Chris Peterson wrote: ↑
Tue Dec 28, 2021 11:17 pm
alter-ego wrote: ↑
Tue Dec 28, 2021 11:07 pm
johnnydeep wrote: ↑
Sun Dec 26, 2021 10:05 pm
Very cool. Better to be in constant sun with the known effect of a constant sun shield than in inconstant shadows. I was wondering what the purpose of that complex orbit was!
So what about the orbit do you think is complex? Are referring to the halo orbit
Yes. But that isn't actually an orbit, because there's nothing at L2 that can be orbited. So it's actually a complex pattern of station keeping that keeps the telescope in the vicinity of L2.
Yes, and my response is tailored to him answering yes as you did.
I interpreted his reply as saying the "orbital" complexity was mostly, if not solely, driven by the complications regarding shadow avoidance and minimizing thermal fluctuations. I disagree. For other reasons, a halo orbit is complex anyway, regardless of mission. As you know, L1, L2 & L3 Lagrange points are dynamically unstable, and a series of orbital thrust maneuvers are required insert it into orbit and to maintain that orbit (station keeping). In detail, halo-orbit designs can vary, but in general, their primary focus is dealing with orbital instabilities, and thrust-controlled loops appear to be a common way to do this. So, when I look at halo orbits for SOHO (L1) and JWST (L2), I can't see any difference in fundamental shapes, and I can't assign any mission-dependent orbital planning just by looking at those halo orbits (maybe JWST's loops are larger to assure 10 years of shadow avoidance?). I don't have doubt that JWST is the most sophisticated and technically demanding telescope mission - managing shadow avoidance (Earth and moon) over 10-year lifetime and the uncertainty of thrust / fuel demands for orbital corrections are critical. I think this sophistication and the new activities for operation at L2 are where most risks and complexities lie. Considered separately, I'd say the halo-orbit design for JWST is relatively straight forward, and fundamentally the same as other halo-orbits.
JWST Orbit SOHO orbit
JWST Orbit wrote:Orbit maintenance
The L2 orbit has an orbit period of 6 months. While orbits about the L2 point are inherently unstable, the orbit size is large and the orbital velocity is low (~1 km/s), so the orbit "decays" slowly. However, JWST's large sun shield, roughly the size of a tennis court, is subject to significant solar radiation pressure which results in both a force and a torque. The direction of solar force varies as the observatory's attitude changes from observation to observation. The solar torque is balanced by reaction wheels, but periodically, the accumulated momentum is dumped by firing thrusters. Because JWST operations are event-driven, the observatory attitude profile and momentum dumping cannot be accurately predicted months in advance. These 2 perturbations increase the acceleration of JWST from its orbit about L2, and necessitates more frequent orbit maintenance (station keeping) maneuvers than other Lagrange orbit missions (which are typically 3–4 times per year).
Accurate orbit determination will require daily tracking measurements over a period of 19 days, so station keeping will be performed every 21 days.
Orbit perturbations along the Sun-L2 axis have the greatest impact on orbit stability. Thrusters are mounted on the spacecraft bus (located on the side of the sun shield facing the Sun); those used for orbit correction are oriented as far away from the sun shield as possible. The sun shield can support a larger sun-pitch angle† for orbit correction than that allowed for science operations. This architecture allows thruster firing at angles up to 90° from the Sun consistent with Sun avoidance restrictions, which is sufficient to provide orbit correction in all cases.
The orbit will be biased to compensate for mean outward forces associated with gravitation of the planets and radiation pressure on the sun shield.
Again, said simply, I think the complexities of L1,L2 & L3 halo orbits are mostly driven by the common, intrinsic dynamic instabilities, not so much the mission criteria.
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