Space Forum / Space Flight / December 2005

Some years ago, a comet made an attempt of this, at Jupiter.  The pictures
were spectacular. See http://seds.lpl.arizona.edu/sl9/sl9.html

Given the speed a comet or asteroid is likely to have, I don't think
it matters whether it hits gas (an atmosphere), or a hard surface.

Hmm. From another perspective, I'd want to find out at what depth
of a typical gas planet does the gas density surpass the density
of the earth crust.

best regards
 Patrick

Several points:

1) You know how when asteroids reach Earth's atmosphere, they're moving
so fast that air friction generates a "fireball" of superheated gases
around them? All the heat energy for that "fireball" is coming from one
source: the asteroid's motion. It trades speed for heat, just like the
brakes on your car or bike.

2) Further, aerodynamic drag increases dramatically the faster you go.
Twice as fast means four times as much drag; three times as fast means
nine times as much drag. In other words, the faster an asteroid is
moving when it enters air, the harder it will "hit the brakes."

You can see this in the difference between heat shields used for
Earth-orbiting capsules (which hit the air at 17500mph), space capsules
returning from the moon (25000mph), and the capsule dropped into
Jupiter's atmosphere (106,000mph). The orbital capsules had the
thinnest heat shields; the Apollo capsules had thicker and heavier heat
shields; the Galileo probe's heat shield was about half shielding by
weight.

3) Denser air means more drag, just like more speed.

4) When an asteroid or comet hits the atmosphere of a gas giant, it
ends up falling longer and faster than an asteroid that will hit Earth.
The gas giants are bigger and more massive than Earth, so their gravity
field is stronger. Rocks falling toward them will move faster.

5) Earth's atmosphere gets thicker toward sea level. It starts out at a
near-vacuum on the edge of space (~100 miles up) and compacts quite a
bit by the bottom. In fact, about 95% of the atmosphere is in the first
20 miles. As you've noted, gas giants have even more atmosphere than
Earth. A lot more. Their atmospheres just keeps getting denser and
thicker and deeper until the gases liquify under their own weight, and
then even turn solid.

Let's put all those ideas together.

When an asteroid or comet hits the atmosphere of a gas giant, it turns
into a meteor, just like it would on Earth. Just like an meteor in
Earth's atmosphere, the meteor in a gas giant's atmosphere "hits the
brakes" as it plows through the gas giant's atmosphere.

But an asteroid or comet hitting a gas giant's atmosphere will be
moving faster than a meteor in Earth's atmosphere (see point 4), so
it'll experience harder braking (point 2). Further, gas giants'
atmospheres get thicker than Earth's (point 5), so the asteroid or
comet will experience even harder braking (point 3).

The end result is that if the asteroid or comet goes deeper into the
gas giant's atmosphere than the uppermost fringes, the asteroid or
comet will come to a complete halt, almost as if it hit solid ground.
The braking will be so hard that G-forces and heat will pulverize even
solid asteroids, let alone comets.

For example, Comet Shoemaker-Levy-9 hit Jupiter some years ago. You can
see the huge fireballs its separate parts made: they're just like
asteroid impacts on Earth.

http://www.mso.anu.edu.au/albums/shoemakerlevy/Impact_G.gif
http://antwrp.gsfc.nasa.gov/apod/image/0011/sl9gevol_hst_big.jpg
http://astro.uchicago.edu/cara/research/ir/spirex/comet.gif
http://jumk.de/astronomie/img/shoemaker.jpg

Incidentally, when the Galileo spacecraft dropped a probe into Jupiter,
the probe went from 106,000mph to a halt in 4 minutes. That's about
20Gs just due to air friction. The probe was much more streamlined than
a comet or asteroid.

However, if the asteroid does just skim through the upper, thinnest
fringes of the atmosphere, it can avoid coming to a halt. This
(probably) happened on Earth in 1972:

http://comets.amsmeteors.org/meteors/1972.html
http://gep.alien.de/ifo/images/ifo_meteor01.jpg

Mike Miller

No.
Basically, once you get to where the mass of the planets atmosphere that
the comet/asteroid intersects exceeds the mass of the asteroid - at
orbital speeds, it's stopped.
In practice, this means that any but grazing encounters plow in.

No.  The atmosphere gets dense quite quickly with depth in relation to
the planet's size, so it's really more liquid than gas.