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Comment by geofffox | Hacker News Reader

Maybe as interesting is where this satellite is: The Lagrange L1 point. The 5 Lagrange points in essence allow orbiting bodies to break Kepler's laws. A satellite a million miles closer to the Sun shouldn't have the same orbital period as the Earth... but it does.

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That's because Kepler only assumes 1-body interaction! With 2-body, or N-body, then you get lagrange points. It doesn't "break" Kepler's law, it is just that Kepler law doesn't apply in 3-body interactions.

This might just be semantics, but you get Lagrange points with the simplest Kepler example, 1 body around the sun, or in other words, a 2 body interaction.

L1 in particular is perfect for measuring the solar wind and Coronal Mass Ejections as an early warning system [0]. DSCOVR (the probe that took this photo) is the successor to ACE which is used to predict Auroras amongst other things [1]. This data is released by NOAA as a forecast [2]. I built an Aurora simulator with a similar API [3], but I think that mainly uses magnetometers.

[0] https://en.wikipedia.org/wiki/Lagrangian_point#Spacecraft_at...

[1] http://www.nesdis.noaa.gov/DSCOVR

[2] http://www.swpc.noaa.gov/products/aurora-30-minute-forecast

[3] https://unop.uk/dev/led-strip-aurora-alerts

Actually DSCOVR is a satellite that was supposed to be used for climate change research, but then Bush was elected... Though, this whole Sun monotoring thing is something that was added twelve years after the sattelite was done, an apparetly what got it flying

So the satellite is not orbiting Earth? What is the advantage of putting the satellite there, as opposed to a regular orbit?

The satellite, like everything in our solar system, is orbiting the sun.

The sun-earth L1 lagrange point is between the sun and the earth (in every two-body gravitational system there are 5 lagrange points, so there are earth-moon lagrange points etc)

These points are interesting because the orbit behaviour of items in them is weird! Specifically, the gravitational effects of the two bodies involved work in tandem to produce interesting effects.

Any bodies positioned at the L1 lagrange point of a two-body system will stay in the same position relative to both the larger bodies. Normally, the closer you get to the thing you are orbiting the faster you orbit. If you are appropriately near another large body (like the earth) that second body "pulls" back on you [edit: as long as you are on the inside of it - if you're on the outside it pulls you forward, and you get the L2 point], causing your orbit to slow down. At L1, those forces are balanced, and the third, smaller body has the exact same orbital period as the second body.

That is, the satellite is always on a straight line drawn between the earth and the sun.

The main reason for it to be there is to measure the solar wind, an early detection system.

It also happens to be in a great position to take pictures of the sunlit side of the earth.

Interesting, thanks. I understand that the satellite needs to have some form of propulsion to keep it at L1 (it is unstable), so it must have a limited life. How long?

From a Q&A session https://www.nasa.gov/content/goddard/qa-on-noaas-dscovr-miss...

The mission is slated for 2 years, and they have fuel for 5. I also happen to know that they reached L1 faster than anticipated thanks to a really nice insertion from the SpaceX Falcon 9 launch vehicle.

This means that the operational life might be even longer, but I haven't found any sources for what that might be.

It's nost just with Lagrange points. Actually, most of the satellites need to carry some fuel for station-keeping - the combined effects of non-uniform mass distribution in the orbited body, solar radiation, gravitational pull of other bodies and even air drag will slowly alter, or even completely degrade your orbit.

See [0] for more: https://en.wikipedia.org/wiki/Orbital_station-keeping

Every satellite needs to use propulsion for station keeping, satellites at LaGrange points included. Orbits aren't as stable in real life as they are in math.

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