Physics/basic orbital capture (#8857)

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---------

Co-authored-by: github-actions <${GITHUB_ACTOR}@users.noreply.github.com>
Co-authored-by: pre-commit-ci[bot] <66853113+pre-commit-ci[bot]@users.noreply.github.com>
Co-authored-by: Christian Clauss <cclauss@me.com>

DIRECTORY.md

@@ -741,6 +741,7 @@
 
 ## Physics
   * [Archimedes Principle](physics/archimedes_principle.py)
+  * [Basic Orbital Capture](physics/basic_orbital_capture.py)
   * [Casimir Effect](physics/casimir_effect.py)
   * [Centripetal Force](physics/centripetal_force.py)
   * [Grahams Law](physics/grahams_law.py)

physics/basic_orbital_capture.py

@@ -0,0 +1,178 @@
+from math import pow, sqrt
+
+from scipy.constants import G, c, pi
+
+"""
+These two functions will return the radii of impact for a target object
+of mass M and radius R as well as it's effective cross sectional area σ(sigma).
+That is to say any projectile with velocity v passing within σ, will impact the
+target object with mass M. The derivation of which is given at the bottom
+of this file.
+
+The derivation shows that a projectile does not need to aim directly at the target
+body in order to hit it, as  R_capture>R_target. Astronomers refer to the effective
+cross section for capture as σ=π*R_capture**2.
+
+This algorithm does not account for an N-body problem.
+
+"""
+
+
+def capture_radii(
+    target_body_radius: float, target_body_mass: float, projectile_velocity: float
+) -> float:
+    """
+    Input Params:
+    -------------
+    target_body_radius: Radius of the central body SI units: meters | m
+    target_body_mass: Mass of the central body SI units: kilograms | kg
+    projectile_velocity: Velocity of object moving toward central body
+        SI units: meters/second | m/s
+    Returns:
+    --------
+    >>> capture_radii(6.957e8, 1.99e30, 25000.0)
+    17209590691.0
+    >>> capture_radii(-6.957e8, 1.99e30, 25000.0)
+    Traceback (most recent call last):
+        ...
+    ValueError: Radius cannot be less than 0
+    >>> capture_radii(6.957e8, -1.99e30, 25000.0)
+    Traceback (most recent call last):
+        ...
+    ValueError: Mass cannot be less than 0
+    >>> capture_radii(6.957e8, 1.99e30, c+1)
+    Traceback (most recent call last):
+        ...
+    ValueError: Cannot go beyond speed of light
+
+    Returned SI units:
+    ------------------
+    meters | m
+    """
+
+    if target_body_mass < 0:
+        raise ValueError("Mass cannot be less than 0")
+    if target_body_radius < 0:
+        raise ValueError("Radius cannot be less than 0")
+    if projectile_velocity > c:
+        raise ValueError("Cannot go beyond speed of light")
+
+    escape_velocity_squared = (2 * G * target_body_mass) / target_body_radius
+    capture_radius = target_body_radius * sqrt(
+        1 + escape_velocity_squared / pow(projectile_velocity, 2)
+    )
+    return round(capture_radius, 0)
+
+
+def capture_area(capture_radius: float) -> float:
+    """
+    Input Param:
+    ------------
+    capture_radius: The radius of orbital capture and impact for a central body of
+    mass M and a projectile moving towards it with velocity v
+        SI units: meters | m
+    Returns:
+    --------
+    >>> capture_area(17209590691)
+    9.304455331329126e+20
+    >>> capture_area(-1)
+    Traceback (most recent call last):
+        ...
+    ValueError: Cannot have a capture radius less than 0
+
+    Returned SI units:
+    ------------------
+    meters*meters | m**2
+    """
+
+    if capture_radius < 0:
+        raise ValueError("Cannot have a capture radius less than 0")
+    sigma = pi * pow(capture_radius, 2)
+    return round(sigma, 0)
+
+
+if __name__ == "__main__":
+    from doctest import testmod
+
+    testmod()
+
+"""
+Derivation:
+
+Let: Mt=target mass, Rt=target radius, v=projectile_velocity,
+     r_0=radius of projectile at instant 0 to CM of target
+     v_p=v at closest approach,
+     r_p=radius from projectile to target CM at closest approach,
+     R_capture= radius of impact for projectile with velocity v
+
+(1)At time=0  the projectile's energy falling from infinity| E=K+U=0.5*m*(v**2)+0
+
+    E_initial=0.5*m*(v**2)
+
+(2)at time=0 the angular momentum of the projectile relative to CM target|
+    L_initial=m*r_0*v*sin(Θ)->m*r_0*v*(R_capture/r_0)->m*v*R_capture
+
+    L_i=m*v*R_capture
+
+(3)The energy of the projectile at closest approach will be its kinetic energy
+   at closest approach plus gravitational potential energy(-(GMm)/R)|
+    E_p=K_p+U_p->E_p=0.5*m*(v_p**2)-(G*Mt*m)/r_p
+
+    E_p=0.0.5*m*(v_p**2)-(G*Mt*m)/r_p
+
+(4)The angular momentum of the projectile relative to the target at closest
+   approach will be L_p=m*r_p*v_p*sin(Θ), however relative to the target Θ=90°
+   sin(90°)=1|
+
+    L_p=m*r_p*v_p
+(5)Using conservation of angular momentum and energy, we can write a quadratic
+   equation that solves for r_p|
+
+   (a)
+    Ei=Ep-> 0.5*m*(v**2)=0.5*m*(v_p**2)-(G*Mt*m)/r_p-> v**2=v_p**2-(2*G*Mt)/r_p
+
+   (b)
+    Li=Lp-> m*v*R_capture=m*r_p*v_p-> v*R_capture=r_p*v_p-> v_p=(v*R_capture)/r_p
+
+   (c) b plugs int a|
+    v**2=((v*R_capture)/r_p)**2-(2*G*Mt)/r_p->
+
+    v**2-(v**2)*(R_c**2)/(r_p**2)+(2*G*Mt)/r_p=0->
+
+    (v**2)*(r_p**2)+2*G*Mt*r_p-(v**2)*(R_c**2)=0
+
+   (d) Using the quadratic formula, we'll solve for r_p then rearrange to solve to
+       R_capture
+
+    r_p=(-2*G*Mt ± sqrt(4*G^2*Mt^2+ 4(v^4*R_c^2)))/(2*v^2)->
+
+    r_p=(-G*Mt ± sqrt(G^2*Mt+v^4*R_c^2))/v^2->
+
+    r_p<0 is something we can ignore, as it has no physical meaning for our purposes.->
+
+    r_p=(-G*Mt)/v^2 + sqrt(G^2*Mt^2/v^4 + R_c^2)
+
+   (e)We are trying to solve for R_c. We are looking for impact, so we want r_p=Rt
+
+    Rt + G*Mt/v^2 = sqrt(G^2*Mt^2/v^4 + R_c^2)->
+
+    (Rt + G*Mt/v^2)^2 = G^2*Mt^2/v^4 + R_c^2->
+
+    Rt^2 + 2*G*Mt*Rt/v^2 + G^2*Mt^2/v^4 = G^2*Mt^2/v^4 + R_c^2->
+
+    Rt**2 + 2*G*Mt*Rt/v**2 = R_c**2->
+
+    Rt**2 * (1 + 2*G*Mt/Rt *1/v**2) = R_c**2->
+
+    escape velocity = sqrt(2GM/R)= v_escape**2=2GM/R->
+
+    Rt**2 * (1 + v_esc**2/v**2) = R_c**2->
+
+(6)
+    R_capture = Rt * sqrt(1 + v_esc**2/v**2)
+
+Source: Problem Set 3 #8 c.Fall_2017|Honors Astronomy|Professor Rachel Bezanson
+
+Source #2: http://www.nssc.ac.cn/wxzygx/weixin/201607/P020160718380095698873.pdf
+           8.8 Planetary Rendezvous: Pg.368
+"""