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1. the path described by an object moving in air or space under the influence of such forces as thrust, wind resistance, and gravity, esp the curved path of a projectile
2. Geometry a curve that cuts a family of curves or surfaces at a constant angle
Collins Discovery Encyclopedia, 1st edition © HarperCollins Publishers 2005


The curve described by a body moving through space, as of a meteor through the atmosphere, a planet around the Sun, a projectile fired from a gun, or a rocket in flight. In general, the trajectory of a body in a gravitational field is a conic section—ellipse, hyperbola, or parabola—depending on the energy of motion. The trajectory of a shell or rocket fired from the ground is a portion of an ellipse with the Earth's center as one focus; however, if the altitude reached is not great, the effect of gravity is essentially constant, and the parabola is a good approximation. See Ballistics

McGraw-Hill Concise Encyclopedia of Physics. © 2002 by The McGraw-Hill Companies, Inc.


(tră-jek -tŏ-ree) The curved path followed by a moving spacecraft, rocket, etc., under the forces of gravity, lift, and/or drag.
Collins Dictionary of Astronomy © Market House Books Ltd, 2006
The following article is from The Great Soviet Encyclopedia (1979). It might be outdated or ideologically biased.



in exterior ballistics, the curve traced in space by the center of mass of a shell or rocket from the moment the projectile leaves the muzzle of the gun or the guide or barrel of the launcher and loses contact with the muzzle, guide, or barrel. The shape of a trajectory depends on the earth’s gravity and rotation as well as on the aerodynamic and reaction forces acting on the shell or rocket in flight.

Figure 1. Elements of a trajectory

Shells move along ballistic trajectories (Figure 1). In the case of small angles of fall (up to 20°), we speak of shallow trajectories and very low-angle fire; for angles of fall greater than 20°, we speak of steep trajectories and high-angle fire. When fired at airborne targets, antiaircraft shells—unlike artillery shells—have trajectories with only an ascending branch; the trajectories of rocket shells and rocket-assisted projectiles (for example, antitank rockets) have one or more powered phases, in which the rocket engines operate, and several free-flight phases. When the total length of the powered phases is small compared with the whole trajectory, the trajectory differs little from a ballistic trajectory; if the flight is controlled over the entire length of the trajectory or a substantial part of it, the trajectory differs considerably from a ballistic one.

Figure 2. The trajectory of a ballistic missile: (OA) launch phase, (AB) insertion phase, (BK) guided phase, (KD) segment outside the atmosphere, (DC) atmospheric phase

The trajectories of ballistic missiles have powered and free-flight phases (Figure 2). In the powered phase of a trajectory the specified speed and tilt angle that a ballistic missile should have at the end of this phase are imparted to the missile. The free-flight phase consists of two segments, a segment outside the atmosphere, in which the missile’s nose cone moves like a freely thrown body, and an atmospheric segment, in which the nose cone is stabilized and approaches the target nose first.


Dmitrievskii, A. A. Vneshnaia ballistika. Moscow, 1972.
Dmitrievskii, A. A. Fizicheskie osnovy poleta raket. Moscow, 1962. (With V. N. Koshev.)




the continuous curve described by a point in motion. If the trajectory is a straight line, the motion of the point is said to be rectilinear; otherwise, it is said to be curvilinear. The shape of a trajectory of a free material particle depends on the forces acting on the particle, the initial conditions of the motion, and the reference frame used. For a constrained particle the trajectory depends also on the constraints that are imposed (seeCONSTRAINT).

For example, a vertical line is the trajectory with respect to the earth (if we neglect its diurnal rotation) of a free particle that is released without an initial velocity and moves under the influence of gravity. If, however, an initial velocity v0 not directed along the vertical is imparted to the particle, then, in the absence of air resistance, the trajectory is a parabola (Figure 1).

Figure 1. A parabolic trajectory

Depending on the initial velocity, the trajectory of a particle moving in a central gravitational field may be an ellipse, a parabola, or a hyperbola; in special cases, the trajectory may be a straight line or a circle. If we regard the earth’s gravitational field as a central field and neglect atmospheric resistance, then the trajectory of a particle given a horizontally directed initial velocity v0 near the surface of the earth (Figure 2) is a circle when Trajectory (orbital velocity), an ellipse when Trajectory, a parabola when Trajectory (escape velocity), and a hyperbola when Trajectory. Here, R is the earth’s radius, g is the acceleration of gravity near the earth’s surface, and the motion is considered with respect to axes that are undergoing translational motion, together with the center of the earth, relative to the stars.

In the case of a body, such as a satellite, all the above remarks apply to the trajectory of the body’s center of gravity. If, however, the direction of v0 is neither horizontal nor vertical and Trajectory, the trajectory of the particle is an arc of an ellipse; this arc intersects the earth’s surface. The center of gravity of a ballistic missile has such a trajectory.

An example of a constrained particle is a small bob suspended on a string (seePENDULUM). If the string is deflected from the vertical and released without an initial velocity, the trajectory of the bob is an arc of a circle. If, however, an initial velocity that does not lie in the plane of the string’s deflection is imparted to the bob, the trajectories of the bob may be curves of rather complicated shape that lie on the surface of a sphere; such trajectories are characteristic of a spherical pendulum. In the special case of a conical pendulum, the trajectory of the bob is a circle lying in the horizontal plane.

Figure 2. Types of trajectories in the earth’s gravitational field

The trajectories of the points of a rigid body depend on the law of the body’s motion. For a translational motion of the body the trajectories of all its points are identical, but in all other cases the trajectories are generally different for the various points of the body. For example, for an automobile wheel on a rectilinear segment of a road, the trajectory of a point on the wheel’s rim relative to the highway is a cycloid, but the trajectory of the wheel’s center is a straight line. Relative to the automobile’s body, the trajectory of a point on the rim is a circle, but the wheel’s center is a fixed point.

The determination of trajectories is important both in theoretical studies and in the solution of many practical problems.


The Great Soviet Encyclopedia, 3rd Edition (1970-1979). © 2010 The Gale Group, Inc. All rights reserved.


The path followed by a seismic wave.
A curve that intersects all the members of a given family of curves at the same angle.
The curve described by an object moving through space, as of a meteor through the atmosphere, a planet around the sun, a projectile fired from a gun, or a rocket in flight.
McGraw-Hill Dictionary of Scientific & Technical Terms, 6E, Copyright © 2003 by The McGraw-Hill Companies, Inc.
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As shown in [1], we can define parallel trajectories and predict contacts using them by means of counting top-signal oscillations ratios.
This speed advantage is more important in case where average length of trajectories is high.
Despite the large number of methods of the MR trajectories formation almost all of them consider the problem of plane motion.
A series of curves and several targets in the robot trajectories generated by TST are shown in Figures 17(a) and 17(b), respectively.
Figure 13 shows the trajectories of optimal maneuvers, random maneuvers, and fixed maneuvers; the green line shows the optimal trajectory, the orange line shows the random trajectory, and the purple line shows the trajectory of fixed maneuvers.
An estimation of the Hausdorff distance between the set of trajectories of the system and the set of trajectories generated by the mixed constrained control functions is obtained (Proposition 6).
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It is required to find at least k-1 other trajectories in the [delta] uncertainty region around it.
Since NURSE can perform several trajectories of different sizes, it can be used by people of any age, anthropomorphic sizes, and anthropometric sizes, including children and elderly people as pointed out in [32, 33].