Future In-Space Operations (FISO) Telecon Colloquium The L1 Orbit Used for Servicing (LOTUS): Enabling Human/Robotic Servicing Missions in the Earth-Moon System Brent Wm. Barbee Future In-Space Operations (FISO) Telecon Colloquium June 16th, 2010

Background NASA-GSFC is currently studying a suite of notional missions to inform a forthcoming congressional report on spacecraft servicing capabilities and concepts The 5th Notional Mission involves human/robotic servicing of a large Sun-Earth L2 (SEL2) telescope in the Earth-Moon system

Mission Profile A large telescope stationed at SEL2 returns to the Earth-Moon system and rendezvouses with a robotic servicing vehicle in a Lyapunov orbit about Earth- Moon L1 (EML1) A crew vehicle carrying astronauts launches to rendezvous with the servicer/telescope stack After servicing is complete, the crew vehicle returns to Earth and the telescope returns to SEL2 The robotic servicer spacecraft remains in orbit for 25 years

Telescope Considerations Minimize telescope maneuver magnitudes Conserve telescope propellant Avoid large thruster-induced accelerations Minimize telescope down-time Avoid excessive travel time to/from SEL2 Telescope is assumed to be a cooperative rendezvous target for the robotic servicer

Robotic Servicer Orbit Robotic servicing vehicle has a 25 year lifetime The orbit it inhabits in the Earth-Moon system must: Be easily accessed by both the crew vehicle and the telescope Require minimal station-keeping ΔV Remain well clear of the Van Allen Belts and GEO

Crew Vehicle Objectives Crew vehicle trajectory should: Maximize available time for servicing Provide a total round-trip flight time (launch to landing) of at most 21 days Offer a free return from launch if possible Stay clear of the Van Allen Belts and GEO Provide safe atmospheric re-entry Maximum atmospheric re-entry velocity of 11 km/s, as per Apollo 10 Notional Orion was assumed for crew vehicle

Robotic Servicer Trajectories

Telescope Trajectories Telescope can travel relatively easily between its SEL2 halo orbit and the EML1 Lyapunov orbit via low-energy transfers ΔV from SEL2 to EML1 = 45 – 50 m/s ΔV from EML1 to SEL2 < 1 m/s Flight time between EML1/SEL2 = 50 – 130 days Faster transfers are possible but require considerable ΔV Some telescope downtime will have to be tolerated in exchange increased lifetime from servicing

Rendezvous at EML1 The robotic servicer can rendezvous with and capture the telescope on the EML1 Lyapunov orbit relatively quickly for modest ΔV costs

Example EML1 Rendezvous The relative motion dynamics between spacecraft on a libration point orbit are completely different from the familiar relative motion dynamics between spacecraft in Earth orbit (LEO, GEO, etc.)

Crew Trajectory Alternatives The first option considered was to send the crew directly to the EML1 orbit and perform servicing there Crew has a free return from launch if necessary However, this only offered ~ 11 days for servicing, which was insufficient for the planned activities Outbound and inbound times are not selectable Additionally, the EML1 orbit experiences eclipses that can be 9 to 12 hours in length

Crew Trajectory Alternatives The second option considered was to place the crew onto a large 21 day long Highly Elliptical Orbit (HEO) about Earth Completely free return for the crew However, bringing the robotic servicer and telescope to this orbit within 1 – 3 days of launch and having them depart within 1 – 3 days of re-entry required > 2,000 m/s of ΔV from the robotic servicer and telescope, which is not permissible

Zero-Velocity Curve Analysis The next approach was to study the restricted three-body dynamics I noticed that there was a large volume of space around Earth that should be accessible from the EML1 orbit for very little ΔV …

The LOTUS Ultra-low departure ΔV from EML1 orbit is easily achieved by the servicer/telescope stack

The LOTUS in the Inertial Frame The LOTUS is a “HEO” with a high perigee Eccentricity of 0.54 Period of ~ 10 days The LOTUS perigee is 83,777 km, well above the Van Allen Belts and GEO

Crew Launch to a LOTUS Apogee

Crew Free Return From Launch The crew always has a free return from launch available from in case LOTUS insertion must be aborted

Servicing on the LOTUS The crew spends 16 days on the LOTUS 1 day is for AR&B with the servicer/telescope stack 15 days for servicing Meets requirements for the notional mission under consideration

Crew Return to Earth

Return to EML1 The LOTUS naturally returns to EML1 ~ 98 days after initial departure The servicer can easily reinsert into the EML1 Lyapunov orbit The telescope can easily continue past EML1 and transfer back to SEL2

Mission Summary Total crew vehicle ΔV (including 100 m/s for AR&B) is 2120 m/s Quite reasonable considering crew vehicles for lunar missions historically had a 2800 m/s capability Well within the notional Orion, Ares I, Ares V capability Total servicing time of 15 days Total round-trip time of 20.54 days

Trajectories in the Inertial Frame

LOTUS Eclipse Analysis Eclipses on the LOTUS are much reduced compared to the EML1 Lyapunov orbit Judicious choice of start date completely avoids eclipses during the LOTUS Worst case eclipse duration on the LOTUS is about 4 hours

Servicing Time Flexibility The 15 day servicing case shown here is only one possibility of many The advantage of the LOTUS is that the crew can arrive at / depart from any points on the LOTUS, making the servicing time selectable With a 21 day round-trip flight time limit, the maximum servicing time available is 19 days Launch into /depart from a LOTUS perigee 0.6 day flight time from launch to LOTUS insertion, same for de-orbit Launch C3 is -7.98 km2/s2 Insertion / De-Orbit ΔV is 1800 m/s Total crew vehicle ΔV of 3700 m/s, not unreasonable for such a low launch C3 and in light of historical and notional future mission capabilities

Summary and Conclusions The LOTUS offers key advantages for human/robotic servicing missions in the Earth-Moon system Selectable time on-orbit for servicing Low ΔV access to/from EML1 and therefore to/from SEL2 Avoidance of eclipses reduces battery size requirements, saving considerable spacecraft mass No orbit maintenance/station-keeping maneuvers required on LOTUS Launch C3 and ΔV requirements consistent with anticipated capabilities Crew always has a free return from launch if necessary LOTUS perigee is well above the Van Allen Belts and GEO Atmospheric re-entry velocity when de-orbiting from any point on the LOTUS is always ≤ 11 km/s