How far away from the earth is halleys comet now.?

For October 26, 2008, Halley's comet is 31.7 AU from earth -- roughly the orbital radius of Neptune.

31.7 AU is equivalent to 4.74 billion kilometers, or 2.95 billion miles.

I got this information from the JPL ephemeris generator:

http://ssd.jpl.nasa.gov/horizons.cgi

For the object, type "Halley" in the box, and select 1P/Halley. When you generate the ephemeris, one of the columns is labeled "Delta." This is the geocentric distance to the comet in astronomical units.

The geocentric distance varies about plus or minus 1 AU during the year as the earth revolves about the sun. Its maximum value will be about 36.1 AU in the summer of 2024. Halley's orbital period is about 76 years, and this is 38 years (half a period) after its last appearance in 1986.

-- edit

The Java viewer mentioned by faesson is an excellent visualization tool, but you should be aware that its precision is quite limited. The JPL ephemeris generator correctly shows that Halley's comet was at perihelion on November 16, 1835. (You have to set the "table settings" to include "heliocentric range" to see this.)

The Java viewer, on the other hand, shows the comet at perihelion on March 9, 1834 -- 1.7 years too early!

I think the reason for the discrepancy is this: The Java viewer probably models the comet with simple elliptical motion. In reality, however, the comet's orbit is modified slightly every time it passes near a planet. (It's really being modified continuously, but some passages have greater effects than others.) The JPL ephemerides take these perturbations into account, but this Java viewer does not.

Of course, this problem applies to *any* modeling of bodies in the solar system. The farther a date lies in the future or the past, the more uncertain is the position of the object; and if an object has a close encounter with a planet, it's very difficult to make accurate predictions beyond that time. That's why it's difficult to predict the chance of an impact by the asteroid Apophis; after its close approach to earth in 2029, it's difficult to make an accurate prediction for its position in 2036.

Halley's orbit is highly elliptical, and focused on the Sun. Its perihelion, its closest distance to the Sun, is just 0.6 AU (between the orbits of Mercury and Venus), while its aphelion, or farthest distance from the Sun, is 35 AU, or roughly the distance of Pluto. Unusually for an object in the Solar System, Halley's orbit is retrograde; it orbits the Sun in the opposite direction to the planets, or clockwise from above the Sun's north pole. Its orbit is highly inclined (18°) to the ecliptic, with much of it lying below the orbits of the planets (assuming Earth's north pole is "up").[14][15]

wanna see where it is?

look at this simulator.

zoom out

http://astro.u-strasbg.fr/~koppen/orbitviewer/hall...