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back to article Euro satellite to count a BEELLLION stars, find origin of Milky Way

The European Space Agency's billion-star mapper Gaia blasted off this morning aboard a Soyuz rocket on its mission to study millions of suns. The satellite has been dispatched to create the most accurate map yet of our galaxy, using accurate measurements of positions and motions of one per cent of the roughly 100 billion stars …

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A succesfull deployment - unfortunately

The previous satellite to do this, Hipparcos, had a problem after launch and the booster stage didn't separate.

However having a couple of tons of booster attached to it meant that its orbit was a lot more stable and there was a suggestion that all future astrometric satellites be launch with a couple of tons of scrap iron bolted to them

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units, units!

1.5m km. I'll try not to be pedantic and correct it to M km or Gm (damn - failed), but come on, use some real units! How much is that in brontosaurussess (brontosauri?)

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DJO
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Re: units, units!

To use a more understandable unit it is 178,997,613 London Bus (Routemaster) bus lengths. (call it 180 MBus)

I'm afraid there is some disagreement about a British Standard Brontosaurus, mainly because it hasn't been called that for ages, it's now known as Apatosaurus.

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Re: units, units!

4,285,714,285,714 Postscript points, the only unit real men use.

Approximately of course

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Anonymous Coward

Re: units, units!

C'mon now - the Reg only works in MEELLIONS, BEELLLIONS and TREELLLIONS (in caps, natch)

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Long term planning

20 years in the making and it's going to take 5 years to 'complete'*, producing a pentabyte of information for astronomers to trawl through.

Ok, so I'm not living in a hotel on the moon, nor do I have a flying car, anti-gravity belt or robot butler, but by gum in 20 to 30 years from now I'll be pretty sure of where I am in the galaxy! **

I'm glad to see that people with vision can still get projects like these off the ground.

ttfn

*Yeah right - 5 years from now the controllers will be thinking 'how many more can we do before it packs in', I suspect it will be going for a long time.

** beware of the total perspective vortex - it might ruin your day.

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Holmes

Re: Long term planning

For one thing, people would have been glad to do this ten years ago. But it's done with public money, lots of it, and that tends to slow things down; plus the public wants its fair share...

For another, knowing your place in the galaxy doesn't really mean you get anything like the total perspective on the universe! It's typical galactic hubris to assume our little milky way is more than one among very many.

And finally, a man should always know his place, no matter where he is in the galaxy.

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Re: Long term planning

It the first time I read anything that says whether this craft has the potential to continue its missions beyond 5 years.

Lets hope the powers at be keep up funding for it.

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Re: Long term planning

Since Gaia's at L2, it'll need fuel-consuming course corrections to remain in a halo orbit there. Gaia also uses nitrogen for aiming rather than reaction wheels, which have an infamous habit of wearing out at awkward times. The amount of propellant and nitrogen is limited. How those limits compare to 5-year periods, I don't know.

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Brilliant!

I cannot begin to imagine what amazing discoveries this might make!

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A pedant writes ...

"Each time, it will plot the position of the sun and its key physical properties, ..."

I thought a star wasn't a 'sun' unless it had planet(s) in orbit?

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ESA TV vs NASA TV style

I watched the launch on the net, plus some of the canned guff they showed beforehand. I must confess I far prefer the rather more minimalist NASA TV style, with a "just the facts, Ma'am" approach. The ESA commentators felt they had to fill the silence all the time, and the beforehand guff looked rather like the work of a wannabe Jean-Luc Godard. A bit like 'Noddy launches a Satellite' instead of an Open University production.

CCTV was much more impressive covering the Chinese moon landing too.

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Re: ESA TV vs NASA TV style

The 10 second 'movie' of a stationary rocket during the countdown is always a useful use of bandwidth.

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Alien

Huh?

"...Gaia is now en route to the stable point at L2..."

Who writes this tripe? L2 is stable like a pin balanced on its point is stable; ie. not very much. I'm also curious why L2? Hiding behind the moon would make sense if the Earth's radio emissions were a problem, but they are apparently using visible light. This only makes sense if they actually intend to spy on that alien base located on the farside.

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Re: Huh?

Well a pin balanced on one end is more stable than an unbalanced pin at, say, 45°. If Gaia is equiped with a whizzbang mega boffin designed stabilizer system it can presumably stay stable with minimum propellant use. This is just my guess (sorry) does anyone know the real reason for using the L2 point (apart from GoogleStreetMapping the far side of the moon of course).

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Re: Huh?

It doesn't sit exactly at L2 it orbits around it - there is still an overall resonance that keeps it around that point.

In fact you don't want it exactly at L2 because the Earth would eclipse the sun and its solar panels wouldn't get any power.

The reason for L2 (rather than something closer) is that the Earth only covers a small patch of the sky. So the Earth doesn't get in the way of the targets and there is no thermal change as the satellite passes through Earth's shadow.

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Re: Huh?

"It doesn't sit exactly at L2 it orbits around it - there is still an overall resonance that keeps it around that point."

You're confusing L2 with L4/5, which are indeed stable points. The other three points require some delta-v corrections to remain on station. However it's less critical than I assumed. Here's what Wikipedia has to say:

"Although the L1, L2, and L3 points are nominally unstable, it turns out that it is possible to find (unstable) periodic orbits around these points, at least in the restricted three-body problem. These periodic orbits, referred to as "halo" orbits, do not exist in a full n-body dynamical system such as the Solar System. However, quasi-periodic (i.e. bounded but not precisely repeating) orbits following Lissajous-curve trajectories do exist in the n-body system. These quasi-periodic Lissajous orbits are what most of Lagrangian-point missions to date have used. Although they are not perfectly stable, a relatively modest effort at station keeping can allow a spacecraft to stay in a desired Lissajous orbit for an extended period of time." http://en.wikipedia.org/wiki/Lagrangian_point

As for sunlight, That page states that at L2 some sunlight does sneak around the edges of the earth, so solar power works.

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Re: Huh?

Yep, there is some sunlight at L2: it is beyond Earth's umbra, and inside Earth's antumbra.

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Re: Huh?

"L2 is stable like a pin balanced on its point is stable; ie. not very much."

L2 is a pretty broad pin. The annual delta-V consumption to maintain an L2 halo orbit is 30 - 100m/s, which compares well with stationkeeping in geostationary orbit (50-55m/s), low Earth orbit (25m/s for a 400km orbit like the ISS), or low lunar orbit (0 to 400m/s). There are a lot of gravitational influences out there to make any orbit imperfectly stable, and sometimes other effects (e.g., low Earth orbit's atmospheric drag.)

"I'm also curious why L2?"

As a sensitive observatory, Gaia is going to L2 to get away from Earth's heat, light, radio noise, and view obstruction.

Heat: Gaia is depending on passively cooling down quite a bit (hence that sunshield parasol), and being in low Earth orbit would bring a lot of infrared heat from multiple angles (since Earth is a big disk to objects in LEO, not a point source). While an astronaut wouldn't feel toasty from Earth's radiating heat, it's enough to make passive storage of mild cryogenic liquids (e.g., liquid oxygen) difficult, or to passive cool an astronomical satellite's instruments. Moving to L2 reduces the cooling problem to the sun, which is a point source easily hidden behind a modest sunshield. (Even heating from zodiacal light can be a problem for REALLY sensitive instruments or passive liquid hydrogen storage, though I don't think Gaia cares.)

Light: glare off Earth is another bother for optical satellites like Gaia. Moving to L2 reduces Earth to (almost) a point source.

Obstruction and orbital problems: As noted above, Earth is a big object to the perspective of a satellite in LEO, blocking out more than a third of the sky. Short orbital periods means satellites are constantly having their views altered, and tidal effects can stabilize (or tumble) satellites in undesired ways. Gaia might find it annoying to be trying to aim at Ceti Alpha 6 when Earth is like, "LOL! Nope, time for gravient gradient stabilization!"

Noise and other disturbances: There's a lot of radio chatter and time spent dodging orbital debris in low Earth orbit.

Higher Earth orbits present radiation issues from the Van Allen Belts or, in the case of geostationary orbits, a lot of radio chatter and crowded traffic lanes. Geostationary is cluttered to about its limit before communication satellites cause radio cross-talk and interference problems.

So: L2. There's no traffic to speak of, no radio interference, no brutish Earth blocking views, no debris to dodge, much simpler tidal / gravity issues, and vast, empty vistas for Gaia to work with.

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Is there any rocket that's better looking than a Soyuz?

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I'm partial to the Delta IV and Delta IV heavy. I like the clean lines.

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