When spacecraft return to Earth, they don’t must shed all their velocity by firing retro-rockets. As a substitute, they use the environment as a brake to decelerate for a comfortable touchdown. Each planet within the Photo voltaic System besides Mercury has sufficient of an environment to permit aerobraking maneuvers, and will enable high-speed exploration missions. A brand new paper seems on the totally different worlds and the way a spacecraft should fly to benefit from this “free lunch” to decelerate on the vacation spot.
Aerobraking makes use of repeated dips into the environment – i.e., atmospheric drag — to step by step sluggish the spacecraft and cut back the scale of the orbit to realize orbit insertion. This method was first utilized by the Magellan mission to Venus in 1993, and has additionally been used on a number of Mars missions, together with the Mars Reconnaissance Orbiter (MRO) in 2006. Whereas aerobraking takes time, it saves on the quantity gas required. In MRO’s case, aerobraking lowered the gas wanted by 600 kilograms (1,300 kilos).
The brand new paper, by Athul Pradeepkumar Girija from the College of Aeronautics and Astronautics at Purdue College, notes that one of many vital dangers related to aerobreaking or aerocapture is the uncertainty within the atmospheric density. For instance, the atmospheric density that MRO truly skilled was a lot totally different than what was predicted by a NASA mannequin known as GRAM (World Reference Atmospheric Mannequin) for Mars.
“At some factors within the environment, we noticed a distinction within the atmospheric density by an element of 1.3, which suggests it was 30% increased than the mannequin,” mentioned Han You, Navigation Crew Chief for MRO, in an article on Universe Immediately in 2006. “That’s fairly a bit, however across the south pole we noticed a good bigger scale issue of as much as 4.5, so which means it was 350% off of the Mars GRAM mannequin.”
To make issues much more difficult, the atmospheric density on Mars and different planets can differ broadly from daily, 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 could exit the environment with out getting captured,” Girija wrote in his new paper. “If the car enters too steep, or the density is far increased than anticipated, the car could bleed an excessive amount of velocity and fail to exit the environment.”
Each eventualities would result in full lack of mission. Subsequently, enough margins should be offered for the steerage system in opposition to 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 by the environment: elevate modulation and drag modulation.
“Elevate modulation includes 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 e mail to Universe Immediately. “Management is achieved by ‘banking’ the car to fly deeper into the denser environment, or increased into the thinner environment. This management technique 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 provides steady management by the atmospheric flight whereas the response management steerage tries to realize the specified location and “exit state situations.”
Drag modulation, then again, is a less complicated management approach by which the management is achieved by steady or discrete (occasional) modulation of the drag space utilizing a deployable system.
“Drag modulation automobiles have L/D = 0, i.e. no lifting functionality,” Girija mentioned. “The commonest variant is a ‘discrete occasion modulation’ the place a deployed drag skirt is jettisoned in the course of the flight, with the jettison time being the one management variable.”
By jettisoning the drag skirt on the right time, Girija defined, it’s potential to focus on a fairly shut exit state situation to what’s ultimate.
“Drag modulation has been proposed as a ‘cheaper’ various to elevate modulation,” Girija mentioned, “by avoiding the usage of RCS thrusters and is especially engaging for small missions. Drag modulation has no flight heritage, although a number of the primary 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 environment a spacecraft enters to succeed in its desired vacation spot. The Theoretical Hall Width (TCW) quantifies the width of the hall, and should be giant sufficient to accommodate a secure touchdown, accounting for atmospheric uncertainties, and in addition present adequate security margin for mission success even in limiting eventualities, resembling mixture of shallow entry and skinny environment.
As a basic rule of thumb, Girija mentioned, elevate modulation supplies almost twice the obtainable entry hall width as drag modulation, and may thus accommodate bigger atmospheric uncertainties. The primary distinction is that whereas drag modulation provides considerably much less management, it’s extra reasonably priced for small missions (lower than $50 million) whereas lifting aeroshells usually price a number of tons of of tens of millions of {dollars}.
Girija says that although the atmospheres of Venus, Mars, and Titan are well-characterized for engineering functions, there could be commonplace density variations of as much as 50%, plus or minus. With no in-situ knowledge, the atmospheres of Uranus and Neptune are usually not as properly characterised, however the GRAM mannequin for them supplies an ordinary 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 obtainable in-situ and distant sensing measurements and supplies an “engineering mannequin for the planetary atmospheres,” Girija mentioned. “For planets resembling Mars and Venus, there’s loads of knowledge (each in situ and distant sensing) and the fashions are thought-about fairly dependable for preliminary engineering design. For Uranus and Neptune, there isn’t a in-situ knowledge obtainable and the fashions are based mostly solely on distant sensing observations in the course of the Voyager flyby.”
However there’s nice variety within the bodily construction and chemical composition of the atmospheric layers of the planets in our photo voltaic system, from the “sizzling thick Venusian CO2 environment 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 are usually not solely essential any aerobraking operations, but in addition to our understanding of the origin, formation, and evolution of the Photo voltaic System.
For Venus’ thick environment, aerocapture utilizing its environment has been proven to be possible utilizing each elevate and drag modulation. Nevertheless, the massive heating charges at Venus make elevate modulation not as fascinating. Girija says that drag modulation with its decrease heating price notably makes it engaging for small satellite tv for pc orbit insertion.
Mars has a comparatively skinny environment 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 environment is properly understood, but in addition has comparatively giant differences due to the season in comparison with Venus, and related uncertainties notably within the thinner higher environment.
Nevertheless, in comparison with Venus, the low gravity and the prolonged environment 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 environment between 50–80 km in altitude, the place a lot of the deceleration happens for aerocapture at Mars. For any mission to the Crimson Planet, the entry proposal must have enough margin for 2 limiting eventualities: shallow entry and skinny environment, and thick environment and steep entry.
Saturn’s largest moon Titan is the one moon in our Photo voltaic 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 environment make it the perfect vacation spot for aerocapture, and these situations present the biggest hall width of any vacation spot in our Photo voltaic System. Since its small measurement makes it notably troublesome to insert orbiters utilizing standard propulsion, aerocapture is a promising various for future missions that may carry out world mapping of Titan’s floor and its lakes and seas. We do have the in-situ knowledge from the Huygens lander, so Girija says that Titan’s density profile is pretty properly constrained, with a couple of 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 a lot 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 environment are fairly totally different when it comes to 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 vital 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. Despite the fact that their distance from Earth presents vital 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 know-how for ice big missions,” he writes. “With aerocapture, each Uranus and Neptune could be equally accessible. Latest research have proven that aerocapture permits considerably shorter flight instances to Uranus than potential with propulsive insertion, particularly with new excessive vitality launch automobiles.”
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 knowledge from an atmospheric probe turns into obtainable, a extra conservative world min-max estimate is really useful to accommodate the worst-case state of affairs.”
The altitude vary of 200–400 km is the realm the place aerocapture could be handiest and Girija says the anticipated density variation of 30% “should be taken as an ‘optimistic’ estimate till in-situ knowledge turns into obtainable. The precise uncertainty could also be a lot increased.”
Girija has written one other paper evaluating elevate and drag modulation for ice big missions, right here: https://arxiv.org/abs/2309.13812.
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 environment or a touchdown. Probably the most essential components of the mission design is the choice of the goal entry flight path angle.
Learn extra particulars about every planet’s aerocapture capabilities at Girija’s paper, printed as a pre-print on arXiv.
