Aerocapture is a ‘free lunch’ in space exploration

When spacecraft return to Earth, they needn’t shed all their velocity by firing retro-rockets. As an alternative, they use the ambiance as a brake to decelerate for a comfortable touchdown. Each planet within the solar system besides Mercury has sufficient of an environment to permit aerobraking maneuvers, and will permit high-speed exploration missions. A brand new paper appears to be like on the completely different worlds and the way a spacecraft should fly to benefit from this “free lunch” to decelerate on the vacation spot.

Aerocapture is an orbital switch maneuver during which a spacecraft makes a single cross via a planetary atmosphere to decelerate and obtain orbit insertion. However, aerobraking makes use of a propulsive burn plus repeated dips into the atmosphere—i.e., atmospheric drag—to regularly gradual the spacecraft and scale back the scale of the orbit to attain orbit insertion.

The new paper posted to the arXiv preprint server, by Athul Pradeepkumar Girija from the College of Aeronautics and Astronautics at Purdue College, notes that one of many important dangers related to aerocapture is the uncertainty within the atmospheric density. Whereas aerobraking takes place within the tenuous higher ambiance of a planetary physique the place the density uncertainties are a lot bigger, aerocapture happens within the deeper ambiance the place the density uncertainties are smaller.

For instance, the atmospheric density that the Mars Reconnaissance Orbiter MRO really skilled when aerobraking for its orbital insertion in 2006 was a lot completely different than what was predicted by a NASA mannequin referred to as GRAM (International Reference Atmospheric Mannequin) for Mars.

“At some factors within the ambiance, we noticed a distinction within the atmospheric density by an element of 1.3, which implies it was 30% greater than the mannequin,” stated Han You, Navigation Group Chief for MRO, in an article on Universe At the moment in 2006. “That is fairly a bit, however across the south pole we noticed a fair bigger scale issue of as much as 4.5, so meaning it was 350% off of the Mars GRAM mannequin.”

For both aerobraking or aerocapture, the atmospheric density on Mars and different planets can differ broadly from day to day, and even orbit to orbit.

“If the car enters too shallow or encounters an environment which is much less dense than the anticipated minimal, spacecraft might exit the ambiance with out getting captured,” Girija wrote in his new paper. “If the car enters too steep, or the density is way greater than anticipated, the car might bleed an excessive amount of velocity and fail to exit the ambiance.”

Each eventualities would result in full lack of mission. Subsequently, enough margins have to be supplied for the steerage system towards these atmospheric uncertainties, along with supply error and aerodynamic uncertainties.

To carry out aerocapture, there are two sorts of aerodynamic management strategies to regulate the speed of vitality depletion because the car flies via the ambiance: raise modulation and drag modulation.

“Elevate modulation entails a ‘lifting’ aeroshell resembling Apollo or Mars Science Laboratory aeroshell, which has a lift-to-drag (L/D) ratio within the vary of 0.24—0.36,” defined Girija in an electronic mail to Universe At the moment. “Management is achieved by ‘banking’ the car to fly deeper into the denser ambiance, or greater into the thinner ambiance. This management methodology requires the usage of high-rate response management thrusters and is routinely used at Earth and Mars, and has intensive heritage in Apollo and MSL (Mars Science Laboratory) missions.”

Elevate modulation affords steady management via the atmospheric flight whereas the response management steerage tries to attain the specified “exit state circumstances.”

Drag modulation, however, is a less complicated management method during which the management is achieved by steady or discrete (occasional) modulation of the drag space utilizing a deployable gadget.

“Drag modulation autos have L/D = 0, i.e. no lifting functionality,” Girija stated. “The most typical variant is a ‘discrete occasion modulation’ the place a deployed drag skirt is jettisoned throughout the flight, with the jettison time being the one management variable.”

By jettisoning the drag skirt on the right time, Girija defined, it’s doable to focus on a fairly shut exit state situation to what’s superb.

“Drag modulation has been proposed as a ‘cheaper’ various to raise modulation,” Girija stated, “by avoiding the usage of RCS thrusters and is especially enticing for small missions. Drag modulation has no flight heritage, although a few of the fundamental applied sciences have been demonstrated in flight experiments such because the Adaptable Deployable Entry and Placement Expertise (ADEPT),” which had a profitable take a look at flight in September of 2018.

One other factor to think about is the entry hall, which is the area of the ambiance a spacecraft enters to achieve its desired vacation spot. The Theoretical Hall Width (TCW) quantifies the width of the hall, and have to be massive sufficient to accommodate a secure touchdown, accounting for atmospheric uncertainties, and likewise present adequate security margin for mission success even in limiting eventualities, resembling mixture of shallow entry and skinny ambiance.

As a normal rule of thumb, Girija stated, raise modulation supplies practically twice the out there entry hall width as drag modulation, and may thus accommodate bigger atmospheric uncertainties. The principle distinction is that whereas drag modulation affords considerably much less management, it’s extra reasonably priced for small missions (lower than $50 million) whereas lifting aeroshells sometimes value a number of a whole bunch of tens of millions of {dollars}.

Girija says that though the atmospheres of Venus, Mars, and Titan are well-characterized for engineering functions, there may be customary density variations of as much as 50%, plus or minus. With no in-situ information, the atmospheres of Uranus and Neptune will not be as nicely characterised, however the GRAM mannequin for them supplies a regular deviation variation of plus or minus 30%. An understanding of the anticipated uncertainties within the density profile is of nice significance to evaluate the danger it poses to a future mission.

The GRAM mannequin makes use of out there in-situ and distant sensing measurements and supplies an “engineering mannequin for the planetary atmospheres,” Girija stated. “For planets resembling Mars and Venus, there’s loads of information (each in situ and distant sensing) and the fashions are thought of fairly dependable for preliminary engineering design. For Uranus and Neptune, there is no such thing as a in-situ information out there and the fashions are primarily based solely on distant sensing observations throughout the Voyager flyby.”

However there’s nice variety within the bodily construction and chemical composition of the atmospheric layers of the planets in our solar system, from the “sizzling thick Venusian CO₂ ambiance to the chilly icy H2-He atmospheres of Uranus and Neptune,” writes Girija, including that measurements such because the noble gasoline abundances and isotopic ratios in these atmospheres will not be solely vital any aerobraking operations, but in addition to our understanding of the origin, formation, and evolution of the solar system.

For Venus’ thick ambiance, aerocapture utilizing its ambiance has been proven to be possible utilizing each raise and drag modulation. Nevertheless, the massive heating charges at Venus make raise modulation not as fascinating. Girija says that drag modulation with its decrease heating fee significantly makes it enticing for small satellite orbit insertion.

Mars has a comparatively skinny ambiance in comparison with the Earth, however a number of missions have efficiently used aerocapture for each orbit insertion and touchdown. Due to the quite a few mission to Mars, the Martian ambiance is nicely understood, but in addition has comparatively massive seasonal differences in comparison with Venus, and related uncertainties significantly within the thinner higher ambiance.

Nevertheless, in comparison with Venus, the low gravity and the prolonged ambiance present bigger TCW at Mars (by an element of two), and Girija says the bigger atmospheric uncertainties can simply be accommodated. The “candy spot” deceleration at Mars is a band of ambiance between 50–80 km in altitude, the place many of the deceleration happens for aerocapture at Mars. For any mission to the Pink Planet, the entry proposal must have enough margin for 2 limiting eventualities: shallow entry and skinny ambiance, and thick ambiance and steep entry.

Saturn’s largest moon Titan is the one moon in our solar system with an environment. With floor liquids and its Earth-like terrain, Titan is an attractive world to review with a future mission. Girija says that Titan’s low gravity and prolonged thick ambiance make it the best vacation spot for aerocapture, and these circumstances present the most important hall width of any vacation spot in our solar system. Since its small measurement makes it significantly tough to insert orbiters utilizing typical propulsion, aerocapture is a promising various for future missions that may carry out international mapping of Titan’s floor and its lakes and seas. We do have the in-situ information from the Huygens lander, so Girija says that Titan’s density profile is pretty nicely constrained, with just a few exceptions.

“The uncertainty within the density profile will increase with altitude, reaches a most of about 40% close to 100 km above the floor after which decreases,” Girija writes. “It’s not clear that is an artifact of the assumptions used within the mannequin, or certainly an actual impact.”

The altitude band of 300–450 km is the place many of the deceleration happens for aerocapture at Titan, with a density variation of about 30%, which is similar to Venus. Girija says that though Venus’ and Titan’s ambiance are fairly completely different by way of their temperature (737K vs. 94K) and chemistry (CO₂ vs. N2), they share a number of bodily similarities, resembling each being comparatively thick, super-rotating atmospheres with the planetary physique rotating slowly and important greenhouse warming within the decrease troposphere.

The ice giants Uranus and Neptune are the final class of planets but to be explored utilizing orbiter spacecraft. Though their distance from Earth presents important mission design challenges, the 2023–2032 Planetary Science Decadal Survey has recognized a Uranus Orbiter and Probe as the highest precedence for a flagship mission within the subsequent decade.

Whereas Uranus and Neptune are each equally compelling scientifically, Girija says that Uranus is much less demanding from a mission design perspective with propulsive insertion. “Aerocapture has been proven to be strongly enhancing to enabling expertise for ice giant missions,” he writes. “With aerocapture, each Uranus and Neptune could be equally accessible. Latest research have proven that aerocapture allows considerably shorter flight occasions to Uranus than doable with propulsive insertion, particularly with new excessive vitality launch autos.”

For each Uranus and Neptune, the GRAM suite supplies a density variation of roughly 30% for the “related altitude ranges which is taken into account an optimistic estimate,” Girija writes. “Till in-situ information from an atmospheric probe turns into out there, a extra conservative international min-max estimate is really helpful to accommodate the worst-case state of affairs.”

The altitude vary of 200–400 km is the realm the place aerocapture could be only and Girija says the anticipated density variation of 30% “have to be taken as an ‘optimistic’ estimate till in-situ information turns into out there. The precise uncertainty could also be a lot greater.”

Girija has written one other paper, additionally posted to the arXiv preprint server, evaluating raise and drag modulation for ice giant missions.

General, Girija says, the aerocapture mission design “should account for the anticipated atmospheric uncertainties to guarantee the steerage scheme can efficiently steer the car to the specified” location within the ambiance or a touchdown. One of the vital essential components of the mission design is the choice of the goal entry flight path angle.