Specific impulse

From Wikipedia, the free encyclopedia

Specific impulse (usually abbreviated I_sp) is a measure of how effectively a rocket uses propellant or a jet engine uses fuel. By definition, it is the total impulse (or change in momentum) delivered per unit of propellant consumed and is dimensionally equivalent to the generated thrust divided by the propellant mass flow rate or weight flow rate. If mass (kilogram, pound-mass, or slug) is used as the unit of propellant, then specific impulse has units of velocity. If weight (newton or pound-force) is used instead, then specific impulse has units of time (seconds). Multiplying flow rate by the standard gravity (g₀) converts specific impulse from the mass basis to the weight basis.

 

A propulsion system with a higher specific impulse uses the mass of the propellant more effectively in creating forward thrust and, in the case of a rocket, less propellant needed for a given delta-v, per the Tsiolkovsky rocket equation. In rockets, this means the engine is more effective at gaining altitude, distance, and velocity. This effectiveness is less important in jet engines that employ wings and use outside air for combustion and carry payloads that are much heavier than the propellant.

Specific impulse includes the contribution to impulse provided by external air that has been used for combustion and is exhausted with the spent propellant. Jet engines use outside air, and therefore have a much higher specific impulse than rocket engines. The specific impulse in terms of propellant mass spent has units of distance per time, which is an artificial velocity called the "effective exhaust velocity". This is higher than the actual exhaust velocity because the mass of the combustion air is not being accounted for. Actual and effective exhaust velocity are the same in rocket engines not utilizing air or other intake propellant such as water.

Specific impulse is inversely proportional to specific fuel consumption (SFC) by the relationship I_sp = 1/(g_o·SFC) for SFC in kg/(N·s) and I_sp = 3600/SFC for SFC in lb/(lbf·hr).

General considerations

The amount of propellant is normally measured either in units of mass or weight. If mass is used, specific impulse is an impulse per unit mass, which dimensional analysis shows to have units of speed, and so specific impulses are often measured in meters per second and are often termed effective exhaust velocity. However, if propellant weight is used, an impulse divided by a force (weight) turns out to be a unit of time, and so specific impulses are measured in seconds. These two formulations are both widely used and differ from each other by a factor of g₀, the dimensioned constant of gravitational acceleration at the surface of the Earth. 

Note that the rate of change of momentum of a rocket (including its propellant) per unit time is equal to the thrust. 

The higher the specific impulse, the less propellant is needed to produce a given thrust during a given time. In this regard a propellant is more efficient the greater its specific impulse. This should not be confused with energy efficiency, which can decrease as specific impulse increases, since propulsion systems that give high specific impulse require high energy to do so.

Thrust and specific impulse should not be confused. The specific impulse is the impulse produced per unit of propellant expended, while thrust is the momentary or peak force supplied by a particular engine. In many cases, propulsion systems with very high specific impulse—some ion thrusters reach 10,000 seconds—produce low thrust.

When calculating specific impulse, only propellant carried with the vehicle before use is counted. For a chemical rocket, the propellant mass therefore would include both fuel and oxidizer. For air-breathing engines, only the mass of the fuel is counted, not the mass of air passing through the engine. 

Air resistance and the engine's inability to keep a high specific impulse at a fast burn rate are why all the propellant is not used as fast as possible. 

A heavier engine with a higher specific impulse may not be as effective in gaining altitude, distance, or velocity as a lighter engine with a lower specific impulse. 

If it were not for air resistance and the reduction of propellant during flight, specific impulse would be a direct measure of the engine's effectiveness in converting propellant weight or mass into forward momentum.

Units

Various equivalent rocket motor performance measurements, in SI and English engineering units

	Specific impulse		Effective exhaust velocity	Specific fuel consumption
	By weight	By mass
SI	= x s	= 9.80665·x N·s/kg	= 9.80665·x m/s	= 101,972/x g/(kN·s)
English engineering units	= x s	= x lbf·s/lb	= 32.17405·x ft/s	= 3,600/x lb/(lbf·hr)

The most common unit for specific impulse is the second, both in SI contexts as well as where imperial or customary units are used. The advantage of seconds is that the unit and numerical value are identical across systems of measurements, and essentially universal. Nearly all manufacturers quote their engine performance in seconds, and the unit is also useful for specifying aircraft engine performance.

The use of metres per second to specify effective exhaust velocity is also reasonably common. The unit is intuitive when describing rocket engines, although the effective exhaust speed of the engines may be significantly different from the actual exhaust speed, which may be due to the fuel and oxidizer that is dumped overboard after powering turbopumps. For airbreathing jet engines, the effective exhaust velocity is not physically meaningful, although it can be used for comparison purposes.

The values expressed in N·s/kg are not uncommon and are numerically equal to the effective exhaust velocity in m/s (from Newton's second law and the definition of the newton). 

Another equivalent unit is specific fuel consumption. This has units of g/(kN·s) or lb/(lbf·hr) and is inversely proportional to specific impulse. Specific fuel consumption is used extensively for describing the performance of air-breathing jet engines.

Specific impulse in seconds

General definition

For all vehicles, specific impulse (impulse per unit weight-on-Earth of propellant) in seconds can be defined by the following equation:

${\displaystyle F_{\text{thrust}}=g_{0}\cdot I_{\text{sp}}\cdot {\dot {m}},}$

where:

• ${\displaystyle F_{\text{thrust}}}$ is the thrust obtained from the engine, in newtons (or pounds force),
• ${\displaystyle g_{0}}$ is the standard gravity, which is nominally the gravity at Earth's surface, in m/s² (or ft/s²),
• ${\displaystyle I_{\text{sp}}}$ is the specific impulse measured in seconds,
• ${\displaystyle {\dot {m}}}$ is the mass flow rate in kg/s (or slugs/s), which is the negative of the time-rate of change of the vehicle's mass (since propellant is being expelled).

The English unit pound mass is more commonly used than the slug, and when using pounds per second for mass flow rate, the conversion constant g₀ becomes unnecessary, because the slug is dimensionally equivalent to pounds divided by g₀:

${\displaystyle F_{\text{thrust}}=I_{\text{sp}}\cdot {\dot {m}}.}$

I_sp in seconds is the amount of time a rocket engine can generate thrust, given a quantity of propellant whose weight is equal to the engine's thrust. 

The advantage of this formulation is that it may be used for rockets, where all the reaction mass is carried on board, as well as airplanes, where most of the reaction mass is taken from the atmosphere. In addition, it gives a result that is independent of units used (provided the unit of time used is the second).

The specific impulse of various jet engines

Rocketry

In rocketry, where the only reaction mass is the propellant, an equivalent way of calculating the specific impulse in seconds is also frequently used. In this sense, specific impulse is defined as the thrust integrated over time per unit weight-on-Earth of the propellant:

${\displaystyle I_{\rm {sp}}={\frac {v_{\text{e}}}{g_{0}}},}$

where

• ${\displaystyle I_{\rm {sp}}}$ is the specific impulse measured in seconds,
• ${\displaystyle v_{\text{e}}}$ is the average exhaust speed along the axis of the engine (in ft/s or m/s),
• ${\displaystyle g_{0}}$ is the standard gravity (in ft/s² or m/s²).

In rockets, due to atmospheric effects, the specific impulse varies with altitude, reaching a maximum in a vacuum. This is because the exhaust velocity isn't simply a function of the chamber pressure, but is a function of the difference between the interior and exterior of the combustion chamber. It is therefore important to note whether the specific impulse refers to operation in a vacuum or at sea level. Values are usually indicated with or near the units of specific impulse (e.g. "sl", "vac").

Specific impulse as a speed (effective exhaust velocity)

Because of the geocentric factor of g₀ in the equation for specific impulse, many prefer to define the specific impulse of a rocket (in particular) in terms of thrust per unit mass flow of propellant (instead of per unit weight flow). This is an equally valid (and in some ways somewhat simpler) way of defining the effectiveness of a rocket propellant. For a rocket, the specific impulse defined in this way is simply the effective exhaust velocity relative to the rocket, v_e. The two definitions of specific impulse are proportional to one another, and related to each other by:

${\displaystyle v_{\text{e}}=g_{0}\cdot I_{\text{sp}},}$

where

${\displaystyle I_{\text{sp}}}$ is the specific impulse in seconds,

${\displaystyle v_{\text{e}}}$ is the specific impulse measured in m/s, which is the same as the effective exhaust velocity measured in m/s (or ft/s if g is in ft/s²),

${\displaystyle g_{0}}$ is the standard gravity, 9.80665 m/s² (in Imperial units 32.174 ft/s²).

This equation is also valid for air-breathing jet engines, but is rarely used in practice. 

(Note that different symbols are sometimes used; for example, c is also sometimes seen for exhaust velocity. While the symbol ${\displaystyle I_{\text{sp}}}$ might logically be used for specific impulse in units of N·s/kg; to avoid confusion, it is desirable to reserve this for specific impulse measured in seconds.) 

It is related to the thrust, or forward force on the rocket by the equation:

${\displaystyle F_{\text{thrust}}=v_{\text{e}}\cdot {\dot {m}},}$

where ${\displaystyle {\dot {m}}}$ is the propellant mass flow rate, which is the rate of decrease of the vehicle's mass. 

A rocket must carry all its fuel with it, so the mass of the unburned fuel must be accelerated along with the rocket itself. Minimizing the mass of fuel required to achieve a given push is crucial to building effective rockets. The Tsiolkovsky rocket equation shows that for a rocket with a given empty mass and a given amount of fuel, the total change in velocity it can accomplish is proportional to the effective exhaust velocity. 

A spacecraft without propulsion follows an orbit determined by its trajectory and any gravitational field. Deviations from the corresponding velocity pattern (these are called Δv) are achieved by sending exhaust mass in the direction opposite to that of the desired velocity change.

Actual exhaust speed versus effective exhaust speed

Note that effective exhaust velocity and actual exhaust velocity can be significantly different, for example when a rocket is run within the atmosphere, atmospheric pressure on the outside of the engine causes a retarding force that reduces the specific impulse, and the effective exhaust velocity goes down, whereas the actual exhaust velocity is largely unaffected. Also, sometimes rocket engines have a separate nozzle for the turbo-pump turbine gas, and then calculating the effective exhaust velocity requires averaging the two mass flows as well as accounting for any atmospheric pressure.

For air-breathing jet engines, particularly turbofans, the actual exhaust velocity and the effective exhaust velocity are different by orders of magnitude. This is because a good deal of additional momentum is obtained by using air as reaction mass. This allows a better match between the airspeed and the exhaust speed, which saves energy/propellant and enormously increases the effective exhaust velocity while reducing the actual exhaust velocity.

Energy efficiency

Rockets

For rockets and rocket-like engines such as ion-drives a higher ${\displaystyle I_{\text{sp}}}$ implies lower energy efficiency: the power needed to run the engine is simply:

${\displaystyle {\frac {dm}{dt}}{\frac {v_{e}^{2}}{2}}}$

where v_e is the actual jet velocity. 

whereas from momentum considerations the thrust generated is:

${\displaystyle {\frac {dm}{dt}}v_{e}}$

Dividing the power by the thrust to obtain the specific power requirements we get:

${\displaystyle {\frac {v_{e}}{2}}}$

Hence the power needed is proportional to the exhaust velocity, with higher velocities needing higher power for the same thrust, causing less energy efficiency per unit thrust. 

However, the total energy for a mission depends on total propellant use, as well as how much energy is needed per unit of propellant. For low exhaust velocity with respect to the mission delta-v, enormous amounts of reaction mass is needed. In fact a very low exhaust velocity is not energy efficient at all for this reason; but it turns out that neither are very high exhaust velocities. 

Theoretically, for a given delta-v, in space, among all fixed values for the exhaust speed the value ${\displaystyle v_{\text{e}}=0.6275\Delta v}$ is the most energy efficient for a specified (fixed) final mass, see energy in spacecraft propulsion. 

However, a variable exhaust speed can be more energy efficient still. For example, if a rocket is accelerated from some positive initial speed using an exhaust speed equal to the speed of the rocket no energy is lost as kinetic energy of reaction mass, since it becomes stationary. (Theoretically, by making this initial speed low and using another method of obtaining this small speed, the energy efficiency approaches 100%, but requires a large initial mass.) In this case the rocket keeps the same momentum, so its speed is inversely proportional to its remaining mass. During such a flight the kinetic energy of the rocket is proportional to its speed and, correspondingly, inversely proportional to its remaining mass. The power needed per unit acceleration is constant throughout the flight; the reaction mass to be expelled per unit time to produce a given acceleration is proportional to the square of the rocket's remaining mass. 

Also it is advantageous to expel reaction mass at a location where the gravity potential is low, see Oberth effect.

Air breathing

Air-breathing engines such as turbojets increase the momentum generated from their propellant by using it to power the acceleration of inert air rearwards. It turns out that the amount of energy needed to generate a particular amount of thrust is inversely proportional to the amount of air propelled rearwards, thus increasing the mass of air (as with a turbofan) both improves energy efficiency as well as ${\displaystyle I_{\text{sp}}}$.

Examples

Specific impulse of various propulsion technologies

Engine	Effective exhaust velocity (m/s)	Specific impulse (s)	Exhaust specific energy (MJ/kg)
Turbofan jet engine (actual V is ~300 m/s)	29,000	3,000	Approx. 0.05
Space Shuttle Solid Rocket Booster	2,500	250	3
Liquid oxygen-liquid hydrogen	4,400	450	9.7
Ion thruster	29,000	3,000	430
VASIMR	30,000–120,000	3,000–12,000	1,400
Dual-stage 4-grid electrostatic ion thruster	210,000	21,400	22,500
Ideal photonic rocket	299,792,458	30,570,000	89,875,517,874

An example of a specific impulse measured in time is 453 seconds, which is equivalent to an effective exhaust velocity of 4,440 m/s, for the Space Shuttle Main Engines when operating in a vacuum. An air-breathing jet engine typically has a much larger specific impulse than a rocket; for example a turbofan jet engine may have a specific impulse of 6,000 seconds or more at sea level whereas a rocket would be around 200–400 seconds.

An air-breathing engine is thus much more propellant efficient than a rocket engine, because the actual exhaust speed is much lower, the air provides an oxidizer, and air is used as reaction mass. Since the physical exhaust velocity is lower, the kinetic energy the exhaust carries away is lower and thus the jet engine uses far less energy to generate thrust (at subsonic speeds). While the actual exhaust velocity is lower for air-breathing engines, the effective exhaust velocity is very high for jet engines. This is because the effective exhaust velocity calculation essentially assumes that the propellant is providing all the thrust, and hence is not physically meaningful for air-breathing engines; nevertheless, it is useful for comparison with other types of engines.

The highest specific impulse for a chemical propellant ever test-fired in a rocket engine was 542 seconds (5,320 m/s) with a tripropellant of lithium, fluorine, and hydrogen. However, this combination is impractical.

Nuclear thermal rocket engines differ from conventional rocket engines in that thrust is created strictly through thermodynamic phenomena, with no chemical reaction. The nuclear rocket typically operates by passing hydrogen gas through a superheated nuclear core. Testing in the 1960s yielded specific impulses of about 850 seconds (8,340 m/s), about twice that of the Space Shuttle engines. 

A variety of other non-rocket propulsion methods, such as ion thrusters, give much higher specific impulse but with much lower thrust; for example the Hall effect thruster on the SMART-1 satellite has a specific impulse of 1,640 s (16,100 m/s) but a maximum thrust of only 68 millinewtons. The variable specific impulse magnetoplasma rocket (VASIMR) engine currently in development will theoretically yield 20,000−300,000 m/s, and a maximum thrust of 5.7 newtons.

Larger engines

Here are some example numbers for larger jet and rocket engines: 

Specific fuel consumption (SFC), specific impulse, and effective exhaust velocity numbers for various rocket and jet engines.

Engine type	Scenario	Spec. fuel cons.		Specific impulse (s)	Effective exhaust velocity (m/s)
		(lb/lbf·h)	(g/kN·s)
NK-33 rocket engine	Vacuum	10.9	308	331	3250
SSME rocket engine	Space shuttle vacuum	7.95	225	453	4440
Ramjet	Mach 1	4.5	130	800	7800
J-58 turbojet	SR-71 at Mach 3.2 (Wet)	1.9	54	1900	19000
Eurojet EJ200	Reheat	1.66–1.73	47–49	2080–2170	20400–21300
Rolls-Royce/Snecma Olympus 593 turbojet	Concorde Mach 2 cruise (Dry)	1.195	33.8	3010	29500
Eurojet EJ200	Dry	0.74–0.81	21–23	4400–4900	44000–48000
CF6-80C2B1F turbofan	Boeing 747-400 cruise	0.605	17.1	5950	58400
General Electric CF6 turbofan	Sea level	0.307	8.7	11700	115000

Elon Musk's Tesla Roadster

From Wikipedia, the free encyclopedia

Elon Musk's Tesla Roadster

Roadster car mounted on Falcon upper-stage; Earth in the background
Names	SpaceX Roadster Starman
Mission type	Test flight
Operator	SpaceX
COSPAR ID	2018-017A
SATCAT no.	43205
Spacecraft properties
Spacecraft type	2008 Tesla Roadster used as a mass simulator, attached to the upper stage of a Falcon Heavy rocket
Manufacturer	Tesla and SpaceX
Launch mass	~1,300 kg (2,900 lb); ~6,000 kg (13,000 lb) including rocket upper stage
Start of mission
Launch date	20:45:00, February 6, 2018 (UTC)
Rocket	Falcon Heavy FH-001
Launch site	Kennedy LC-39A
Orbital parameters
Reference system	Heliocentric
Eccentricity	0.25571
Perihelion	0.98613 au (147,523,000 km)
Aphelion	1.6637 au (248,890,000 km)
Inclination	1.077°
Period	1.525 year
Epoch	1 May 2018

Elon Musk's Tesla Roadster is an electric sports car that served as the dummy payload for the February 2018 Falcon Heavy test flight and became an artificial satellite of the Sun. "Starman", a mannequin dressed in a spacesuit, occupies the driver's seat. The car and rocket are products of Tesla and SpaceX, both companies founded by Elon Musk. The 2008-model Roadster was previously used by Musk for commuting to work, and is the first production car in space.

The car, mounted on the rocket's second stage, acquired enough velocity to escape Earth's gravity and enter an elliptical heliocentric orbit crossing the orbit of Mars. The orbit reaches a maximum distance from the Sun at aphelion of 1.66 astronomical units (au). During the early portion of the voyage outside the Earth's atmosphere, live video was transmitted back to the mission control center and live-streamed for slightly over four hours.

Advertising analysts noted Musk's sense of brand management and use of new media for his decision to launch a Tesla into space. While some commenters voiced concern that the car contributed to space debris, others saw it as a work of art. Musk explained he wanted to inspire the public about the "possibility of something new happening in space," as part of his larger vision for spreading humanity to other planets.

Background

Musk's Tesla Roadster parked outside SpaceX in 2010

In March 2017, SpaceX's founder, Elon Musk, said that because the launch of the new Falcon Heavy vehicle was risky, it would carry the "silliest thing we can imagine". In June 2017, one of his Twitter followers suggested that the silly thing be a Tesla Model S, to which Musk replied "Suggestions welcome!". In December 2017 he announced that the payload would be his personal "midnight cherry Tesla Roadster". Later that month, photos of the car were taken and publicly released prior to payload encapsulation. 

One of the test flight objectives was to demonstrate that the new rocket could carry a payload as far as the orbit of Mars. NASA had declined SpaceX's offer to carry a scientific payload.

Following the successful launch, the Roadster became the first standard roadworthy vehicle sent into space. Three special-purpose off-road vehicles had previously been sent to the Moon: the lunar rovers of Apollo 15, 16, and 17 in the 1970s.

Roadster as payload

The Roadster is permanently attached to the upper stage of the Falcon Heavy rocket.

The car was permanently mounted on the rocket in an inclined position above the payload adapter. Tubular structures were added to mount front and side cameras.

Positioned in the driver's seat is "Starman", a full-scale human mannequin clad in a SpaceX pressure spacesuit. It was placed with the right hand on the steering wheel and the left elbow resting on the open window sill. The mannequin was named after the David Bowie song "Starman" and the car's sound system was set before launch to continuously loop the Bowie song "Space Oddity".

There is a copy of Douglas Adams' novel The Hitchhiker's Guide to the Galaxy in the glovebox, along with references to the book in the form of a towel and a sign on the dashboard that reads "DON'T PANIC!". A Hot Wheels miniature Roadster with a miniature Starman is mounted on the dashboard. A plaque bearing the names of the employees who worked on the project is placed underneath the car, and a message on the vehicle's circuit board reads "Made on Earth by humans". The car also carries a copy of Isaac Asimov's Foundation trilogy on a 5D optical disc, a proof of concept for high-density long-lasting data storage, donated to Musk by the Arch Mission Foundation.

Trajectory

Animation of SpaceX Roadster's trajectory.
   SpaceX Roadster    Sun  ·    Mercury ·    Venus  ·    Earth  ·    Mars 

 

Falcon Heavy liftoff from pad LC-39A

 

Orbit of the Roadster, with the planets of the inner Solar System for context. Its aphelion is ~250 million kilometres (1.66 au).

 

The US Office of Commercial Space Transportation issued the test flight's launch license on February 2, 2018. The rocket lifted off from Launch Complex 39A at Kennedy Space Center at 15:45 EST (20:45 UTC) on February 6. The upper stage supporting the car was initially placed in an Earth parking orbit. It spent six hours coasting through the Van Allen radiation belts, thereby demonstrating a new capability requested by the U.S. Air Force for direct insertion of heavy intelligence satellites into geostationary orbit. Then, the upper stage performed a second boost to reach the desired escape trajectory.

The launch was live streamed, and video feeds from space showed the Roadster at various angles, with Earth in the background, thanks to cameras placed inside and outside the car, on booms attached to the vehicle's custom adaptor atop the upper stage. Musk had estimated the car's battery would last over 12 hours, but the live stream ran for just over four hours, thus ending before the final boost out of Earth orbit. The images were released by SpaceX into the public domain on their Flickr account.

Following the launch, the rocket stage carrying the car was given the Satellite Catalog Number 43205, named "TESLA ROADSTER/FALCON 9H", along with the COSPAR designation 2018-017A. The JPL Horizons system publishes solutions for the trajectory as target body "-143205".

The Roadster is in a heliocentric orbit that crosses the orbit of Mars and reaches a distance of 1.66 au from the Sun. With an inclination of roughly 1 degree to the ecliptic plane, compared to Mars' 1.85° inclination, this trajectory by design cannot intercept Mars, so the car will neither fly by Mars nor enter an orbit around Mars. This was the second object launched by SpaceX to leave Earth orbit, after the DSCOVR mission to the Earth–Sun L₁ Lagrangian point. Nine months after launch, the Tesla had travelled beyond the orbit of Mars, reaching aphelion at 12:48 UTC on November 9, 2018, at a distance of 248,892,559 km (1.664 au) from the Sun. The maximum speed of the car relative to the Sun will be 121,005 km/h (75,189 mph) at perihelion.

Even if the rocket had targeted an actual Mars transfer orbit, the car could not have been placed into orbit around Mars, because the upper stage that carries it is not equipped with the necessary propellant, maneuvering, and communications capabilities. This flight simply demonstrated that Falcon Heavy is capable of launching significant payloads towards Mars in potential future missions.

Cultural impact

The car in space quickly became a topic for Internet memes. Western Australia Police distributed a picture of a radar gun aimed at the Roadster whilst above Australia. Škoda produced a parody video of a Škoda Superb being driven to Mars (a village in central France). An attempt was made by Donut Media to launch a Hot Wheels-sized Tesla Model X to the stratosphere using a weather balloon.

Some news reports observed a similarity between the real pictures of a car orbiting the Earth and the title sequence of the 1981 animation film Heavy Metal, where a space traveler lands on Earth in a two-seater Chevrolet Corvette convertible.

The SpaceX launch live stream reached over 2.3 million concurrent viewers on YouTube, which made it the second most watched live event on the platform, behind another space-related event: Felix Baumgartner's jump from the stratosphere in 2012.

Reactions

The choice of the Roadster as a dummy payload was variously interpreted as a shrewd marketing move for Tesla, a work of art, or a contribution to space debris.

Marketing move

Musk was lauded as a visionary marketer and brand manager by controlling both the timing and the content of his corporate public relations. After the launch, Scientific American said using a car was not entirely pointless, in the sense that something of that size and weight was necessary for a meaningful test. "Thematically, it was a perfect fit" to use the Tesla car, and there was no reason not to take the opportunity to remind the auto industry that Musk was challenging the status quo in that arena, as well as in space. Advertising Age agreed with Business Insider that the Roadster space launch was the "greatest ever car commercial without a dime spent on advertising", demonstrating that Musk is "miles ahead of the rest" in reaching young consumers, where "mere mortals scrabble about spending millions to fight each other over seconds of air time", Musk "just executes his vision." Alex Hern, technology reporter for The Guardian, said the choice to launch a car was a "hybrid of genuine breakthrough and nerd-baiting publicity stunt" without "any real point beyond generating good press pics", which should not detract from the much more important technological milestone represented by the launch of the rocket itself.

Lori Garver, a former NASA deputy director, initially said the choice of payload for the Falcon Heavy maiden flight is a gimmick and a loss of opportunity to further advance science—but later clarified that "I was told by a SpaceX VP (vice president) at the launch that they offered free launches to NASA, Air Force etc. but got no takers."

Musk responded to the critics explaining he wanted to inspire the public about the "possibility of something new happening in space," as part of his larger vision for spreading humanity to other planets.

Work of art

The mannequin known as "Starman", seated in the Roadster

Alice Gorman, a lecturer in archaeology and space studies at Flinders University in Australia, said that the Roaster's primary purpose is symbolic communication, that "the red sports car symbolises masculinity – power, wealth and speed – but also how fragile masculinity is." Drawing on anthropological theories of symbols, she argues that "The car is also an armour against dying, a talisman that quells a profound fear of mortality." Gorman wrote that "the spacesuit is also about death. [...] The Starman was never alive, but now he's haunting space."

The Verge likened the Roadster to a "Readymade" work of art, such as Marcel Duchamp's 1917 piece Fountain, created by placing an everyday object in an unusual position, context and orientation.

Space debris

Orbital debris expert Darren McKnight stated that the car poses no risk because it is far from Earth orbit. He added: "The enthusiasm and interest that [Musk] generates more than offsets the infinitesimally small 'littering' of the cosmos." Tommy Sanford, director of the Commercial Spaceflight Federation, opined that the car and its rocket stage are no more "space junk" than the mundane material usually launched on other test flights. Mass simulators are often deliberately placed in a graveyard orbit or sent on a deep space trajectory, where they are not a hazard. Hugh Lewis, an expert in space debris at the University of Southampton, tweeted "Intentionally launching a car to a long-lived orbit is not what you want to hear from a company planning to fly 1000s satellites in LEO."

The Planetary Society was concerned that launching a non-sterile object to interplanetary space may risk biological contamination of a foreign world. Scientists at Purdue University thought it was the "dirtiest" man-made object ever sent into space, in terms of bacteria amount, noting the car was previously driven on Los Angeles freeways. Although the vehicle will be sterilized by solar radiation over time, some bacteria might survive on pieces of plastic which could contaminate Mars in the distant future.

Orbit tracking

The car and the upper stage were passivated by intentionally removing remaining chemical and electrical energy, at which point they ceased transmitting telemetry. Based on optical observations made using a robotic telescope at the Warrumbungle Observatory, Dubbo, Australia and refinement of the orbit, a close re-encounter with Earth (originally predicted for 2073) is not possible. In October 2020, the car will make its closest approach to Mars, about 6.9 million kilometres (4.3 million miles) away, well outside the planet's gravitational sphere of influence.

The Virtual Telescope Project observed the Tesla two days after its launch, where it had a magnitude of 15.5, comparable to Pluto's moon Charon. The Roadster was automatically spotted and logged by the Asteroid Terrestrial-impact Last Alert System (ATLAS) telescope operated by the University of Hawaii.^[73] The car was observed by the Deimos Sky Survey (DeSS) at a distance of 720,000 km (450,000 mi) with a flashing effect suggesting spinning.

Roadster photographed with a 0.43 m telescope of Dubbo Observatory in Australia, on 8 February 2018, 16:29–16:50 UTC, at a distance of 550,000 km (1.4 Lunar distances) from Earth. Varying brightness suggests spinning.

Through measuring changes in apparent brightness of the object, astronomers have determined that the Roadster is rotating with a period of 4.7589 ± 0.0060 minutes (i.e. 4 minutes, 46 seconds). By February 11, 2018, astrometry measurements from 241 independent observations had been collated, refining the positions to within one-tenth of an arcsecond—more accurate than for most observations of objects in space.

Future predictions

Simulations over a 3-million-year timespan found a probability of the Roadster colliding with Earth at approximately 6%, or with Venus at approximately 2.5%. These probabilities of collision are similar to those of other near-Earth objects. The half-life for the tested orbits was calculated as approximately 20 million years, but with trajectories varying significantly following a close approach to the Earth–Moon system in 2091.

Musk had originally speculated that the car could drift in space for a billion years. According to chemist William Carroll, solar radiation, cosmic radiation, and micrometeoroid impacts will structurally damage the car over time. Radiation will eventually break down any material with carbon–carbon bonds, including carbon fiber parts. Tires, paint, plastic and leather might last only about a year, while carbon fiber parts will last considerably longer. Eventually, only the aluminum frame, inert metals, and glass not shattered by meteoroids will remain.