ArtsAutosBooksBusinessEducationEntertainmentFamilyFashionFoodGamesGenderHealthHolidaysHomeHubPagesPersonal FinancePetsPoliticsReligionSportsTechnologyTravel

Basic Concepts Of Orbital Spaceflight

Updated on March 13, 2011

The basic concepts of orbital spaceflight are easy to understand, and this knowledge will give you a better understanding of space-related news, history and current events.

The common conception of orbital flight is that of an object flying through space, following a circular or elliptical path around another object. That description is accurate, but incomplete. It leaves out the most important element of orbital flight: gravity.

An orbit is the path of an object in space as it moves around another object due to the force of gravity. Orbital flight is not powered flight, it is the result of a careful balance between gravity and momentum. A rocket is used to carry a spacecraft into space, but once the spacecraft has achieved sufficient altitude and velocity, the rocket engine is turned off, and often discarded. At this point, the spacecraft will be held in orbit by gravity - the same force that holds the moon in orbit and makes the planets revolve around the sun.

Newton's Cannon

In 1687, in a book titled Principia Mathematica, the physicist Isaac Newton presented the first explanation of orbital motion. It is still one of the simplest and clearest explanations available.

Newton's Cannon.
Newton's Cannon.

Newton imagined a canon on top of a very high mountain. A cannonball is shot out, which travels for a distance, but eventually gravity pulls it down and it strikes the ground. On a second shot, more gunpowder is used, and the cannonball travels further before striking the ground. In each case, the cannonball follows a curved path to the ground.

The surface of the earth is also curved. If enough gunpowder were used, Newton suggested, the curvature of the cannonball's trajectory would be the same as the curvature of the earth. The cannonball would be falling, but it would never reach the ground. The cannonball would "fall" all the way around the world and strike the cannoneer in the back of the head.

Momentum

Momentum is the tendency of a moving object to stay in motion. When you are in a moving vehicle that stops suddenly, it is your own momentum that throws you forward.

Momentum is related to velocity. The greater velocity an object has, the more momentum it has.

This is the definition of an orbit. The momentum of the cannonball and the pull of gravity are balanced. The cannonball is in a continuous state of free fall, and will remain so until affected by another force (Newton's theoretical example assumes the cannon is high enough that the resistance of earth's atmosphere can be ignored).

NASA has an online app that shows Newton's cannon in action. You can plug in different amounts of gunpowder and see the resulting trajectory of the cannonball: Shoot a Cannonball into Orbit!

The Space Shuttle is launched into orbit. Photo courtesy of NASA.
The Space Shuttle is launched into orbit. Photo courtesy of NASA.

Orbital Velocity

The velocity needed to maintain a given orbit is called orbital velocity. The velocity required depends on the altitude of the orbit. The closer an orbiting spacecraft is to earth, the higher the velocity required to remain in that orbit. To give you an idea of the speeds required, maintaining a circular orbit at an altitude of 100 miles requires a velocity of 17,478 miles per hour.

Getting Into Orbit

Rockets are used to give a spacecraft sufficient altitude and velocity to achieve orbit. A rocket is launched vertically, but as it ascends it adjusts its trajectory to become more horizontal. This puts the spacecraft in the proper position for orbit.

When the desired altitude and velocity have been reached, the rocket engine is cut off. If altitude and velocity are correct, the spacecraft will be in orbit. The path of the orbit is determined by the spacecraft's velocity.

Once in the desired orbit, the spacecraft's velocity must be carefully monitored. Changes in velocity will change the path of the orbit. In fact, this is how a spacecraft in orbit returns to earth. Small rockets, known as retrorockets, fire briefly against the direction of flight, changing the velocity of the spacecraft. This changes the path of the spacecraft's orbit to one that will bring the spacecraft back into the atmosphere.

One of the iconic images of the Space Age - Astronaut Ed White floating in space on America's first spacewalk. Photo courtesy of NASA.
One of the iconic images of the Space Age - Astronaut Ed White floating in space on America's first spacewalk. Photo courtesy of NASA.

Weightlessness

Weightlessness is an interesting side effect of being in orbit. A common misconception is that objects in space float because there is no gravity in space. The term "weightlessness" reinforces this idea, but the word is a misnomer. In fact, the pull of gravity on objects in orbit is diminished only by a small amount. For example, an astronaut that weights 160 pounds on earth will weigh about 140 pounds while in orbit.

The astronaut feels weightless because he is in a constant state of free fall. Few of us have ever been in free fall long enough to notice, but a person in free fall doesn't feel his own weight. Only when something is opposing the pull of gravity do we experience the feeling of weight. On earth, gravity pulls us into something - the ground or the floor, for example - and this opposition to gravity is why we feel weight.

Everything in orbit with the astronaut floats around him because it all is continuously falling at the same rate as he is. Although travelling at thousands of miles per hour, these objects are not moving at all relative to each other, and they appear to be floating in space, or "weightless".

Apogee and Perigee

Not all orbits are circular. Many take the shape of an ellipse. The point of highest altitude in an elliptical orbit is called the apogee, and the lowest point is the perigee. A circular orbit is just a special case of an elliptical orbit where the apogee and perigee are the same.

In a circular orbit, the velocity of an object is constant, while velocity varies throughout an elliptical orbit. In such an orbit, velocity is greatest at the perigee, and lowest at apogee.

Atmospheric drag

Earth's atmosphere doesn't end suddenly at the edge of space. It gradually thins out. A spacecraft in low-earth orbit will experience drag, or resistance, from the thin atmosphere it encounters. This drag will cause an orbit to deteriorate, or decay, over time. All current manned space activity takes place in low-earth orbit, and must take drag into account. The International Space Station, for example, must periodically be boosted to its proper altitude and velocity to compensate for the effects of atmospheric drag.

Alan Shepard is launched on a suborbital spaceflight on May 5, 1961. Photo courtesy of NASA.
Alan Shepard is launched on a suborbital spaceflight on May 5, 1961. Photo courtesy of NASA.

Suborbital flight

The first two manned launches of America's space program, by Alan Shepard and Gus Grissom in 1961, were not orbital flights. The rockets available at the time were powerful enough to carry astronauts into space, but could not provide the velocity needed to achieve orbit. These flights were known as suborbital flights.

By 1962, more powerful rockets were available, and there were no more manned suborbital flights for several decades. In 2004, SpaceShipOne, the first privately funded spacecraft to take a human into space, made three suborbital flights. The plan is for a similar spacecraft, SpaceShipTwo, to begin taking tourists on suborbital space flights, possibly beginning in 2011.

Space tourism has renewed interest in suborbital spaceflight, which is now being considered for other commercial applications, such as transportation, experimentation in weightlessness, and even extreme sports. The Suborbital Institute is a good place to learn more about the potential benefits of suborbital spaceflight.

Comments

    0 of 8192 characters used
    Post Comment

    • GD Nunes profile imageAUTHOR

      Glen Nunes 

      6 years ago from Cape Cod, USA

      Well, JT, I'm afraid I have to shatter my image. I haven't actually read all of Principia Mathematica, although I have read parts of Book I, including the part where Newton describes how an object might orbit the earth. I mostly skimmed through the parts of Book I that were filled with formula and diagrams. I haven't read the other Principia books, or any Galileo or Euclid. Have you read them? Just curious - how old is the son who is learning this?

    • JT Walters profile image

      JT Walters 

      6 years ago from Florida

      You are the first person I have ever encountered that had read Newton's Principia Mathematica. I just finsihed techingt his book to my son. I had to teach him all about retrogrades and it was quite interesting. Have you read Gallileo as well as Euclid to?

      Great Article by the way. You have enlghtened and I am surpirsed anyone else has read this bok.

      JT

    • GD Nunes profile imageAUTHOR

      Glen Nunes 

      7 years ago from Cape Cod, USA

      sciburg, you are correct, of course! I've made the correction to the hub. Thanks for bringing it to my attention.

    • profile image

      sciburg 

      7 years ago

      Great explanation! Not sure about one thing though. You state that, for an elliptical orbit, "velocity is greatest at the apogee, and lowest at perigee". Isn't it the other way around?

    • GD Nunes profile imageAUTHOR

      Glen Nunes 

      8 years ago from Cape Cod, USA

      I was born in 1960, and the astronauts were my heroes when I was growing up. I've never lost my interest in space exploration. Unfortunately, things didn't turn out the way we thought they would. When I was a kid I thought for sure that by 2010 there would be humans on Mars, and we'd all be vacationing at hotels in orbit.

    • Instgtr profile image

      Instgtr 

      8 years ago from Mesa, Arizona

      Interesting article - how did you get interested in spaceflight?

    working

    This website uses cookies

    As a user in the EEA, your approval is needed on a few things. To provide a better website experience, hubpages.com uses cookies (and other similar technologies) and may collect, process, and share personal data. Please choose which areas of our service you consent to our doing so.

    For more information on managing or withdrawing consents and how we handle data, visit our Privacy Policy at: https://hubpages.com/privacy-policy#gdpr

    Show Details
    Necessary
    HubPages Device IDThis is used to identify particular browsers or devices when the access the service, and is used for security reasons.
    LoginThis is necessary to sign in to the HubPages Service.
    Google RecaptchaThis is used to prevent bots and spam. (Privacy Policy)
    AkismetThis is used to detect comment spam. (Privacy Policy)
    HubPages Google AnalyticsThis is used to provide data on traffic to our website, all personally identifyable data is anonymized. (Privacy Policy)
    HubPages Traffic PixelThis is used to collect data on traffic to articles and other pages on our site. Unless you are signed in to a HubPages account, all personally identifiable information is anonymized.
    Amazon Web ServicesThis is a cloud services platform that we used to host our service. (Privacy Policy)
    CloudflareThis is a cloud CDN service that we use to efficiently deliver files required for our service to operate such as javascript, cascading style sheets, images, and videos. (Privacy Policy)
    Google Hosted LibrariesJavascript software libraries such as jQuery are loaded at endpoints on the googleapis.com or gstatic.com domains, for performance and efficiency reasons. (Privacy Policy)
    Features
    Google Custom SearchThis is feature allows you to search the site. (Privacy Policy)
    Google MapsSome articles have Google Maps embedded in them. (Privacy Policy)
    Google ChartsThis is used to display charts and graphs on articles and the author center. (Privacy Policy)
    Google AdSense Host APIThis service allows you to sign up for or associate a Google AdSense account with HubPages, so that you can earn money from ads on your articles. No data is shared unless you engage with this feature. (Privacy Policy)
    Google YouTubeSome articles have YouTube videos embedded in them. (Privacy Policy)
    VimeoSome articles have Vimeo videos embedded in them. (Privacy Policy)
    PaypalThis is used for a registered author who enrolls in the HubPages Earnings program and requests to be paid via PayPal. No data is shared with Paypal unless you engage with this feature. (Privacy Policy)
    Facebook LoginYou can use this to streamline signing up for, or signing in to your Hubpages account. No data is shared with Facebook unless you engage with this feature. (Privacy Policy)
    MavenThis supports the Maven widget and search functionality. (Privacy Policy)
    Marketing
    Google AdSenseThis is an ad network. (Privacy Policy)
    Google DoubleClickGoogle provides ad serving technology and runs an ad network. (Privacy Policy)
    Index ExchangeThis is an ad network. (Privacy Policy)
    SovrnThis is an ad network. (Privacy Policy)
    Facebook AdsThis is an ad network. (Privacy Policy)
    Amazon Unified Ad MarketplaceThis is an ad network. (Privacy Policy)
    AppNexusThis is an ad network. (Privacy Policy)
    OpenxThis is an ad network. (Privacy Policy)
    Rubicon ProjectThis is an ad network. (Privacy Policy)
    TripleLiftThis is an ad network. (Privacy Policy)
    Say MediaWe partner with Say Media to deliver ad campaigns on our sites. (Privacy Policy)
    Remarketing PixelsWe may use remarketing pixels from advertising networks such as Google AdWords, Bing Ads, and Facebook in order to advertise the HubPages Service to people that have visited our sites.
    Conversion Tracking PixelsWe may use conversion tracking pixels from advertising networks such as Google AdWords, Bing Ads, and Facebook in order to identify when an advertisement has successfully resulted in the desired action, such as signing up for the HubPages Service or publishing an article on the HubPages Service.
    Statistics
    Author Google AnalyticsThis is used to provide traffic data and reports to the authors of articles on the HubPages Service. (Privacy Policy)
    ComscoreComScore is a media measurement and analytics company providing marketing data and analytics to enterprises, media and advertising agencies, and publishers. Non-consent will result in ComScore only processing obfuscated personal data. (Privacy Policy)
    Amazon Tracking PixelSome articles display amazon products as part of the Amazon Affiliate program, this pixel provides traffic statistics for those products (Privacy Policy)