10 Outlook

In addition to the tests of gravity, new LLR instruments would be of great benefit to studies of the interior structure of the Moon. Tighter constraints resulting from more complete tracking could aid in the search for a solid inner core. The second-degree tidal lunar Love numbers are detected by LLR, as well as their phase shifts. From past measurements, a fluid core of ∼ 20% of the Moon’s radius is indicated. A lunar tidal dissipation of Q = 30 ± 4 has been reported to have a weak dependence on tidal frequency [76]. Evidence for the oblateness of the lunar fluid-core/solid-mantle boundary may be reflected in a century-scale polar wobble frequency. The lunar vertical and horizontal elastic tidal displacement Love numbers h2 and l2 are known to be no better than 25% of their values, and the lunar dissipation factor Q and the gravitational potential tidal Love number k2 no better than 11%. These values have been inverted jointly for structure and density of the core [30, 29], implying a liquid core and regions of partial melt in the lunar mantle.

Lunar interior studies have arguably suffered the most from the clustering of the Apollo arrays on the central portion of the moon. The rediscovery of the Lunokhod 1 array should greatly improve the situation. In addition, placing retroreflectors far from the Apollo arrays, at a pole or a limb, would improve the measurements by up to a factor of 4 at the same level of ranging precision as is currently performed [34].

The advancement of active laser ranging systems also opens up the possibility of precision ranging beyond the Moon. Laser ranging to Mars can be used to measure the gravitational time delay as Mars passes behind the Sun relative to the Earth. With 1 cm precision ranging, the PPN parameter γ can be measured to about 10–6, ten times better than the Cassini result [70]. The Strong Equivalence Principle polarization effect is about 100 times larger for Earth-Mars orbits than for the lunar orbit. With 1 cm precision ranging, the Nordtvedt parameter, η = 4β − γ − 3, can be measured to between 6 × 10–6 and 2 × 10–6 for observations over ten years [2]. Combined with the time delay measurements this leads to a measurement of PPN parameter β to the 10–6 level. Mars ranging can also be used in combination with lunar ranging to get more accurate limits on the time variation of the gravitational constant.

The ephemeris of Mars itself is known to meters in plane, but hundreds of meters out-of-plane [31]. Laser ranging would get an order of magnitude better estimate, significant for interplanetary navigation. Better measurements of Mars’ rotational dynamics could provide estimates of the size of its core [26]. Mars’ elastic tidal Love number is predicted to be less than 10 cm, within reach of laser ranging. There is also an unexplained low value of Q, inferred from the secular decay of Phobos’ orbit that is a constraint to the present thermal state of the Mars interior [10]. Laser ranging to Phobos would help solve this mystery.

LLR remains one of the best tests of gravity in the weak field and promises to continue to be a key tool for many years to come. The five lunar retroreflectors remain visible today and continue to produce valuable data. Advances in ranging technology have finally reached the point where the precision of the data is being limited by the physical characteristics of the lunar arrays. The natural next step in LLR is to place new retroreflectors and/or laser transponders on the Moon at sites far from the Apollo arrays that have a high enough return rate to take advantage of the SLR network of ground stations. With the retroreflectors and transponder technology available today, these new instruments could easily support laser ranging and advances in ground station technology for another productive 40 years of LLR.

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