juvva wrote:Varoon Shekhar wrote:Just to clear up for lay people like myself
- there is no perigee and apogee with Chandrayaan-2 at this time, right? It is headed straight for the moon. Then there is the 'lunar capture', which leads to a new set of perigees and apogees around the moon, until it sets in at a more or less 100km orbit indefinitely.
CY-2 is still in elliptical orbit around the earth, this orbit will get it close to the approaching moon around 20th August and enable lunar capture.
Few comments. For lay people as well as serious ones (with good background in physics/math).
- Juvva is basically correct but let me add and amplify:
1. Like everything in physics,
NO orbit is *exactly* elliptical but it is a *very* good approximation and sufficient to *understand* basics. Just like we say earth is a sphere but closer we get (or more details) there are mountains and other irregularities. This does not matter if we are looking from space but can matter a lot if we are climbing the mountain.
2. For most planets, (or moon or sats around earth)
we can simplify by not considering complications and treat it as a two body problem. Thus we can say moon goes around an elliptical orbit (nearly circular orbit) around earth. But there are perturbations due to effect of Sun, (and even Jupiter and Venus which are quite noticeable.
3. If
we look from earth, the orbit of CY2 looks like figure 1 (figure constructed from actual data). Each part of the "orbit" is ellipse (or nearly an ellipse). After each rocket firing, there is different ellipse.
4. When CY2 reaches moon - before LOI, - ignoring moon - the path is such that perigee is where TLI was rocket was fired and apogee is pretty close to moon.
5. After moon's "disturbance"+ LOI rocket - the orbit changes to near circular orbit virtually identical to moon.
(Or course all those ellipses have little "ripples/perturbations/variation" from true ellipse)
Figure 1:
Hope this helps.
If you are looking from the Moon..
(See figure 2 - Sorry, the data for last 3 rocket firings are not available for this picture.. so posting a picture but *not* zooming in for the "orbits" around moon.. so they look like a point only) (You have to imagine that at the end point CY2 is going into elliptical orbit around the moon).
- CY2 looks like a sat orbiting earth (closely at first and then "escaping" and reaching towards moon.
- It is approaching the moon in nearly hyperbolic orbit.
- Near perigee (closest approach to moon) LOI starts.
- The nearly hyperbolic orbit becomes elliptical orbit with very large e.
- After a few more rocket firings - the orbit becomes more circular ending in a circular orbit. (about 100 Km)
- Vikram separates and assumes a elliptical orbit (perigee 30 km or so)
- Vikram soft lands etc..
Figure 2.
*** Added later: (Created a new diagram)
Added later: I zoomed on the CY2 orbit from Moon's perspective. (I am using a plane where north pole of the moon is pointing upwards.)
From this point of view - CY2 is approaching from right -( almost a straight line, but it is a part of ellipse in X-Y plane -- I am showing Y-Z plane). After the first burn, the orbit now is ellipse, it changes inclination.. almost polar wrt to moon now.. and the orbit is less elliptical after the second burn.
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****
If you are looking from Sun:
Nothing much happens - small ripples - everything within few hundred thousand Km over the elliptical orbit of earth (about 150 million Km).
