Re: Could a larger comet impact change the mass of ear

Let’s assume that you want to try and increase the Earth’s mass by 1 
percent, or 1/100th of it’s current mass.  How big would the impactor have 
to be?  And then, do we see such large comets or asteroids in the Solar 
System?

The Earth’s mass is roughly 6 x 10^24 kg (6.6 x 10^21 tons).  We would need 
an impactor (either comet or asteroid) that is 6 x 10^22 kg in mass to 
increase the Earth’s mass by 1% (assuming all the material is retained 
during the impact).  Now, comets are mostly made up of ice and dust, while 
asteroids are more like rocks, so they have different densities.  Comets 
have densities around 1.5 g/cc while asteroids have densities from 2 to 4 g/
cc (I’ll use 3 g/cc); this means that a comet of any given mass needs to be 
bigger than an asteroid of the same mass.  The end result is that a comet 
with a mass of 6 x 10^22 kg would have a radius of about 2000 km (1242 
miles) and an asteroid of the same mass would have a radius of about 1700 km 
(1056 miles).  (For more information on comets and asteroids, check out the 
National Space Science Data Center web page – link below.)

I was able to find two measurements for the size of comets.  Halley’s Comet 
was measured to be 16 x 8 x 7 km in size and the comet Encke was measured to 
be between 0.4 and 4 km in size.  In general, the solid parts of comets are 
very small and I would estimate that there are no comets larger than 100 km 
across in our Solar System.  In any case, there are not any comets that are 
large enough to add 1% of the Earth’s mass to the Earth if they hit us.

The largest asteroid is the asteroid Ceres with a diameter of 912 km (radius 
of 456 km), which means that it is approximately 0.01% the weight of the 
Earth.  Again, there are no known asteroids in our Solar System that are 
large enough to deliver 1% of the Earth’s mass to the Earth during an 
impact.

In any case, bodies that are 1% the mass of the Earth probably will not 
destroy the Earth.  The Earth is quite huge for our region of the Solar 
System and it has survived a number of large impacts in its past.  For 
example, the best current theory for the origin of our Moon is from a giant 
impact.  Basically, when the Earth was only a few tens of millions of years 
old a mars-sized body (a large body approximately 10% the mass of the Earth) 
slammed into the newly formed Earth.  The material that was thrown off of 
the Earth recondensed in orbit around the Earth and formed the Moon.  Since 
the Earth was so young when this happened, there is no crater that we could 
identify as being from this Moon-forming event.  This is probably the 
largest impact that the Earth could have withstood without being blown up 
entirely.  The name for this theory is the Giant Impact Hypothesis.  A great 
web page explaining all this (with lots of cool artwork depicting this giant 
impact) can be found through the Planetary Science Institute (link below).

So, the short answer to your question is that there are no bodies in our 
Solar System that are really large enough to add a lot of mass to the Earth.  
However, a secondary question that you are asking refers to what happens 
when a comet impacts the Earth.  Since a comet is very ice-rich, friction 
with the Earth’s atmosphere will cause it to heat up.  The water trapped 
inside the comet will boil and produce steam that will probably blow the 
comet apart above the Earth’s surface (depending on the comet’s size and 
speed).  This is what is thought to have happened during the Tunguska event 
over Siberia on June 30, 1908.  The explosion causes all the trees in the 
area to be uprooted and blown over and even knocked people and animals to 
the ground.  A web page about a recent expedition to the Tunguska region has 
lots of good photos of the trees (see the Photo Gallery, "Photos about 
Tunguska" at the University of Bologna, Italy, link below).  Here are two 
great general articles about the Tunguska event, with lots of photos and 
maps:

"Target Tunguska" by Andrew Chaikin, Sky & Telescope 1984, pp. 18-21.
"Journey to Tunguska" by Roy A. Gallant, Sky & Telescope 1994, pp. 38-43.

Web Pages:
National Space Science Data Center’s page on asteroids and comets:
http://nssdc.gsfc.nasa.gov/planetary/planets/asteroidpage.html

Planetary Science Institute’s page on the Giant Impact Hypothesis:
http://www.psi.edu/projects/moon/moon.html