Starship Asterisk*

by **bystander** » Thu Sep 17, 2009 1:14 am

There was an interesting article about Jupiter and its gravity well at Astronomy.com. It even talked about Hilda and the latest impact.

http://www.astronomy.com/asy/default.aspx?c=a&id=8616

by **neufer** » Thu Sep 17, 2009 12:16 am

Chris Peterson wrote:
karlhein wrote:Someone in this string was talking about the 'climatic effect' on the Jovian atmosphere. I seem to recall reading a long time ago that someone had estimated that the presence of Jupiter decreased the number of impacts on the Earth, secondary to the size, location and gravitational influence of Jupiter; and that this may have had a favorable effect on the climate and the evolution life on Earth.
Jupiter can sweep up some incoming comets. But it can also divert material towards us, and it is the primary gravitational perturber in the Solar System, certainly responsible for a lot of the smaller stuff that hits us. I've never seen anything one way or the other that really convinced me that Jupiter could be considered either a positive or negative force (in the balance) for how conditions evolved on Earth.

Certainly the Earth's cometary situation is probably only acerbated by Jupiter(; just consider how close we came to being smashed by a Jupiter captured Halley!).

OTOH, Jupiter must certainly be considered a plus in so far as asteroids are concerned.

http://en.wikipedia.org/wiki/Asteroid wrote:
<<It is believed that planetesimals in the main asteroid belt evolved much like the rest of the Solar Nebula until Jupiter neared its current mass, at which point excitation from orbital resonances with Jupiter ejected over 99% of planetesimals in the belt. Both simulations and a discontinuity in spin rate and spectral properties suggest that asteroids larger than approximately 120 km (75 mi) in diameter accreted during that early era, whereas smaller bodies are fragments from collisions between asteroids during or after the Jovian disruption.

In the Nice model, a large number of Kuiper Belt objects are captured in the outer Main Belt, at distances greater than 2.6 AU. Most were subsequently ejected by Jupiter, but those that remained may be the D-type asteroids, and possibly include Ceres.>>

by **Chris Peterson** » Mon Sep 14, 2009 11:13 pm

karlhein wrote:Someone in this string was talking about the 'climatic effect' on the Jovian atmosphere. I seem to recall reading a long time ago that someone had estimated that the presence of Jupiter decreased the number of impacts on the Earth, secondary to the size, location and gravitational influence of Jupiter; and that this may have had a favorable effect on the climate and the evolution life on Earth. Is this true, or am I just having memory flashes?

Jupiter can sweep up some incoming comets. But it can also divert material towards us, and it is the primary gravitational perturber in the Solar System, certainly responsible for a lot of the smaller stuff that hits us.

I've never seen anything one way or the other that really convinced me that Jupiter could be considered either a positive or negative force (in the balance) for how conditions evolved on Earth.

by **karlhein** » Mon Sep 14, 2009 11:00 pm

Someone in this string was talking about the 'climatic effect' on the Jovian atmosphere. I seem to recall reading a long time ago that someone had estimated that the presence of Jupiter decreased the number of impacts on the Earth, secondary to the size, location and gravitational influence of Jupiter; and that this may have had a favorable effect on the climate and the evolution life on Earth. Is this true, or am I just having memory flashes?

by **geckzilla** » Thu Aug 06, 2009 1:44 pm

by **apodman** » Thu Aug 06, 2009 1:19 pm

geckzilla wrote:I'm going to go impact my head into a door frame now.

Easy, Jean-Luc, we need that head for the next mission.

by **geckzilla** » Thu Aug 06, 2009 11:30 am

Heh, you act like it was a peaceful, happy event. I'm going to go impact my head into a door frame now.

by **Storm_norm** » Thu Aug 06, 2009 4:03 am

I know, I know, not trying to convince. just trying to make a point.
even a kid can see that this impact that the object had on jupiter is at the bottom of the impact scale, if there is such a scale. the event is insulting to the word "impact".
if I were the word impact, and I knew the types of astronomical impacts that go in the universe, i'd be downright insulted that my name was even used.
Jupiter absorbed the comet.
Jupiter enveloped the comet.
the comet was assimilated, ingested, swallowed whole.

the comet entered the cloudy mist and became part of the collective.

by **Chris Peterson** » Thu Aug 06, 2009 2:40 am

Storm_norm wrote:from webster's online dictionary.org.
http://www.websters-online-dictionary.o ... ion/impact

1. A single collision of one mass in motion with a second mass which may be either in motion or at rest. 2. Specifically, the action or event of an object, such as a rocket, striking the surface of a planet or natural satellite, or striking another object; the time of this event, as in from launch to impact. 3. To strike a surface or an object.4. Of a rocket or fallaway section: To collide with a surface or object, as in the rocket impacted 10 minutes after launch
the connotation of the word "impact" is that the comet or object made contact with a surface. I'd like to argue that any object that falls into the depths of the jupiter gases doesn't actually make contact with a surface.

You can argue it all you want, but I don't think you're going to convince many people that this wasn't a collision. A comet striking Jupiter certainly meets the very first definition given- one mass in motion striking another. There is no requirement for a "surface". Astronomically, we talk of all sorts of collisions. Star systems form because diffuse clouds of gas and dust collide. This is normal usage of the term.

by **Storm_norm** » Thu Aug 06, 2009 2:04 am

from webster's online dictionary.org.
http://www.websters-online-dictionary.o ... ion/impact

1. A single collision of one mass in motion with a second mass which may be either in motion or at rest. 2. Specifically, the action or event of an object, such as a rocket, striking the surface of a planet or natural satellite, or striking another object; the time of this event, as in from launch to impact. 3. To strike a surface or an object.4. Of a rocket or fallaway section: To collide with a surface or object, as in the rocket impacted 10 minutes after launch

the connotation of the word "impact" is that the comet or object made contact with a surface. I'd like to argue that any object that falls into the depths of the jupiter gases doesn't actually make contact with a surface. instead, jupiter incorporates the comets and other types of space debris into itself.
the scar that was left on jupiter is a blemish in comparison to a real celestial impact like theia impacting the ancient earth. or the Herschel crater on the Saturn moon Mimas.
Jupiter has been swallowing up comets, asteroids and other dust in the solar system for billions of years. its a naturally occuring event that hardly deserves the term "impact."
those comets, those asteroids are just bugs on jupiter's highway to increasing its size and content.

impact ?? nah, not when describing a celestial meeting of comet bugs and Jupiter.
Jupiter eats comets for breakfast. it doesn't impact them.

by **The Code** » Mon Aug 03, 2009 8:37 pm

neufer

I stand corrected...You are the man to ask...

You very much, do know your stuff.

I am sorry for thinking otherwise.

Quality posts thanks.

Mark

by **geckzilla** » Mon Aug 03, 2009 12:19 pm

You don't have to hit something solid to make an impact, nor does size matter.

a dictionary author wrote: im·pact (ĭm'păkt')
n.
1. The striking of one body against another; collision.
2. The force or impetus transmitted by a collision.
3. The effect or impression of one thing on another: still gauging the impact of automation on the lives of factory workers.
4. The power of making a strong, immediate impression: a speech that lacked impact.

by **Storm_norm** » Mon Aug 03, 2009 8:08 am

we are calling the black mark on jupiter an "impact scar"

since Jupiter is pretty much composed of gas, what exactly did the comet (or object) impact?

or is "impact" just the generic term in this case?

Does this mean that every tiny spec of dust that encounters our atmosphere get the term "impact" ?

by **rigelan** » Mon Aug 03, 2009 3:05 am

I ride my bike with sunglasses at all times for the same reason.

apodman wrote:
grump wrote:When I rode I wore an open face helmet - hitting a (big) fly at 60mph stung a bit, and was fatal for the fly. What was really interesting was riding through a swarm of bees at that speed.
Like Ralph Nader's Corvair, bees can be unsafe at any speed. My first bee sting, acquired while riding a bicycle with my mouth open, was on the inside of my upper lip. Very inconvenient. Never rode with my mouth open again.

by **apodman** » Mon Aug 03, 2009 1:50 am

grump wrote:When I rode I wore an open face helmet - hitting a (big) fly at 60mph stung a bit, and was fatal for the fly. What was really interesting was riding through a swarm of bees at that speed.

Like Ralph Nader's Corvair, bees can be unsafe at any speed. My first bee sting, acquired while riding a bicycle with my mouth open, was on the inside of my upper lip. Very inconvenient. Never rode with my mouth open again.

by **grump** » Mon Aug 03, 2009 1:45 am

emc wrote:
mark swain wrote:emc

Feel the breeze at 100mph on a motor bike?

Now speed your bike up to 65,000mph and feel that breeze... (Just an example ..)

Mark
As a matter of fact, I have felt 100mph+ breeze on a motor bike. Interesting that you should bring it up... more interesting that I am still alive, but that is another story not the least bit astronomical.

I like neufer's and Chris Peterson's comments on the 65k+ mph... I expect hitting a bug would also be somewhat catacylsmic.

When I rode I wore an open face helmet - hitting a (big) fly at 60mph stung a bit, and was fatal for the fly. What was really interesting was riding through a swarm of bees at that speed.

by **bystander** » Sat Aug 01, 2009 11:03 pm

emc wrote:I like neufer's and Chris Peterson's comments on the 65k+ mph... I expect hitting a bug would also be somewhat catacylsmic.

Hey, Ed! No worries! They don't have windows on motor bikes.

by **emc** » Sat Aug 01, 2009 10:50 pm

mark swain wrote:emc

Feel the breeze at 100mph on a motor bike?

Now speed your bike up to 65,000mph and feel that breeze... (Just an example ..)

Mark

As a matter of fact, I have felt 100mph+ breeze on a motor bike. Interesting that you should bring it up... more interesting that I am still alive, but that is another story not the least bit astronomical.

I like neufer's and Chris Peterson's comments on the 65k+ mph... I expect hitting a bug would also be somewhat catacylsmic.

by **neufer** » Thu Jul 30, 2009 10:11 pm

Pete wrote:
neufer wrote:Actually, any deflection at all from a rear end collision will cause the parabolic orbit to pass in front of Jupiter thereby transferring cometary energy to Jupiter energy and inserting the comet into an elliptical orbit around the sun.
Isn't it also possible for a comet approaching from the rear to gain energy from Jupiter and leave the solar system? (something like this, where the Sun is off the bottom of the diagram)?

Yes, where the perijovian point is also behind Jupiter then Jupiter slows down and the comet gains energy.

Note, however, that this is more than made up for by the advancement of the perijovian point for the trajectory coming from the other side.

All this probably means is that the elliptical orbit insertion cross section is simply shifted off center from Jupiter based upon the direction of the incoming comet. The off center cross section for an off center incoming trajectory might still be as large or larger as it would be otherwise.

by **Pete** » Thu Jul 30, 2009 9:37 pm

neufer wrote:If that were the case then Jupiter's moons are in a lot of trouble

Aren't you really referring to the much smaller Roche limit?

Oops! Yeah, I was confused. Fortunately for the moons, they're within Jupiter's Hill sphere ("sphere of influence").

neufer wrote:The most common type of collision would have these velocity vectors
roughly perpendicular to each other to form a [1,sqrt(2),sqrt(3)] triangle.

Thanks.

neufer wrote:Actually, any deflection at all from a rear end collision will cause the parabolic orbit to pass in front of Jupiter thereby transferring cometary energy to Jupiter energy and inserting the comet into an elliptical orbit around the sun.

Isn't it also possible for a comet approaching from the rear to gain energy from Jupiter and leave the solar system? (something like this, where the Sun is off the bottom of the diagram)?

by **neufer** » Thu Jul 30, 2009 3:05 pm

Pete wrote:Very cool analyses, neufer.

The elliptical insertion area you compute for Jupiter, 200 solar areas, corresponds to 20 000 Jupiter areas, or about sqrt(2e4 / pi) * 7e7 m / (5.2 AU) = 0.7% of Jupiter's distance from the Sun. For comparison, the radius of Jupiter's Hill sphere (inside of which a body loosely held together will be tidally disrupted) is (1.898e27 kg / 3 / 1.989e30 kg)^(1/3) = 7% of Jupiter's distance from the Sun. It seems from this that any weakly bound object scattered by Jupiter will also be tidally ripped apart.

If that were the case then Jupiter's moons are in a lot of trouble

Aren't you really referring to the much smaller Roche limit?

Pete wrote:
neufer wrote:We know what the closing speed of a typical Oort cloud comet on Jupiter would be: v ~ 13.07 sqrt(3) km/s
(Jupiter orbital speed = 13.07 km/s).
I'm feeling unusually dense... Where does the sqrt(3) factor come from?

Jupiter is traveling at 13.07 km/s
The Oort Cloud parabolic comet is traveling at 13.07sqrt(2) km/s

The most common type of collision would have these velocity vectors
roughly perpendicular to each other to form a [1,sqrt(2),sqrt(3)] triangle.

However, for Jupiter & Neptune the cross section for "slow motion" Oort cloud comet rear ends collision is so large that most side & head on collisions can be effectively ignored. (Note that when the deflected comet returns to Jupiter in its elliptical orbit the relative velocities are exactly the same as in the original collision so that the large "slow motion" cross sections are retained even for side collisions. )

Pete wrote:
neufer wrote:However the closing speed of an Oort cloud comet coming up from behind Jupiter is: v ~ 13.07 [sqrt(2)-1] km/s
I understand that sqrt(2) - 1 comes from the fact that the parabolic velocity at Jupiter's distance is just sqrt(2) v_J.

Yes, It's just as if a returning 25,000 mph Apollo capsule had collided with the 18,000 mph Sky Lab.

Pete wrote:
neufer wrote:The amazing thing is that the surface escape velocity is 10 times the relative comet velocity (v) and
one must go out to ~ 100 Jupiter radii for the escape velocity to be comparable to the relative comet velocity [a roughly sufficient condition to inject the comet into an (e)lliptical (o)rbit [...]
In the derivation for effective collisional cross-section, the quantity 2GM/R (planet mass and radius) was replaced with v_esc, the *surface* escape velocity. Does the formula still hold if you use *local* v_esc at arbitrary distances from Jupiter? I have trouble understanding how the distance at which local escape speed roughly equals comet speed is an estimate of the e.o. cross section.

Actually, any deflection at all from a rear end collision will cause the parabolic orbit to pass in front of Jupiter thereby transferring cometary energy to Jupiter energy and inserting the comet into an elliptical orbit around the sun. Hence, one could make the case that the Hill sphere cross section is the relevant value here.

However, my proposed smaller cross section will virtually guarantee an elliptical cometary orbit that returns closely to the "scene of the crime" within a reasonable time frame.

by **Pete** » Thu Jul 30, 2009 5:17 am

Very cool analyses, neufer.

The elliptical insertion area you compute for Jupiter, 200 solar areas, corresponds to 20 000 Jupiter areas, or about sqrt(2e4 / pi) * 7e7 m / (5.2 AU) = 0.7% of Jupiter's distance from the Sun. For comparison, the radius of Jupiter's Hill sphere (inside of which a body loosely held together will be tidally disrupted) is (1.898e27 kg / 3 / 1.989e30 kg)^(1/3) = 7% of Jupiter's distance from the Sun. It seems from this that any weakly bound object scattered by Jupiter will also be tidally ripped apart.

neufer wrote:We know what the closing speed of a typical Oort cloud comet on Jupiter would be: v ~ 13.07 sqrt(3) km/s
(Jupiter orbital speed = 13.07 km/s).

I'm feeling unusually dense... Where does the sqrt(3) factor come from?

neufer wrote:However the closing speed of an Oort cloud comet coming up from behind Jupiter is: v ~ 13.07 [sqrt(2)-1] km/s

I understand that sqrt(2) - 1 comes from the fact that the parabolic velocity at Jupiter's distance is just sqrt(2) v_J.

neufer wrote:The amazing thing is that the surface escape velocity is 10 times the relative comet velocity (v) and
one must go out to ~ 100 Jupiter radii for the escape velocity to be comparable to the relative comet velocity [a roughly sufficient condition to inject the comet into an (e)lliptical (o)rbit [...]

In the derivation for effective collisional cross-section, the quantity 2GM/R (planet mass and radius) was replaced with v_esc, the *surface* escape velocity. Does the formula still hold if you use *local* v_esc at arbitrary distances from Jupiter? I have trouble understanding how the distance at which local escape speed roughly equals comet speed is an estimate of the e.o. cross section.

by **neufer** » Wed Jul 29, 2009 8:56 pm

neufer wrote:I did a more careful analysis using Pete's formula with backside impinging angles > 0º .

The amazing thing is that the surface escape velocity is 10 times the relative comet velocity (v) and
one must go out to ~ 100 Jupiter radii for the escape velocity to be comparable to the relative comet velocity
[a roughly sufficient condition to inject the comet into an (e)lliptical (o)rbit... one that returns to
impinge precisely on Jupiter's orbit for many other shots at Jupiter's solar sized collisional cross section].

Hence, the collisional cross section (initial & return) is comparable to the area of the sun while
the initial elliptical orbit insertion cross section is comparable to 100 times the area of the sun!
Code: Select all
(solar area) sigma        comet impinging
collision   e.o.          angle    speed
-------------------------------------------------
1.0         200            10º	 6.05 km/s
0.26         50            41º   12.1 km/s
-------------------------------------------------
Here, Pete's own calculation of an effective doubling of the collisional cross section
for relative comet velocity ~ escape velocity repeats itself but for this time for the
situation where these velocities are actually equivalent ... at ~ 100 Jupiter radii!

Thus the significant elliptical orbit insertion cross section
σ = 2 x (100^2) Jupiter cross sections = 200 solar areas.

The backside of Jupiter proved to be such a wonderful "seductive flame" for attracting & killing comets that I thought I would look at the solar system's first line of defense against comets: Neptune. Like Jupiter, Neptune's surface escape velocity is roughly an order of magnitude greater than the velocity of an Oort Cloud comet impinging on the planet's back side. Hence Neptune is simply a miniature version of Jupiter with all cross sections scaled down by a factor of 8 (~ Jupiter's area/Neptune's area):

Code: Select all

(solar area) sigma        comet impinging
collision   e.o.          angle    speed
-------------------------------------------------
0.125        25            06º	 2.35 km/s
0.032         6            37º    4.7 km/s
-------------------------------------------------

Neptune diameter: 3.86 earth diameters
Sun's diameter: 109 earth diameters

23.5 km/s : escape velocity at Neptune's surface
5.43 km/s : circular orbital velocity at Neptune
7.68 km/s : parabolic comet orbit velocity at Neptune

v = 7.68 - 5.43 = 2.25 km/s for 0º backside impinging angle
v = 2.35 km/s for 6º backside impinging angle
v = 4.7 km/s for 37º backside impinging angle

by **neufer** » Tue Jul 28, 2009 1:35 am

Hubble Captures Jupiter's New Spot
Source: CBC News 07/27/09 1:00PM

<<This image provided by NASA's Hubble Space Telescope taken with it's Wide Field Camera 3 on Thursday July 23, 2009 shows the sharpest visible-light picture taken of the impact feature (dark spot) and "backsplash" of material from a small object that plunged into Jupiter's atmosphere and disintegrated. This is a natural color image of Jupiter as seen in visible light. The spot is a debris plume associated with turbulence in Jupiter's atmosphere, NASA reported, estimating it was caused by the impact of an object about the size of several football fields.>>

by **neufer** » Mon Jul 27, 2009 9:21 pm

neufer wrote:Nice piece of work, Pete.

We know what the closing speed of a typical Oort cloud comet on Jupiter would be: v ~ 13.07 sqrt(3) km/s
(Jupiter orbital speed = 13.07 km/s).

This gives an increase of Jupiter's cross section by about a factor of 8 => 1000 earth cross sections

However the closing speed of an Oort cloud comet coming up from behind Jupiter is: v ~ 13.07 [sqrt(2)-1] km/s

This gives an increase of Jupiter cross section by about a factor of 122 => 15,000 earth cross sections > the area of the sun!

I did a more careful analysis using Pete's formula with backside impinging angles > 0º .

The amazing thing is that the surface escape velocity is 10 times the relative comet velocity (v) and
one must go out to ~ 100 Jupiter radii for the escape velocity to be comparable to the relative comet velocity
[a roughly sufficient condition to inject the comet into an (e)lliptical (o)rbit... one that returns to
impinge precisely on Jupiter's orbit for many other shots at Jupiter's solar sized collisional cross section].

Hence, the collisional cross section (initial & return) is comparable to the area of the sun while
the initial elliptical orbit insertion cross section is comparable to 100 times the area of the sun!

Code: Select all

(solar area) sigma        comet impinging
collision   e.o.          angle    speed
----------------------------------
1.0         200            10º	 6.05 km/s
0.26         50            41º   12.1 km/s

Here, Pete's own calculation of an effective doubling of the collisional cross section
for relative comet velocity ~ escape velocity repeats itself but for this time for the
situation where these velocities are actually equivalent ... at ~ 100 Jupiter radii!

Thus the significant elliptical orbit insertion cross section
σ = 2 x (100^2) Jupiter cross sections = 200 solar areas.
-------------------------------------------------
Values used:

Jupiter diameter: 10.86 earth diameters
Sun's diameter: 109 earth diameters

60.5 km/s : escape velocity at Jupiter's surface
13.07 km/s : circular orbital velocity at Jupiter
18.48 km/s : parabolic comet orbit velocity at Jupiter

v = 18.48 - 13.07 = 5.41 km/s for 0º backside impinging angle
v = 6.05 km/s for 10º backside impinging angle
v = 12.1 km/s for 41º backside impinging angle

Starship Asterisk*

Jupiter's New Impact Scar (2009 July 23)

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