# Aerospace
## 1 Introduction  
Small satellites come at a fraction of the cost of large interplanetary orbiter missions and could enable a new revolution in our exploration of the Solar System, as with small satellites in Low Earth Orbit (LEO) [1]. Traditionally, planetary exploration has been performed through large programs such as the $500M Discovery-class [2], the $1B New Frontiers-class [3], and the $2-4B large Flagship-class missions [4]. NASA's Planetary Science Division (PSD) has recognized the value and impact of SmallSats, from CubeSats to ESPA-class satellites, for high-value science missions to support exploration in addition to large missions [5]. In 2017, PSD solicited concepts using small spacecraft under the Small Innovative Missions for Planetary Exploration (SIMPLEX) program, within a cost cap of USD 55 million [6]. Compared to small satellites in LEO, which have very limited or no need for propulsion, interplanetary small satellites require propulsion systems that can deliver substantial \(\Delta V\) capability for two maneuvers. The first is to escape the Earth's sphere of influence if the spacecraft is only delivered to LEO or GTO and is not launched as a secondary payload on a planetary mission or a dedicated small-launch vehicle. The second is to achieve orbit insertion, which can be several hundred m/s to a few km/s of \(\Delta V\) depending on the science orbit. Figure 1 shows the required impulsive orbit insertion \(\Delta V\) at Mars and Venus as a function of the orbit apoapsis altitude. The required \(\Delta V\) increases sharply, with a decrease in apoapsis altitude, as one moves towards low-circular orbits. For example, the required \(\Delta V\) to achieve a 200 \(\times\) 300 km orbit at Mars and Venus is approximately 2000 and 3500 m/s, respectively. This presents a significant challenge for small spacecraft, as large \(\Delta V\) requirements can drive up both spacecraft mass and cost. A recent NASA studyshowed that the \(\Delta V\) required is the largest cost driver for small missions, and minimizing the required \(\Delta V\) can allow for smaller and cheaper spacecraft and lower total mission costs [7]. There are three methods to achieve orbit insertion--(1) chemical propulsion; (2) electric propulsion; and (3) aerocapture. Figure 2 shows the dry-to-wet mass ratio as a function of \(\Delta V\) for various propulsion systems computed using the rocket equation. With chemical bi-propellant systems, the maximum achievable \(\Delta V\) is about 2600 m/s, beyond which the required propellant mass exceeds 60%. Electric propulsion provides a high specific impulse and keeps the propellant mass fraction small, but its low thrust level implies orbit transfers are slow. While a typical Earth-Mars transfer with chemical propulsion takes 12-15 months, an SEP trajectory takes approximately 24-28 months before entering operational orbit [8]. The long flight times can increase operational costs for small spacecraft. In addition, SEP requires large solar arrays, which can drive up the dry mass and cost [7]. Figure 1: Orbit insertion \(\Delta V\) as a function of the apoapsis altitude at Mars and Venus from trajectories with \(V_{\infty}\) = 2.5 and 3.5 km/s, respectively, periapsis altitude = 200 km. Figure 2: Dry-to-wet mass ratio as a function of \(\Delta V\) from the rocket equation. The dotted horizontal line demarcates the region below which the propellant mass fraction exceeds 60%. The third option is aerocapture, which allows orbit insertion with a very small \(\Delta V\), enabling small spacecraft direct access into a low-circular orbit immediately upon arrival. The reduction in \(\Delta V\) required minimizes spacecraft mass and can potentially reduce costs and thus enable constellations of multiple low-cost small spacecraft at Mars and Venus. Drag-modulation aerocapture, a technique that uses atmospheric drag to decelerate spacecraft, has been proposed as a solution to enable small, low-cost satellites to achieve orbit insertion without a large propulsion system [9; 10; 11; 12].