Comments on: Found: One of Neptune's Asteroid Stalkers

By: Eliza Strickland

Eliza Strickland — Wed, 18 Aug 2010 17:29:33 +0000

Thanks Brian Too — fixed the formatting & link!

– Eliza, DISCOVER online news editor

By: Brian Too

Brian Too — Tue, 17 Aug 2010 23:12:07 +0000

@5. Joseph Calamia,

Your link is flawed, it generates a 404 Page Not Found error. The problem appears to be that the terminating bracket was included in the link address, whereas you meant that as formatting text in your message. I’m surmising.

Fortunately the WMAP site gives you a search box right on the error page, you can type in Lagrange, and it takes you right there.

By: Joseph Calamia

Joseph Calamia — Tue, 17 Aug 2010 15:32:49 +0000

Brian Too, thanks.

From this Wilkinson Microwave Anistopy Probe outreach page: the L4 and L5 points are home to stable orbits so long as the mass ratio between the two large masses exceeds 24.96.

In this case, the two masses are Neptune and the Sun. The ratio of the Sun’s mass to Neptune’s is about 19,000. The ratio of Earth’s mass to our moon’s is about 81.

By: Eliza Strickland

Eliza Strickland — Tue, 17 Aug 2010 15:28:56 +0000

@ Brian Too : Yes! Two of Saturn's moons, Tethys and Dione, have been shown to have smaller moons at their Lagrangian points.

By: Brian Too

Brian Too — Tue, 17 Aug 2010 01:54:21 +0000

I hadn’t thought of this before, but I guess every planet has Lagrange points.

Do major satellites also have gravitationally significant Lagrange points too? Such as the Moon, Titan, Ganymede, and so forth?

By: Joseph Calamia

Joseph Calamia — Sat, 14 Aug 2010 02:20:02 +0000

ChH. Thanks so much for the correction. You are right; 2008 LC18 is almost the same distance from Neptune and the sun. I’m sorry for my mistake–I’ve updated the post above. The bodies’ motions–and their associated “apparent” forces such as the centrifugal and Coriolis effects–also enter into the balancing game to create these stable points: http://www.physics.montana.edu/faculty/cornish/lagrange.html
http://esamultimedia.esa.int/docs/edu/HerschelPlanck/EN_13e_L_Points_EarthMoonSystem.pdf

Thanks again,
Joe

By: ChH

ChH — Fri, 13 Aug 2010 23:17:54 +0000

oops – Netpune is not “much closer”
The L4 and L5 Lagrange points are almost exactly the same distance from the sun as from the planet stabilizing that orbital slot. The three points form an equilateral triangle.

Objects near L4 or L5 orbit the sun, but after having their orbits perterbed by massive bodies like other planets, Netpune’s weak but consistent gravitation force causes them to return to their former slot – therefore the orbit is stable.
L1, L2 and L3, by comparison, are unstable – an object could orbit the sun at the same period as Neptune despite being closer or further – but if perterbed will not tend to return to those spots. That’s why satellites such as WIND, SOHO, ACE and Planck – all at Earth-Sun L1 or L2 – must station-keep to remain in those orbits.