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The Shallow Sky

Trojans in STEREO

Akkana Peck


Saturn continues to give a lovely show in the western sky, visible most of the evening. Saturn’s so pretty that you just never get tired of looking at it. And it’s a good thing, because all the rest of the planets except Pluto – Jupiter, Venus, Mars, Uranus and Neptune – are in the morning sky.

Pluto season officially starts this month, with opposition on the 23rd. The bad news: it’s in Sagittarius, right in the heart of the Milky Way, in a very crowded star field. That means there are lots of nearby stars to potentially point the way – but it also makes it tough to figure out which of those dim points is the planet. A fun challenge for experienced or aspiring Plutocrats!

There may not be many planets in our night sky, but an interesting pair of spacecraft was in the news last month, a NASA mission called STEREO.

It’s a solar observation mission, designed to view the sun in stereo. The two spacecraft will view the sun from different angles, creating images that can be combined to give a three-dimensional view of the sun and a better understanding of solar storms, specifically the huge storms known as coronal mass ejections. CMEs eject billions of tons of particles into space and can cause magnetic storms and electromagnetic interference once the particles reach our atmosphere.

The STEREO spacecraft have been flying since 2006 (and has also discovered 20 comets along the way) but they’re just now entering an interesting phase of their mission: they’re nearing the Earth’s Lagrangian regions L4 and L5.

The points are named after Joseph-Louis Lagrange, who in the 1700s came up with a way of describing orbital mechanics. His equations predicted that a two-body system, such as the Earth and the Sun, would create stable five points where a third object could orbit without its orbit being perturbed by the first two bodies. These points came to be known as Lagrangian points.

Three of them were already known. L1 is a point between the Earth and Sun (but much closer to the Earth) where the gravitational attraction of the two bodies is cancelled out: the Earth is pulling just as hard on an object at L1 as the Sun is. L2 is on the opposite side of the Earth from the Sun, about four times as far away as the moon’s orbit. L3 is on the opposite site of the Sun from us, roughly at the point the Earth will be half a year from now.

But the last two Lagrangian points were more interesting and unexpected. They lie along Earth’s orbit, but 60 degrees ahead of and behind Earth’s current position. That puts them at about 93 million miles away. And unlike the other Lagrangian points, they’re highly stable: if you put an object at L4 or L5 and perturb it a little bit, it will eventually be pulled back to the Lagrangian point. In the 1970s, an organization called the L5 Society argued that we should put a space station at L4 or L5, but that idea never got off the ground.

I’ve been talking about points, but that’s really a fib. Lagrangian “points” are based on a circular orbit with no other bodies (like the moon, or Jupiter) nearby. In the real world, with elliptical orbits and lots of other disturbances, Earth has Lagrangian regions.

Other planets do too, and those regions can hold objects in them. L4 and L5 are sometimes called a planet’s “Trojan points”, and Jupiter’s Trojan points contain quite a few asteroids, called “Trojan asteroids”.

Some people think that Earth’s Trojan points originally contained something bigger than asteroids: a Mars-sized planet called Theia, which coalesced at Earth’s L4 or L5 point, then was perturbed away by some other passing planet and ended up on a collision course with Earth: the impact thought to have created the moon, discussed by Dr. Kevin Zahnle at SJAA a few months ago.

Which brings me to the excuse for last month’s press release. The STEREO pair aren’t really that close to Earth’s theoretical L4 and L5 points – STEREO A will make its closest pass to L4 in September, while STEREO B passes closest to L5 in October.

So why was there a press release about this in April? Well, the spacecraft have just started to scan the regions with their cameras looking for asteroid-sized leftovers from Theia. They haven’t found anything yet ... but as we move toward September and October, they should get increasingly better views of the region, and it should be interesting to see what they find.


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