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Escape velocity

From Academic Kids

For the video game title, see Escape Velocity (computer game).

In physics, for a given gravitational field and a given position, the escape velocity is the minimum speed an object without propulsion, at that position, needs to have to move away indefinitely from the source of the field, as opposed to falling back or staying in an orbit within a bounded distance from the source. The object is assumed to be influenced by no forces except the gravitational field; in particular there is no propulsion, as by a rocket, there is no friction, as between the object and the Earth's atmosphere (these conditions correspond to freefall) and there is no gravitational radiation. This definition may need modification for the practical problem of two or more sources in some cases. In any case, the object is assumed to be a point with a mass that is negligible compared with that of the source of the field, usually an excellent approximation. It is commonly described as the speed needed to "break free" from a gravitational field.

One somewhat counterintuitive feature of escape velocity is that it is independent of direction, so that "velocity" is a misnomer; it is a scalar quantity and would more accurately be called "escape speed". The simplest way of deriving the formula for escape velocity is to use conservation of energy, thus: in order to escape, an object must have at least as much kinetic energy as the increase of potential energy required to move to infinite height.

Defined a bit more formally, "escape velocity" is the initial speed required to go from an initial point in a gravitational potential field to infinity with a residual velocity of zero, relative to the field. Conversely, an object starting at rest and at infinity, dropping towards the attracting mass, would reach its surface moving at the escape velocity. In common usage, the initial point is on the surface of a planet or moon. On the surface of the Earth the escape velocity is about 11.2 kilometres per second. However, at 9000 km altitude in "space", it is slightly less than 7.1 km/s.

For a body rotating about its axis the escape velocity with respect to the surface does depend on direction. E.g., for the Earth the rotational velocity is 465 m/s to the east at the equator, and the escape velocity to the east, with respect to the Earth's surface, is ca. 10.7 km/s.

Contents

List of escape velocities

  Location   with respect to Ve
on the Sun, the Sun's gravity: 617.5 km/s
on Mercury, Mercury's gravity: 4.4 km/s
at Mercury, the Sun's gravity: 67.7 km/s
on Venus, Venus' gravity: 10.4 km/s
at Venus, the Sun's gravity: 49.5 km/s
at the Earth, the Earth's gravity: 11.2 km/s
on the Moon, the Moon's gravity: 2.4 km/s
at the Moon, the Earth's gravity: 1.4 km/s
at the Earth/Moon, the Sun's gravity: 42.1 km/s
on Mars, Mars' gravity: 5.0 km/s
at Mars, the Sun's gravity: 34.1 km/s
on Jupiter, Jupiter's gravity: 59.5 km/s
at Jupiter, the Sun's gravity: 18.5 km/s
on Saturn, Saturn's gravity: 35.5 km/s
at Saturn, the Sun's gravity: 13.6 km/s
on Uranus, Uranus' gravity: 21.3 km/s
at Uranus, the Sun's gravity: 9.6 km/s
on Neptune, Neptune's gravity: 23.5 km/s
at Neptune, the Sun's gravity: 7.7 km/s
on Pluto, Pluto's gravity: 1.3 km/s
at Pluto, the Sun's gravity: 6.7 km/s
at the solar system,   the Milky Way's gravity:   ~1000 km/s

Due to the atmosphere it is not useful and hardly possible to give an object near the surface of the Earth a speed of 11.2 km/s, as these speeds are too far in the hypersonic regime for most practical propulsion systems. For an actual escape orbit a spacecraft is first placed in low Earth orbit and then accelerated to the escape velocity at that altitude, which is a little less, ca. 10.9 km/s. The required extra velocity, however, is less because the spacecraft has already been accelerated to about 8 km/s.

Calculating an escape velocity

In the simple case of the escape velocity from a single body, it can be calculated by setting the kinetic energy equal to minus the gravitational potential energy. This is because the positive kinetic energy is needed to increase the negative gravitational potential energy to zero, which applies when the object is at an infinite distance.

<math> \begin{matrix}\frac12\end{matrix} mv_e^2=\frac{GMm}{r}<math>
<math>v_e = \sqrt{\frac{2GM}{r}} = \sqrt{\frac{2\mu}{r}}<math>

where <math>v_e<math> is the escape velocity, G is the gravitational constant, M is the mass of the body being escaped from, m is the mass of the escaping body (factors out), and r is the distance between the centre of the body and the point at which escape velocity is being calculated, and μ is the standard gravitational parameter .

The escape velocity at a given height is √2 times the speed in a circular orbit at the same height, compare (14) in circular motion. This corresponds to the fact that the potential energy with respect to infinity of an object in such an orbit is minus two times its kinetic energy, while to escape the sum of potential and kinetic energy needs to be at least zero.

For a body with a spherically-symmetric distribution of mass, the escape velocity <math>v_e<math> from the surface (in m/s) is approximately 2.364×10−5 m1.5kg−0.5s−1 times the radius r (in metres) times the square root of the average density ρ (in kg/m3), or:

<math>v_e \approx 2.364 \times 10^{-5} r \sqrt \rho.\,<math>

Escape velocity given "g" and "r"

The Earth's escape speed can be derived from "g", the acceleration at the Earth's surface with calculus. It is not necessary to know the gravitational constant G or the mass M of the Earth. Let

r = the Earth's radius, and
g = the acceleration of gravity at the Earth's surface.

Above the Earth's surface, the acceleration of gravity is governed by Newton's inverse-square law of universal gravitation. Accordingly, the acceleration of gravity at height s above the center of the Earth (where s > r) is g(r/s)2. The weight of an object of mass m at the surface is gm, and its weight at height s above the center of the Earth is gm(r/s)2. Consequently the energy needed to lift an object of mass m from height s above the Earth's center to height s + ds (where ds is an infinitesimal increment of s) is gm(r/s)2 ds. Since this decreases sufficiently fast as s increases, the total energy needed to lift the object to infinite height does not diverge to infinity, but converges to a finite amount. That amount is the integral of the expression above:

<math>\int_r^\infty gm (r/s)^2 \, ds

=gmr^2 \int_r^\infty s^{-2}\,ds =gmr^2 \left[-s^{-1}\right]_{s:=r}^{s:=\infty} <math>

<math>=gmr^2\left(0-(-r^{-1})\right)=gmr.<math>

That is how much kinetic energy the object of mass m needs in order to escape. The kinetic energy of an object of mass m moving at speed v is (1/2)mv2. Thus we need

<math> \begin{matrix}\frac12\end{matrix} mv^2=gmr.<math>

The factor m cancels out, and solving for v we get

<math>v=\sqrt{2gr\,}.<math>

If we take the radius of the Earth to be r = 6400 kilometres and the acceleration of gravity at the surface to be g = 9.8 m/s2, we get

<math>v=\sqrt{2\left(9.8\ {\mathrm{m}/\mathrm{s}^2}\right)(6.4\times 10^6\ \mathrm{m})}\cong 11\,200\ \mathrm{m}/\mathrm{s}.<math>

This is just a bit over 11 kilometers per second, or a bit under 7 miles per second, as Isaac Newton calculated.

Multiple sources

The escape velocity from a position in a field with multiple sources is derived from the total potential energy per kg at that position, relative to infinity. The potential energies for all sources can simply be added. For the escape velocity this results in the square root of the sum of the squares of the escape velocities of all sources separately.

For example, at the Earth's surface the escape velocity for the combination Earth and Sun is Template:Sqr(11.2² + 42.1²) = 43.6 km/s. As a result, to leave the solar system requires a speed of 13.6 km/s relative to Earth in the direction of the Earth's orbital motion, since the speed is then added to the speed of 30 km/s of that motion.

Gravity well

In the hypothetical case of uniform density, the velocity that an object would achieve when dropped in a hypothetical vacuum hole from the surface of the Earth to the center of the Earth is the escape velocity divided by √2, i.e. the speed in a circular orbit at a low height. Correspondingly, the escape velocity from the center of the Earth would be √1.5 times that from the surface.

A refined calculation would take into account the fact that the Earth's mass is not uniformally distributed as the center is approached. This gives higher speeds.

See also Gravitational potential energy.

Orbit

If a freefalling body at any position has the escape velocity for that position, this is the case for the whole orbit. If the gravity source is a spherically symmetric body the orbit is (part of) a parabola with the center of the source as focus (parabolic trajectory), or part of a straight line through the source. When moving away from the source it is called an escape orbit, otherwise a capture orbit. Both are also known as C3 = 0 orbit.

If the body has the escape velocity with respect to the Earth, this is not enough to escape the Solar System, so near the Earth the orbit resembles a parabola, but further away it bends into an elliptical orbit around the Sun.

See also

External link

da:Undvigelseshastighed de:Fluchtgeschwindigkeit es:Velocidad de escape eu:Ihes-abiadura fr:Vitesse de libration it:Velocit di fuga he:מהירות מילוט nl:Ontsnappingssnelheid ja:宇宙速度 nn:Unnsleppingsfart pl:Prędkość ucieczki pt:Velocidade de escape fi:Pakonopeus sv:Flykthastighet zh:宇宙速度

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