Venus & Mercury, closer to the Sun, orbit more quickly, in 225 & 88 Earth days, respectively.

Mars is further out & takes 687 days to orbit; Jupiter 4332, Saturn 10759, Uranus 30685, & Neptune 60,189 days.

Ditto for non-planets like Pluto. Cough 😳

Image: Wikipedia/WP

18/
The relationship between the distance of a planet from the Sun & its orbital period was discovered by Johannes Kepler.

His third law says the square of the orbital period of a planet is proportional to the cube of its orbital semi-major axis.

19/

So, if your orbit is a bit further from the Sun than Earth's, you'll orbit a bit more slowly.

So, at 1.5 million km further out than Earth (i.e. 1% larger than Earth's orbit), you'd take 1.5% longer to go around the Sun, i.e. 370.74 days.

20/
So that means you'd fall behind Earth & slowly drift away from it over the years.

Except (& this is what Lagrange found), when you account for Earth's gravity as well, things change – it adds extra pull into the equation.

21/
As Kepler would have you drift slowly behind Earth, Earth's gravity pulls you back into line. And if for some reason, you started to drift *ahead* of Earth, the same would happen: Earth's additional gravity would pull you back along the orbit & hold you in place.

22/
Those are the uphill parts of the saddle: it's hard to drift along the orbit from L2.

But if you start falling towards Earth radially, you're in trouble: Earth's gravity gets stronger & pulls you harder. You fall off L2 & towards Earth.

23/
Similarly, if you move further away from Earth radially, its gravitational pull gets weaker with distance & you just keep falling away.

That explains the downhill slopes of the saddle at L2, as well as at L1 & L3.

That's why L1, L2, & L3 are not stable equilibria.

24/
But if you can find a way of staying near L2 or L1, then you'll co-orbit the Sun with Earth in 365.25 days & at a nice distance of about 1.5 million kilometres.

Not too close to Earth's heat & not too far to mean low data rates.

25/
Now obviously L1 is no use for #JWST, because it’s 1.5 million km towards the Sun, so will have the hot Sun always on one side & the warm Earth always on the other.

A kind of lopsided toaster.

26/
L1 is good for solar missions, studying the Sun from an unchanging viewpoint (ISEE-3, SOHO, ACE, Genesis, WIND, Chang'e 5), missions that view Earth (DSCOVR), or just for stability away from Earth (LISA Pathfinder).

27/
But L2 is the site of choice for many astronomy missions, including past ones like WMAP, Herschel, & Planck, present ones like @esagaia & #JWST, & future ones like Euclid, NGRST, Ariel, LiteBIRD, plus as the holding point for Comet Interceptor.

28/
@ESAGaia Keeping both Earth & the Sun on one side of the spacecraft lets them use a sunshield to block the heat, allowing the other side to get very cold & dark.

All the while not too far from Earth, so lots of data can be sent back. For example, #JWST can send 28Mbps from L2.

29/
@ESAGaia But because L2 isn't stable (remember the saddle?), you can't just plonk a spacecraft there.

Instead, you go into an orbit *around* L2. Yes, around an empty point in space, but where gravity, centripetal force, & maths act to create a kind of stealth "Planet Lagrange" 🙂

30/
There are whole families of different orbits which can be used around L2.

For example, Gaia is in a so-called Lissajous orbit, which looks like this as seen from Earth, rising above & falling below the ecliptic, & moving ahead & behind Earth.

esa.int/ESA_Multimedia…

31/
With the MCC-2 burn, JWST will enter a halo orbit around L2, a special sub-class of Lissajous orbit.

It's ~1.6 million km wide along the Earth orbit direction, but rises & falls ~400,000km out of the plane, & is tilted. Each orbit takes ~6 months.

jwst-docs.stsci.edu/jwst-observato…

32/
So, while an object strictly at L2 would always rise to its highest point in the night sky at midnight (think about it 🙂), in reality, #JWST will move around by +/- 30º or so at midnight over the course of six months.

33/

As #JWST moves in its orbit around L2 & Earth's axis tilts during the year, this would make for an interesting artificial analemma, either for optical or radio observers, the latter monitoring the data being beamed back.

Photo: Anthony Ayiomamitis

en.wikipedia.org/wiki/Analemma

34/
Another key aspect of this halo orbit around L2 is the out-of-the-ecliptic part, which ensures that #JWST never experiences an eclipse where Earth blocks the Sun.

As #JWST is solar-powered, this would not be a good thing 😱

35/
Of course, the Sun & Earth are not the only forces acting at L2 in reality: there's also the gravity of the Moon & planets to consider, plus the solar radiation pressure.

These will all act to perturb #JWST's halo orbit & it will need to be adjusted every ~21 days.

36/
This will be done with small SCAT thrusters on #JWST, using a mix of hydrazine & nitrogen tetroxide.

Those are on the hot side of the observatory, to avoid any possible contamination of the optics. That means they can essentially only push #JWST away from Earth.

37/
Thus to avoid the possibility of accidentally pushing #JWST over the top of the L2 saddle & away from Earth forever (because there are no thrusters on the cold side to push it back), the whole #JWST orbit is slightly biased to stay on the Earth side of the saddle.

38/
Because the orbit is large & the orbital speed around L2 low, only ~1km/s, changes are slow & easily controlled with those thruster burns.

All to keep #JWST where it needs to be, in the Goldilocks spot able to stay cold & deliver lots of science data back home.

39/
Which brings us back to MCC-2, the burn that will inject #JWST into that halo orbit around L2 today.

It's a small one, just ~1.5 metres/sec velocity adjustment, using little fuel, similar in some ways to the routine ones that'll come later.

40/

But once the MCC-2 burn is complete, I think it’ll be fair to say that we’re “home”, & that another major step will have been taken on #JWST’s journey, a step closer to the science we’ve all been waiting for so long.

41/
There’s still much work to be done by the teams, aligning, focussing, & phasing the optics, cooling the instruments & commissioning them, & first science won’t come until June.

But still, huge positive progress so far for the @nasa/@esa/@csa_asc #JWST 🙇‍♂️🖖🤘

42/42
@NASA @esa @csa_asc Coda: I also promised a "when" at the start of this thread.

MCC-2 is due to take place at 14:00 Baltimore time, so in about 5 minutes time 🙂

There's a press event an hour later to get the latest news: nasa.gov/press-release/…
This should continue the broken thread until the end (42 tweets in all), if I’m lucky 🤞

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More from @markmccaughrean

Jan 24
A few hours from now, #JWST will make its Mid-Course Correction 2 (MCC-2) burn, injecting it into its operational orbit around the Sun-Earth L2 point, ~1.5 million kilometres away.

What, why, how, when?!

A thread.

1/
First, a reminder: #JWST was launched on #Ariane5 #VA256 from Europe's spaceport in French Guiana on 25 December 2021. The #Ariane5 put it on a near-perfect trajectory towards L2 & two subsequent JWST Mid-Course Corrections have tweaked that.

2/
But why such a long journey to a place that's about four times further away from Earth than the Moon?

By contrast, the Hubble Space Telescope is in a low Earth orbit ~535km above the surface, making it accessible to several servicing missions over the past 31 years.

3/
Read 19 tweets
Jan 8
As the start of the last major #JWST deployment approaches, the starboard primary mirror wing, it's time for a thread about what that helps enable – excellent spatial resolution.

It's #SharpnessSaturday (yes, the hashtag symbol also denotes a "sharp" in music 🙂)
So what do we mean by "spatial resolution"?

It's a way of quantifying the sharpness of an image scene, the amount of detail visible at small scales, or at some rather fundamental level, how close two things can be in a scene & still be separated.

2/
For astronomers, that's often simplified to saying "how close can two stars of equal brightness be on the sky & still separable or resolvable?"

That's not to say the stars need actually be close in space, but just how they appear on the sky.

3/
Read 34 tweets
Jan 8
And … as one third of you got right, the correct answer is “Glass” 🙂

Yes, even though the beryllium mirrors of #JWST are coated with a highly infrared reflective 100nm layer of gold, that in turn is coated with a thin layer of SiO2 (aka silica) to protect it from dings.

1/
Gold is soft & easily scratched, hence the overcoat. Silica is used in many applications, but in this form, it’s reasonable to refer to it as glass.

Of course, almost all photons hitting #JWST pass straight through before hitting & getting reflected by the gold, but still.

2/
A little more information here

jwst.nasa.gov/content/observ…

and here

laserfocusworld.com/test-measureme…

and thanks again to @apolitosb for posing the question. What we haven’t found out yet is how thick the SiO2 layer is – probably similar to the 100nm of gold, but more exactly … 🤷‍♂️

3/3
Read 4 tweets
Jan 7
Today’s the day – #JWST starts spreading the wings around its eyes 👀

And yes, I know this joke would make more sense if this was a pit viper rather than a cobra, but they don’t have a deployable hood 🐍🤷‍♂️

1/
That is, cobras are members of the elapid family of snakes, whereas pit vipers, including rattlesnakes, are from the crotaline family.

And what pit vipers share with boas & pythons is an ability to sense the infrared, which cobras lack.

2/
Not with their eyes, but via specialised “pit organs” near their snout. These hold a thin membrane with many nerve endings & blood vessels: the former sense infrared light (aka heat) between 5 & 30 microns, & the latter cool the membrane to refresh it.

en.m.wikipedia.org/wiki/Infrared_…
Read 14 tweets
Jan 6
Morning. As we near the end of #JWST’s deployments (& how mad is that?! 😱), the big focus (😉) will naturally be on the primary mirror wings swinging into place 🔭

But don’t forget the aft deployable radiator, key to the instruments keeping their cool as they do science 😎

1/
Unfortunately I don’t have time today for a megathread on the physics of the radiator, as I’m going to be driving my son back to university in Groningen.

But there’s one equation to keep in mind with the ADR: sigma.A.T^4

2/
That’s the amount of power radiated by a black body in Watts & can be used to determine how big that radiator & others on the roof of the Integrated Science Instrument Module (ISIM) need to be to shed the heat generated by the instruments.

3/
Read 12 tweets
Jan 2
As we wait for the #JWST sunshield tensioning to begin after a day of well-deserved down time for the mission team, let’s talk in a bit more detail about how having a big, cold telescope helps us detect faint things.

Yes, folks, it’s Signal-To-Noise Sunday 😬

1/
Now, a health & safety warning – this could get a bit technical, maybe mathematical, & possibly even quantum mechanical 😱

I don’t know – I haven’t planned this thread at all, so it’ll just come out as a stream of consciousness. But hopefully a semi-intelligible one 🤷‍♂️🙂

2/
In the optical & infrared parts of the electromagnetic spectrum (i.e. light), as well as at higher energies like the ultraviolet, X-rays, & gamma rays, we typically thing of it comprising photons, individual packets or quanta, like little bullets of energy.

3/
Read 38 tweets

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