The new Space Tourist: Astronaut or Astronaut*?Follow article
VSS Unity firing its rocket engine and heading towards the edge of Space.
Image credit: Virgin Galactic
A trip into Space (for a mere $250,000)
Space tourism became a reality today (11th July 2021) when Virgin Galactic’s VSS Unity spacecraft gave the boss Richard Branson a brief lift above the atmosphere and a few minutes of ‘weightlessness’. It’s taken 17 years from the creation of the company in 2004 to develop the concept of a rocket-powered spacecraft launched from a high-flying jet aircraft to the point where it’s safe to sell ride-tickets to passengers with no astronaut training. The concept is definitely not new, with all the pioneering work carried out by the USAF and NASA on the X15 project back in the 1950s. The X15 rocket plane was carried aloft by a modified B-52 bomber and then released, with rocket ignition seconds later causing the X15 to soar almost vertically, up to 350,000 feet (67 miles) at speeds of up to Mach 6.7 (5140 mph). This was no joyride and test pilots died pushing the limits of what was possible. One who survived went on to become the first man to set foot on the Moon: Neil Armstrong. From watching the first Unity 22 passenger flight live, it looks like this ride into Space is about as exciting as traveling on a bus to Skegness (but with better views out of the windows). The comparatively relaxed ride is no doubt due to the less extreme numbers: maximum speed Mach 3.5 (2600 mph) and apogee (maximum altitude) of about 50 miles. That and having pilots more concerned with returning in one piece than smashing the speed/altitude record set by the last guy. There is one element of the flight that only lasts a few seconds but would definitely see anyone’s adrenaline levels peak. A video of Unity’s first powered test flight shows these moments very clearly at the start:
I think the interval of maximum terror must be from the point when you feel the spacecraft detach from the mothership and freefall for a few seconds before the rocket engine fires and you’re kicked back in your seat by nearly 6g of acceleration. I’ll bet that wait feels like a lifetime, despite knowing that the pilots have practiced this manoeuvre and landed safely without an engine even having been installed. The drop followed by the acceleration up to Mach 3.5 is probably the most exciting part of the flight.
However, the controversy surrounding the altitude which you need to exceed in order to qualify as an Astronaut spoils the party somewhat.
Where does Space begin?
You would have thought that the definition of reaching Space should have been resolved by now. Well, it hasn’t and the media hype surrounding the rivalry between Virgin Galactic and Jeff Bezos’ Blue Origin company has intensified. The first crewed flight of New Shepard, a conventional rocket/capsule arrangement for carrying passengers is slated for 20th July. But, unlike Unity, it can travel higher than an ‘official’ boundary line known as the Kármán line. This puts the boundary of Space at 100 km (62 miles or 330,000 feet) above sea-level and is accepted by the whole world – except the USA. The US FAA and the USAF place the boundary at 80 km (50 miles or 260,000 feet) instead. So, Unity passengers qualify as Astronauts under US rules only, while New Shepard customers are well clear as their vehicle can get almost as high as the X15! To most people, this might seem to be a ridiculous detail; but if you’ve handed over hundreds of thousands of dollars for a seat only to find it only makes you an “Astronaut” or an Astronaut*, then you might just get a bit upset about it.
What makes an astronaut?
I have a solution: let’s say that to call yourself an astronaut you need to complete at least one orbit of the Earth. That deals with any arguments about altitudes as 100 km is considered to be the minimum for a stable Earth orbit. It also ensures that you have to travel in a spacecraft propelled by a ‘proper’ rocket booster capable of reaching orbital speeds. See The Scourge of Space Junk, Part 1: Orbital Mechanics for more detail on the topic of orbital mechanics.
The feeling of weightlessness appears as soon as the booster engine is shut-down: an event called Main-Engine Cut-Off (MECO). The sensation is maintained as the spacecraft coasts onwards with a ballistic trajectory and ends on the way down when drag forces cause downward acceleration to cease. You don’t need to go into Space to experience the effect: a diving aircraft or even a free-falling lift can (briefly) provide the same sensation.
Even official documents often refer to ‘weightlessness’ or ‘zero-gravity’ in Earth orbit. These terms are at best misleading, or at worst, nonsense, So I thought l would expand on the comments made in the Space Junk article:
There is no such thing as ‘zero-gravity’
All bodies of large mass like the Earth or Moon create gravitational forces which gradually diminish with distance. The acceleration due to Earth’s gravity is 9.81 m/s2 at sea-level. At 100 km altitude it’s still about 9.5 m/s2. So, the apparent weightlessness inside the cabin of Unity is not due to a zero-gravity condition. After all, how can the spacecraft and its occupants get back to Earth if they are stuck in a mysterious zone with no gravity?
Weightlessness is a sensory illusion
Lie on a sofa and close your eyes: how do you know that gravity is holding you down on the sofa or put another way, that you possess weight? Answer: you can feel the sofa cushion pressing against your back. As the Action-at-a-distance Force called gravity (which you can’t feel) pulls you down against the cushion, the cushion pushes back with an equal and opposite Contact Force (which you can feel). That’s Newton’s third law of motion.
Now imagine yourself lying on your Unity couch after being dropped from the mothership VMS Eve. As the craft falls away, and before the rocket engine ignites, you will feel a lot lighter. That’s because both Unity and yourself are accelerating downwards under the influence of gravity. However, the effect of gravity acting on Unity is partially counteracted by atmospheric drag. Inside, you feel the full 1g of gravity which is why you feel lighter but not weightless – your back is still being pressed against the couch. Then the rocket motor lights up and suddenly Unity is shooting upwards, accelerating at over three times the rate of gravity. You’re crushed back into the couch by this ‘g-force’ because the couch is being accelerated directly by the rocket thrust along with the rest of the spacecraft. But not you: you are not fixed to the couch. It feels like you are being pushed into the couch, but in fact, it’s the latter pushing against your back, overcoming your inertia and accelerating your body forward to the same velocity as Unity. At MECO the rocket thrust stops, the atmospheric drag is much reduced and so the only force remaining is Earth’s gravity of 1g acting upon both you and Unity. Gravity now asserts itself, slowing both the spacecraft and its passengers to a halt at apogee. From MECO to apogee you’ll feel ‘weightless’ and float about the cabin because there’s now no difference in the forces acting upon you and Unity. Your body thinks its floating. In fact, relative to the spacecraft you are floating, while you and it move under the influence of the same accelerating force.
After apogee, the spacecraft starts to descend, but until it encounters strong drag forces again in the thicker lower atmosphere, apparent weightlessness in the cabin remains. The feeling of weight returns as the spacecraft descends into the thicker atmosphere. Eventually, Unity’s downward acceleration is reduced to zero and the craft continues to fall at a constant velocity termed Terminal Velocity. But inside Unity, you continue to accelerate at 1g because obviously, you are not experiencing any atmospheric drag. So as the craft approaches terminal velocity, the couch begins to press against your back until once again, you feel your 1g ‘weight’. Your weight never went away – you just couldn’t sense it.
The crew of the orbiting International Space Station experience the same weightless sensation because they too are falling towards Earth. But because the ISS components and subsequent crew were propelled onto a course tangential to the Earth’s curvature at a velocity of around 17,000 mph, they ‘fall’, but so slowly that their trajectory matches the curvature of the Earth. This means that the distance between them and the surface never changes or, in other words, they orbit at a constant altitude. The crew feels weightless all the time because gravity is the only force that acts upon them and the ISS around them.
The first fully-crewed flight of New Shepard on 20th July will no doubt see arguments of what makes an astronaut surface again. In my view, the title of ‘Astronaut’ should be reserved for those with appropriate professional training and experience, After all, you wouldn’t call yourself a doctor because you once visited someone in hospital, now would you?
In my next post, I'll talk about New Shepard, SpaceX Falcon 9 and what it takes to get into orbit.
If you're stuck for something to do, follow my posts on Twitter. I link to interesting articles on new electronics and related technologies, retweeting posts I spot about robots, space exploration and other issues.