## 3 The Advantages and Disadvantages of Lasers over Microwaves in Deep Space Operations  
The principal advantage of lasers in deep space operations is that the divergence of the transmitted optical beam is proportional to the ratio of the wavelength \(\lambda\) to the transmit aperture \(D\). The Deep Space Network Ka-band microwave antennas operate at sub-centimeter wavelengths, and the resulting beam divergence is roughly 4-5 orders of magnitude larger for a comparable antenna diameter operating at popular laser wavelengths in the visible and near infrared. The corresponding reduction in received energy at the Earth terminal greatly reduces the achievable data bandwidth. For example, microwave communications bandwidths from the NASA New Horizons Flyby Mission to Pluto to Earth were typically limited to between 1 and 4 KHz, and the acquired data took months to download. A second advantage of lasers is that charged particles in the interplanetary solar plasma cannot follow optical frequencies. Thus, there is no need for solar plasma models or corrections via dual wavelength methods and no propagation delays due to Earth's ionosphere. Overall, laser systems require both lower power transmitters and smaller optical antennas (telescopes) and are therefore more lightweight and consume less prime power for a given link. On the negative side, the low beam divergence at optical frequencies requires more precise pointing knowledge and tracking at both terminals whose optical axes must remain coaligned during wideband laser communications. For interplanetary links, coalignment is further complicated by the enormous distances involved (up to 40 AU for the outermost planets) and the finite speed of light, requiring point ahead angles to compensate for the rapid and different orbital velocities of the Earth and subject planet about the Sun; therefore, initial acquisition of the opposite planet will likely rely on optical tracking of the sunlit planet combined with accurate orbital models to determine the proper look-ahead angles. Finally, if the laser beams are too narrow, they may illuminate a relatively small portion of the planetary surface, especially in the case of the giant planets, Jupiter and Saturn. Figure 2: Computation of the instantaneous range R and the clock offset dt via Asynchronous Laser Ranging and Time Transfer. Multiple measurements of the range provide the time derivative of R between the Earth and spaceborne clocks that appears in the equation for the clock offset, dt. Table 1 lists the relevant solar system parameters needed to assess required look-ahead angles related to planetary rotation. Rotational velocities about the polar axis of the planets also cause periodic interruptions in link availability and can further complicate acquisition and tracking. As a result, each terminal must point to where the opposite terminal will be when the light arrives or, in hunter's terminology, \"lead the duck\". Furthermore, unlike microwaves, link availability can be negatively affected by weather and clouds at an Earth ground terminal. Large aperture astronomical telescopes located on mountaintops can greatly mitigate this problem but not totally eliminate it. A possible alternative means of eliminating atmospheric effects and interruptions in link availability is to use a large Earth-orbiting telescope, such as NASA's 2.4 m aperture Hubble Space Telescope ([PERSON] et al., 2016), and relay the high bandwidth planetary data to an Earth station via conventional high bandwidth microwave systems. At or near the minimum Earth-planet range, the Earth, as viewed from the planet, is solidly located in front of the solar disk and the background solar radiation will likely overwhelm the planetary terminal, while at or near the maximum Earth-planet range, the Sun will physically block both links. One can probably assume that the need for high bandwidth communications implies that the deep space terminal is not a lander but instead is orbiting the planet of interest and collecting voluminous data to be transmitted back to Earth.