[KevinDavis]

I guess..

[callisto]

Does that mean it doesn’t orbit the earth but follows?

Yes. It will drift behind Earth as it circles the Sun. The Spitzer Space telescope pioneered the use of Earth-trailing orbits. An advantage of this type of orbit is the clarity of the star field provided.

[cripplecreek]

An Earth-trailing heliocentric orbit with a period of 372.5 days provides the optimum approach to meeting of the combined Sun-Earth-Moon avoidance criteria within the Boeing 7925-10L (Delta-II) launch vehicle capability (launch videos). In this orbit the spacecraft slowly drifts away from the Earth and is at a distance of 0.5 AU (worst case) at the end of four years. Telecommunications and navigation for the mission are provided by NASA's Deep Space Network (DSN).

Another advantage of this orbit is that it has a very-low disturbing torque on the spacecraft, which leads to a very stable pointing attitude. Not being in Earth orbit means that there are no torques due to gravity gradients, magnetic moments or atmospheric drag. The "largest" external torque then is that caused by solar pressure. This orbit also avoids the high radiation dosage associated with an Earth orbit, but from time to time is subject to solar flares

Launch Vehicle and Orbit

Orbit diagrams and details at the site.

[GeronL]

how far behind Earth will it trail?