that OSIRIS-REx used four approach maneuvers between August 17 and November 27, 2018, to reduce its velocity relative to Bennu from about 1,100 mph (491 m/s) to 0.10 mph (0.04 m/s). These figures describe Bennu-relative speed, not speed relative to Earth or the Sun. NASA’s final-approach images were taken from about 40 miles (65 km) away.
| Maneuver | Bennu-relative speed change | NASA’s reported detail |
|---|---|---|
| AAM-1 | About 1,100 mph (491 m/s) to 313 mph (140 m/s) | Designed to reduce the approach speed. |
| AAM-2 | About 315 mph (141 m/s) to 11.8 mph (5.2 m/s) | Reported starting value differs slightly from AAM-1’s rounded endpoint. |
| AAM-3 | About 11.7 mph (5.2 m/s) to 0.24 mph (0.11 m/s) | Completed in two burns because of constraints on pointing science instruments too close to the Sun. |
| Overall approach | About 1,100 mph (491 m/s) to 0.10 mph (0.04 m/s) | NASA reported four maneuvers over the approach period. |
The individual maneuver values are rounded figures from separate NASA reports, so they should not be read as one perfectly continuous speed record. NASA also described the approach maneuvers as targeting the trajectory for arrival and the start of proximity operations.
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Slowing down is only part of the arrival problem. The spacecraft must also approach from a useful direction, navigate using observations, and retain enough control to make corrections. A NASA technical report on Bennu operations describes two large early burns followed by maneuvers using progressively lower-thrust systems; it says finer control was needed below 200 km. Navigation also had to stay within a range and solar-phase-angle corridor that supported observations.
That is why the spacecraft does not simply wait until the asteroid looks close and apply one enormous brake. Mission teams plan the trajectory and timing in advance, make corrections as the encounter develops, and use navigation results to guide the next steps. Bennu’s four-maneuver approach is one mission’s design, not a rule for every asteroid mission.
Independent reader supportYour contribution helps us test, update, and keep practical guides available for everyone.Rendezvous is not landing
Matching the asteroid’s motion allows a spacecraft to remain nearby or conduct orbital and proximity operations. It does not mean the spacecraft has landed. Surface contact or sample collection is a separate operation that comes after arrival and depends on the mission’s design.
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For OSIRIS-REx, arrival at Bennu was followed by proximity operations. The approach speed reduction enabled the spacecraft to work near the asteroid; it was not itself a landing maneuver.
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