What is a comet ?

Comet Halley orbits the sun with a period of 76 years and, in 1986,had a distance of closestapproach to the Sun, its perihelion distance R_p ,of 8.9 xx 10^10m .Table 13-3 shows that this is between the orbits of Mercury and Venus. (a) What is the comet's farthest distance from the Sun, which is called its aphelion distance R_a ?

Halley's comet has a period of 76 years and in the year 1986, had a distance of closest approach to the sun equal to 8.9xx10^(-10) m. What is the comet's farthest distance from the sun if the mass of sun is 2xx10^(30) kg and G=6.67xx10^(-11) MKS units?

When does a comet become visible to us ?