This article originally appeared in 2009, but I’ve updated it and added this video.
The ground feels firm and solid beneath your feet. Of course, the Earth is rotating, turning once on its axis every day. Fortunately gravity keeps you firmly attached to the planet, and because of momentum, you don’t feel the movement – the same way you don’t feel the speed of a car going down the highway. But how fast does the Earth rotate?
You might be surprised to know that a spot on the surface of the Earth is moving at 1675 km/h or 465 meters/second. That’s 1,040 miles/hour. Just think, for every second, you’re moving almost half a kilometer through space, and you don’t even feel it.
Want to do the calculation for yourself? The Earth’s circumference at the equator is 40,075 km. And the length of time the Earth takes to complete one full turn on its axis is 23.93 hours.
Wait, 23.93 hours? Isn’t a day 24 hours? Astronomers calculate a day in two ways. There’s the amount of time it takes for the Earth to complete one full rotation on its axis, compared to the background stars. Imagine you were looking down at the Earth from above the North Pole. You’d see the Earth turn once completely in 23 hours and 56 minutes. Astronomers call this a sidereal day.
And then there’s the time it takes for the Sun to return to the same spot in the sky. Since the Earth is orbiting the Sun, we actually need an extra 4 minutes each day to return the Sun to the same spot. Astronomers call this a solar day.
Then we divide the length of a day into the distance a point on the equator travels in that period: 40,075 km/23.93 hours = 1,675 km/hour, 465 meters/second.
The speed of the Earth’s rotation changes as you go North or South away from the equator. Finally, when you reach one of the Earth’s poles, you’re taking a whole day to just turn once in place – that’s not very fast.
Because you’re spinning around and around on the Earth, there’s a force that wants to spin you off into space; like when you spin a weight on a string. But don’t worry, that force isn’t very strong, and it’s totally overwhelmed by the force of gravity holding you down. The force that wants to throw you into space is only 0.3% the force of gravity. In other words, if the Earth wasn’t spinning, you would weigh 0.3% more than you do right now.
Space agencies take advantage of the higher velocities at the Earth’s equator to launch their rockets into space. By launching their rockets from the equator, they can use less fuel, or launch more payload with the same amount of fuel. As it launches, the rocket is already going 1,675 km/hour. That makes it easier to reach the 28,000 km/hour orbital velocity; or even faster to reach geosynchronous orbit.
We have written many articles about the Earth for Universe Today. Here’s an article about why the Earth rotates.
This article originally appeared in Universe Today in July, 2012, but it’s been updated with a related video.
The planet Mars is one of the brightest objects in the night sky, easily visible with the unaided eye as a bright red star. Every two years or so, Mars and Earth reach their closest point, called “opposition”, when Mars can be as close as 55,000,000 km from Earth. And every two years, space agencies take advantage of this orbital alignment to send spacecraft to the Red Planet. How long does it take to get to Mars?
The total journey time from Earth to Mars takes between 150-300 days depending on the speed of the launch, the alignment of Earth and Mars, and the length of the journey the spacecraft takes to reach its target. It really just depends on how much fuel you’re willing to burn to get there. More fuel, shorter travel time.
History of Going to Mars:
The first spacecraft ever to make the journey from Earth to Mars was NASA’s Mariner 4, which launched on November 28, 1964 and arrived at Mars July 14, 1965, successfully taking a series of 21 photographs. Mariner 4’s total flight time was 228 days.
The next successful mission to Mars was Mariner 6, which blasted off on February 25, 1969 and reached the planet on July 31, 1969; a flight time of only 156 days. The successful Mariner 7 only required 131 days to make the journey.
The NASA team threw in every bit of data they could to model the Mars Curiosity landing. Credit: NASA
Mariner 9, the first spacecraft to successfully go into orbit around Mars launched on May 30, 1971, and arrived November 13, 1971 for a duration of 167 days. This is the same pattern that has held up for more almost 50 years of Mars exploration: approximately 150-300 days.
Here are some more examples:
Viking 1 (1976) – 335 days
Viking 2 (1976) – 360 days
Mars Reconnaissance Orbiter (2006) – 210 days
Phoenix Lander (2008) – 295 days
Curiosity Lander (2012) – 253 days
Why Does it Take So Long?:
A top-down image of the orbits of Earth and Mars. Credit: NASA
When you consider the fact that Mars is only 55 million km away, and the spacecraft are travelling in excess of 20,000 km/hour, you would expect the spacecraft to make the journey in about 115 days, but it takes much longer. This is because both Earth and Mars are orbiting around the Sun. You can’t point directly at Mars and start firing your rockets, because by the time you got there, Mars would have already moved. Instead, spacecraft launched from Earth need to be pointed at where Mars is going to be.
The other constraint is fuel. Again, if you had an unlimited amount of fuel, you’d point your spacecraft at Mars, fire your rockets to the halfway point of the journey, then turn around and decelerate for the last half of the journey. You could cut your travel time down to a fraction of the current rate – but you would need an impossible amount of fuel.
How to Get to Mars with the Least Amount of Fuel:
The primary concern of engineers is how to get a spacecraft to Mars, on the least amount of fuel. Robots don’t really care about the hostile environment of space, so it makes sense to decrease the launch costs of the rocket as much as possible.
NASA engineers use a method of travel called a Hohmann Transfer Orbit – or a Minimum Energy Transfer Orbit – to send a spacecraft from Earth to Mars with the least amount of fuel possible. The technique was first proposed by Walter Hohmann who published the first description of the maneuver in 1925.
Instead of pointing your rocket directly at Mars, you boost the orbit of your spacecraft so that it’s following a larger orbit around the Sun than the Earth. Eventually that orbit will intersect the orbit of Mars – at the exact moment that Mars is there too.
If you need to launch with less fuel, you just take longer to raise your orbit, and increase the journey to Mars.
Other Ideas to Decrease the Travel Time to Mars:
Although it requires some patience to wait for a spacecraft to travel 250 days to reach Mars, we might want a completely different propulsion method if we’re sending humans. Space is a hostile place, and the radiation of interplanetary space might pose a longterm health risk to human astronauts. The background cosmic rays inflict a constant barrage of cancer-inducing radiation, but there’s a bigger risk of massive solar storms, which could kill unprotected astronauts in a few hours. If you can decrease the travel time, you reduce the amount of time astronauts are getting pelted with radiation, and minimize the amount of supplies they need to carry for a return journey.
Go Nuclear:
One idea is nuclear rockets, which heat up a working fluid – like hydrogen – to intense temperatures in a nuclear reactor, and then blast it out a rocket nozzle at high velocities to create thrust. Because nuclear fuels are far more energy dense than chemical rockets, you could get a higher thrust velocity with less fuel. It’s proposed that a nuclear rocket could decrease the travel time down to about 7 months
Go Magnetic:
Another proposal is a technology called the Variable Specific Impulse Magnetoplasma Rocket (or VASIMR). This is an electromagnetic thruster which uses radio waves to ionize and heat a propellant. This creates an ionized gas called plasma which can be magnetically thrust out the back of the spacecraft at high velocities. Former astronaut Franklin Chang-Diaz is pioneering the development of this technology, and a prototype is expected to be installed on the International Space Station to help it maintain its altitude above Earth. In a mission to Mars, a VASIMR rocket could reduce the travel time down to 5 months.
Go Antimatter:
Perhaps one of the most extreme proposals would be to use an antimatter rocket. Created in particle accelerators, antimatter is the most dense fuel you could possibly use. When atoms of matter meet atoms of antimatter, they transform into pure energy, as predicted by Albert Einstein’s famous equation: E = mc2. Just 10 milligrams of antimatter would be needed to propel a human mission to Mars in only 45 days. But then, producing even that minuscule amount of antimatter would cost about $250 million.
Artist’s concept of Antimatter propulsion system. Credit: NASA/MFSC
Future Missions to Mars:
Even though some incredible technologies have been proposed to shorten the travel time to Mars, engineers will be using the tried and true methods of following minimum energy transfer orbits using chemical rockets. NASA’s MAVEN mission will launch in 2013 using this technique, as well ESA’s ExoMars missions. It might be a few decades before other methods become common techniques.
We record the Weekly Space Hangout every Friday at 12 pm Pacific / 3 pm Eastern. You can watch us live on Google+, Cosmoquest or listen after as part of the Astronomy Cast podcast feed (audio only).
We record the Weekly Space Hangout every Friday at 12 pm Pacific / 3 pm Eastern. You can watch us live on Google+, Cosmoquest or listen after as part of the Astronomy Cast podcast feed (audio only).
Every Sunday night we connect up a bunch of telescopes into a live Google+ Hangout and showcase the night sky. In addition to a team of astronomers and their scopes, we usually have a few PhD astronomers on hand to explain the objects that we’re looking at. Each episode typically lasts about an hour and we’re able to see a few dozen objects. If you want to get the gist of what a Virtual Star Party looks like, check out this 60-second version put together by my co-host, Scott Lewis.
I’m not sure if Scott will have the energy to do these every week, but I love how this worked out.
I just wanted to remind everyone, if you don’t want advertising mixed in with your space news, you can remove them permanently with a 1-time donation to Universe Today. I’ll switch you over to a Member Account, and all the ads will be automatically removed for your viewing pleasure.
Here’s a question… how long does it take sunlight to reach Earth? This sounds like a strange question, but think about it. Sunlight travels at the speed of light. Photons emitted from the surface of the Sun need to travel across the vacuum of space to reach our eyes.
The short answer is that it takes sunlight an average of 8 minutes and 20 seconds to travel from the Sun to the Earth.
If the Sun suddenly disappeared from the Universe (not that this could actually happen, don’t panic), it would take a little more than 8 minutes before you realized it was time to put on a sweater.
Here’s the math. We orbit the Sun at a distance of about 150 million km. Light moves at 300,000 kilometers/second. Divide these and you get 500 seconds, or 8 minutes and 20 seconds.
This is an average number. Remember, the Earth follows an elliptical orbit around the Sun, ranging from 147 million to 152 million km. At its closest point, sunlight only takes 490 seconds to reach Earth. And then at the most distant point, it takes 507 seconds for sunlight to make the journey.
But the story of light gets even more interesting, when you think about the journey light needs to make inside the Sun.
You probably know that photons are created by fusion reactions inside the Sun’s core. They start off as gamma radiation and then are emitted and absorbed countless times in the Sun’s radiative zone, wandering around inside the massive star before they finally reach the surface.
What you probably don’t know, is that these photons striking your eyeballs were ACTUALLY created tens of thousands of years ago and it took that long for them to be emitted by the sun.
Once they escaped the surface, it was only a short 8 minutes for those photons to cross the vast distance from the Sun to the Earth
As you look outward into space, you’re actually looking backwards in time.
The light you see from your computer is nanoseconds old. The light reflected from the surface of the Moon takes only a second to reach Earth. The Sun is more than 8 light-minutes away. And so, if the light from the nearest star (Alpha Centauri) takes more than 4 years to reach us, we’re seeing that star 4 years in the past.
There are galaxies millions of light-years away, which means the light we’re seeing left the surface of those stars millions of years ago. For example, the galaxy M109 is located about 83.5 million light-years away.
If aliens lived in those galaxies, and had strong enough telescopes, they would see the Earth as it looked in the past. They might even see dinosaurs walking on the surface.
Many people are still having a tough time wrapping their head around Google+. They go there and none of their friends are around. To solve this problem, I’ve created my Super Science Circle. This is a pre-made circle containing 400 people who actively post on Google+ about science topics. There are some big names there, including astronauts, professors, researchers, students, journalists, and lots of science enthusiasts. I carefully curate this list to make sure each and every person on the list is active and regularly posts about science.
So, if you want to understand Google+, and get over the idea that it’s a ghost town, all you need to do is import this circle and you’ll have a ready-made community overflowing with amazing content.
Emily Coren is an accomplished illustrator and scientist, and she’s figured out how to bring those two skills together to make science easier to understand: science illustrations.
Emily completed a series last year on how adaptive optics works. This is the technology that increases the resolution of both telescopes and microscopes, and she gave us permission to reprint the series here on Universe Today.
We interrupt your regular Weekly Space Hangout with this extra special edition to cover to the two asteroid-related events: the Russian meteor explosion and the close pass of Asteroid 2012 DA14.
We record the Weekly Space Hangout every Friday at 12 pm Pacific / 3 pm Eastern. You can watch us live on Google+, Cosmoquest or listen after as part of the Astronomy Cast podcast feed (audio only).
