Laser ranging to Mars Shapiro delay: –can measure (1+  )/2 effect to ~ 2  level with 1 cm range precision to Mars –translates to 4  determination of  : 10  better than Cassini Strong EP: –If sun’s grav. self energy (few parts in 10 6 ) or radiant energy in photons (10 -7 ) falls at an anomalous rate toward Jupiter, orbits are polarized toward Jupiter: more so the farther from the sun –at Mars, this polarization is 850  meters (  = 4  -  - 3 is Nordtvedt parameter) –1 cm precision gets  to few times precision

Shapiro delay to Mars Figure shows range rate from Shapiro delay alone (i.e., after taking out orbital range rate) Thus centimeter range precision two days in a row gives measurement this is a pdf conversion from postscript. Looks bad. Original from John Anderson. I can try to get a better one.

Laser Ranging as a NAV-COMM tool Optical comm links can serve multiple purposes. laser links between spacecraft and to earth can perform a variety of functions: –optical communication –navigation via range measurements –fundamental physics via range measurements Any optical communications link referenced to a good clock can be utilized to provide accurate range measurements Transponder “beacons” throughout solar system would provide a navigational network (GPS equivalent) and allow solar system “grand fit” to nail down gravitational physics

Interplanetary Laser Link Milestone Farthest laser link established May 2005: 24 million km MESSENGER spacecraft en- route to Mercury Used laser altimeter as asynchronous transponder (advertise John Degnan’s talk here) 20 cm range precision achieved – precision pathbreaking result important for interplanetary ranging, laser comm, navigation MET=mission elapsed time MLA=mercury laser altimeter GGAO= goddard obs ground station Smith et al., 2006: Science, 311, 53

3 lengths & 1 angle measured Apparent position of the Target spacecraft Reference spacecraft Earth Sun D R-Earth ~ 2 AU  ~ 1º D T-Earth ~ 2 AU Geometric redundancy allows for accurate measurement of relativistic gravitational light deflection to 1 part in D R-T ~ 0.03 AU True position of the Target spacecraft ISS Expected accuracies: LATOR: the Laser Test of Relativity CWS redo this idea

LATOR concept three spacecraft/platforms form triangle whose edges are measured via laser ranging in Euclidean geometry, these three measurements uniquely determine angles within triangle in curved spacetime of solar system, interferometric measurement of the interior angle at one vertex provides measure of spacetime curvature two spacecraft 1  apart on the far side of the sun allows measurement of curvature to < 10 -8, translating into similar precision measurement of 