How to find Mass of Comets



The most famous Halleys comet visits Earth every 76 years.
Comets are basically made up of rock and ice (mostly in equal proportions). A comet has a nucleus and a long tail.
Ever imagined how is the mass of comets determined??
About comets we can know very less details. The orbit of comets can be traced easily by telescopes and using mathematics. Most comets have elliptical orbits like planets. Like planets comets revolve around the sun and have a time period.


Thus time period can't give us enough details about the mass of comets as the time periods are very large. Mass of an astronomical object is the basic pillar to find about all its properties.
The mass of comets can be estimated with an error of maximum 40% from Earth.
Scientists study about the composition of comets just as they do about planets. They study about the proportion of ice to rock and average density is estimated.
The dimensions of comets are calculated (length x height x width) and the estimate of its mass is obtained.
However this is not an accurate method and thus gives error. A comet is not any cuboidwl object to estimate volume as l x b x h. Also sometimes the front view of comet maybe different than the back and assuming the same dimensions for both is not a goid assumption.

To make an accurate estimate of its mass researchers send a probe (spaceship that lands on foreign astronomical bodies) . The probe is directed to comet. We know the orbital path and velocity of comets. Thus its easy to decide the trajectory of the comet.

When the probe is close to comet (100km) there is influence of the gravity on it. The initial velocity of probe is noted at this point and the change in velocity is also noted. The change gives an idea about force experienced by the probe (ignoring other objects in surroundings. Comets have isolated paths generally.) This gives idea about mass of comets more accurately. Till date only few comets have been invaded by machines.

Finding mass of comet outside our galaxy is still a challenge as comets being small objects are not visible if too far ( the glowing tail may be visible). As also it's difficult to send a probe there.

Also comets don't have a permanent orbit. The comets whose volatile contents once end i.e, they no more have a tail are classified as asteroids. 

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