Is the starshade going to be positioned in a fixed location?

[roineust]

Is the starshade going to be positioned in a fixed location in space or is it going to be moved around space on regular basis, in relation to where its coupled telescope will be pointing at? The reason for asking this is that i read (Methods, last section) that the starshade is going to be located at a distance of 72,000km away from its coupled telescope. If the starshade has to move in such a radius around its coupled telescope on regular basis, isn't that a lot of rocket fuel needed to make such maneuvers many times?

 

[mfb]

It has to be moved. With some planning, ion thrusters and a lot of time you can do this with tiny amounts of fuel.

Cassini made about 100 close fly-bys at Titan and many fly-bys of other moons with minimal delta_v, using the gravitational attraction of the moons to change the orbit.

 

[roineust]

How come there is such a significance in putting the James Webb at a Lagrange point, while its starshade can be satisfied with ion thruststers? Is it because the starshade weighs much less? Is it because 72,000km is still within the Lagrange point?

 

[mfb]

Getting the star shade to L2 would still need a large rocket, but moving it around once it is there would not. Moving JWST relatively to the Lagrange point wouldn't need a big rocket either, although that would be less efficient than moving the lighter star shade.

 

[stefan r]

Is the starshade going to be positioned in a fixed location in space or is it going to be moved around space on regular basis, in relation to where its coupled telescope will be pointing at? The reason for asking this is that i read (Methods, last section) that the starshade is going to be located at a distance of 72,000km away from its coupled telescope. If the starshade has to move in such a radius around its coupled telescope on regular basis, isn't that a lot of rocket fuel needed to make such maneuvers many times?

JWST will be on a Lissajous orbit (halo orbit) around L2:

The telescope will circle about the L2 point in a halo orbit, which will be inclined with respect to the ecliptic, have a radius of approximately 800,000 kilometers (500,000 mi), and take about half a year to complete.

The star shade could lead, follow, orbit inclined, or make a larger or smaller halo orbit. You could use a lot of ellipses if the 72,000 is flexible. You could line up the same star 2 times per year with minimal maneuver.
JWST plans to use 2 to 4 m/s fuel per year to stay in orbit. You could time that to help line ups. Starshade could bring extra fuel to get better shots.

 

[roineust]

2 to 4 m/s fuel

What does that term mean?

As well,
1. Would the starshade use fuel as well? More than 2 to 4 m/s per year or less?
2. Isn't a very important reason to positioning the JWT in a Lagrange point, is to save up fuel? If the answer is that it is, is 72000km wawy from the JWT, still within an area that has these fuel saving properties?

 

[stefan r]

What does that term mean?

Delta-v. Is a way of measuring thrust needed to change orbits. Getting to Sun-Earth Lagrange 2 will take around 12,000 m/s.

1. Would the starshade use fuel as well? More than 2 to 4 m/s per year or less?
2. Isn't a very important reason to positioning the JWT in a Lagrange point, is to save up fuel? If the answer is that it is, is 72000km wawy from the JWT, still within an area that has these fuel saving properties?

Satelites orbiting L2 are orbiting a point in space. If they are both circling the same point why would they have different fuel requirements?

I assume that astronomers will eventually pick specific stars that are not perfectly lined up with the plane of the orbit. Getting into the ideal location would take some fuel. Staying slightly off of an ideal halo orbit would take a little more fuel (delta-v). The estimated price listed on wikipedia was $750 million for one starshade. It would probably lighter weight than the JWST so more delta-v might mean less fuel consumed.

 

[mfb]

Satelites orbiting L2 are orbiting a point in space. If they are both circling the same point why would they have different fuel requirements?

The star shade would maneuver to block the starlight of target stars, while JWST would just make sure it stays close to L2.

Here is a detailed discussion. The star shade would get an ion thruster and one ton of xenon (see page 12), enough to move between many different targets over its lifetime of a few years (many km/s delta_v).