Europe’s galactic survey spacecraft, Gaia, recently arrived at its deep space location, ahead of conducting its primary mission of observing one billion stars. Gaia is now in a location known as Lagrange point 2 (L2), one of a number of destinations of interest for a number of future spacecraft, from JWST through to the potential “Gateway” station.
Its mission is to create a highly accurate 3D map of our Milky Way Galaxy by repeatedly observing a billion stars to determine their precise positions in space and their motions through it. The resulting census will allow astronomers to determine the origin and the evolution of our Galaxy.
The five year mission will be conducted 1.5 million kilometers from Earth, in a deep space orbit that is of a Lissajous-type around the second Lagrange point (L2).
Following launch, Gaia performed a thruster burn to set course to its destination, ahead of the recent critical maneuver that boosted Gaia into its 263,000 x 707,000 x 370,000 km, 180 day-long orbit around L2, refined by a small course correction this week.
“Entering orbit around L2 is a rather complex endeavor, achieved by firing Gaia’s thrusters in such a way as to push the spacecraft in the desired direction whilst keeping the Sun away from the delicate science instruments,” noted David Milligan, Gaia spacecraft operations manager.
Orbiting L2 is technically like orbiting around “nothing”, as explained specialists on ESA’s site this week.
“Lagrange points are special – it’s true there’s nothing there,” added Markus Landgraf, a mission analyst at ESOC, ESA’s operations centre in Darmstadt, Germany.
“They are points where the gravitational forces between two masses, like the Sun and Earth, add up to compensate for the centrifugal force of Earth’s motion around the Sun, and they provide uniquely advantageous observation opportunities for studying the Sun or our Galaxy.”
There are five Lagrangian locations where the gravitational forces and the orbital motion of the spacecraft, Sun and planet interact to create a stable location from which to make observations.
A spacecraft placed in the L2 location is 1.5 million kilometres directly ‘behind’ the Earth as viewed from the Sun and is classed as an ideal place from which to observe the larger Universe.
A spacecraft in this location does not have to orbit Earth and so is spared from sweeping in and out of our planet’s shadow, heating up and cooling down, and distorting its view.
Spacecraft making use of L2 include Herschel, Planck, and now Gaia – with the James Webb Space Telescope (JWST) set to join them later this decade.
L2 also provides a moderate radiation environment, which helps extend the life of the instrument detectors in space.
Despite the obvious benefits to spacecraft located in the L2 region, these orbits are fundamentally unstable, as further explained by ESA.
“We’ll have to conduct stationkeeping burns every month to keep Gaia around L2, otherwise perturbations would cause it to ‘fall off’ the point,” added Gaia Operations Manager David Milligan.
ESA’s flight dynamics team will utilize their suite of mathematical models and software tools to keep a close eye on Gaia’s orbit, tools they have created themselves via their own mission experience.
“That is where expertise and experience are indispensable to reconsider the assumptions and then start all over,” says Frank Dreger, Head of Flight Dynamics.
“There’s no commercial source for this sort of software or expertise – it’s been built up over many years at ESOC and represents a capability that is rare in the world and unique in Europe.”
Future use of EML2:
With JWST already set to head out to L2, the possibility of using another L2 location known as Earth-Moon L2 (EMl2) to host an Exploration Platform has been touted over recent years, as much as the plan has failed to advance from the study stage to become part of the current near-term Exploration Roadmap.
EML-2 stays fixed with the Moon as it rotates around the Earth, whereas Gaia’s Earth-Sun L-2 stays fixes with the Earth as it rotates around the Sun.
Mainly referred to as the Exploration Gateway – but also known as the Exploration Platform or L2 Waypoint – the concept calls for the launch of several modules for construction at the International Space Station (ISS), prior to departing to EML 2, serving as a deep space outpost.
Several conceptual versions of the Gateway have been complied since it first started to gain mentions in the Exploration debate last year.
The existing hardware would involve an orbiter external airlock, an MPLM (Multi-Purpose Logistics Module) habitat module, and an international module, linked by the Node 4/DHS (Docking Hub System) at the orbital outpost.
With the modules ranging from only 11mt to 13mt in mass, most Exploration Platform concepts cite the use of existing launch vehicles for lofting the hardware to the ISS.
However, the concepts that involve the use of a large amount of existing hardware are understood to be the most viable, with costs further mitigated by the use of existing launchers to loft the hardware to the ISS.
Although the extension of the ISS to 2024 – and likely to eventually be until 2028 – increases the potential for barter agreements and providing the ISS with another exploration role, NASA’s current plan only reaches as far as 2021, the current date for the second SLS and Orion mission that will visit a captured asteroid.
Technically, the Gateway would provide a staging post for missions to the Lunar Surface, NEAs, Mars and potentially other destinations. While a return to the Moon – which would likely take advantage of EML-1 – has been firmly ruled out by NASA’s current leadership, long duration missions to asteroids continues to be classed as the main goal ahead of missions to Mars.
Notably, the Boeing company already outlined a conceptual approach involving the EML2 Gateway at a Global Exploration Workshop. Under this Boeing plan, Solar Electric Propulsion would be used by NASA for NEA missions – a technology also cited by other companies in tandem with a Gateway.
Per an amalgamation of the proposals, the SEP propulsion system would be gradually developed over the next 10 years, although a demonstration flight would be capable of readiness by 2014.
Meanwhile, a NASA docking system, Spacecraft boom, triple panel SEP module, Solar Array mast, and Alpha-joint (similar to the ISS’ Beta joint) would be developed between 2016 and 2020 – all leading to the creation of a 320 kW SEP operational spacecraft for NEA missions by 2022.
Under the Boeing notional plan, a 2024 NEA mission to NEA2008EV5 would depart not from Earth but from the ISS-EP at the EML2 point.
Using the new SEP technology, transit from the EML2 point to the NEA of interest would take approximately 100 days with SLS’ third stage used to “kick start” the stage and shorten the trip. SLS would be involved with the Gateway plan.
Investigations at the NEA would last for approximately 30 days before a ~235-day trip back to Earth for a total mission duration of roughly one year.
The current status of the Gateway plans is currently unknown, as NASA teams concentrate on both the near-term missions and the creation of “enabling” technology to allow them to create the following missions that are set to launch in the mid-2020s.
(Images: Via L2 content, Boeing, ESA and NASA)
(L2 is – as it has been for the past several years – providing full exclusive Exploration Planning coverage. To join L2, click here: http://www.nasaspaceflight.com/l2/)
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