Distances in the Universe and Space Travel
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Distances in the Universe and Space Travel. Earth and Moon. Diameters: D Earth = 12,700 km D Earth = 4 x D Moon Average distance from the Earth to the Moon: 384,400 km 30 x D Earth 1.28 light seconds. Altitudes of Space Shuttle and Satellites. Space Shuttle: 200-1000 km

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Earth and moon
Earth and Moon


DEarth = 12,700 km

DEarth = 4 xDMoon

Average distance from the Earth to the Moon:

384,400 km

30 xDEarth

1.28 light seconds

Altitudes of space shuttle and satellites
Altitudes of Space Shuttle and Satellites

Space Shuttle:

200-1000 km

Hubble Space Telescope:

600 km

International Space Station:

340 km

geosynchronous satellites (always above same location on the Earth):

36,000 km

10% of the distance to the Moon

Earth and sun
Earth and Sun


DEarth = 12,700 km

DSun = 1,400,000 km = 110 xDEarth

Average distance from the Earth to the Sun:

150,000,000 km

11,800 xDEarth

8.3 light minutes

defined as 1 astronomical unit (AU)

Kuiper Belt and Oort Cloud

Kuiper Belt: near the orbit of Pluto (30-100 AU)

Oort Cloud: 50,000 AU from Sun almost 1 light year

Voyager 1

Launched in 1977

Beyond orbit of Pluto in 1990’s

Now 100 AU from Earth = 14 light hours

Traveling at 15 km/s = 0.005% speed of light

= 5 light years in 100,000 years

Beyond the nearest stars
Beyond the Nearest Stars

Distance to center of Milky Way:

25,000 light years

Diameter of Milky Way:

100,000 light years

Distance to nearest large galaxy (Andromeda):

2 million light years

Most distant parts of the known Universe:

45 billion light years

Distances vs. Speed

The time required for travel to other planets in the solar system and to other stars is determined by the distances to those destinations and the velocities of our spacecraft. The ultimate speed limit for any ship is the speed of light (300,000 km/s). The distances to stars are large, even compared to the speed of light, making space travel a lengthy endeavor.

Distances vs. Speed

The fastest space probes currently exploring the solar system travel at velocities of 17 km/s, which is less than 0.01% of the speed of light. At this speed, it takes several years to reach the outer planets and 70,000 years to reach the nearest star. However, current spacecraft have not been designed for travel to stars. It may be feasible to build ships that could reach speeds of 10% of the speed of light. At these speeds, the travel time would be much lower for the nearest stars, but still very long for more distant parts of the universe:

nearest star = 40 years

center of our galaxy = 125,000 years

nearest large galaxy = 20,000,000 years

The Twin Paradox

Based on his theory of special relatively, Einstein postulated that time passes more slowly as one approaches the speed of light. This prediction was described in a famous thought experiment called the “twin paradox” (which is not actually a true paradox). In this story, one twin travels to a star at nearly the speed of light. After returning home, the twin find that he appears much younger than his sibling who stayed home. For instance, imagine that a person travels to the nearest star (4 light years) at 99% of the speed of light. For people on Earth, his roundtrip would take 8 years, while only 1 year would have passed for the traveler. In effect, it’s one-way time travel!

Challenges of Manned Missions to Mars

Mars is the next logical destination for a manned mission beyond the Moon. A mission of this kind faces many daunting challenges:

  • physical effects of prolonged exposure to cosmic rays

  • physical effects of prolonged weightlessness

  • psychological effects of isolation

  • social effects of small, crowded environment

  • lack of medical facilities

  • technology (propulsion, life support, energy, etc.)

  • cost

Cosmic Rays

Cosmic rays are energetic subatomic particles (e.g., protons) that originate from solar flares and outside of the solar system. Because they travel so fast (sometimes near the speed of light), these particles damage DNA as they pass through the body. Extended exposure to cosmic rays causes neurological damage and an increased risk of cancer.

Parker, 2006, Scientific American

Cosmic Rays

The atmosphere and magnetic field of the Earth prevent most cosmic rays from reaching surface. The magnetic field also offers some protection for astronauts in low-Earth orbit (e.g., space shuttle). However, no natural protection is available for the Moon and beyond.

Cosmic Rays

Exposure to cosmic rays was not a concern for the Apollo missions because they lasted only several days. But long-term exposure through permanent lunar bases and Mars missions (lasting 2 years) poses a serious risk for astronauts.

For spacecraft to Mars, one option is to surround the ship with shielding, perhaps made of water. However, this would greatly increase the mass of the spacecraft, requiring much more fuel.

Parker, 2006, Scientific American

Cosmic Rays

As a second option for protection from cosmic rays, a spacecraft could be designed to include a magnetic field that would act as a shield. However, a sufficiently strong magnetic field would require enormous amounts of power and would greatly increase the mass of the ship.

Also, the crew would be immersed in the magnetic field during the voyage. The effects of long-term exposure to strong magnetic fields on the body are unknown.

Parker, 2006, Scientific American

Cosmic Rays

By firing a beam of (negative) electrons away from the spacecraft, it could be given a net positive charge that would repel positively charged cosmic rays (like protons).

However, an electric field of this kind would require an enormous electric current, and it would attract many negative particles from space that are just as bad as the positive cosmic rays.

Parker, 2006, Scientific American

Cosmic Rays

Because the atmosphere of Mars is so thin, it provides little protection against cosmic rays. So even after arriving on Mars, proper shielding would be important for long-term visits.

If the living areas were placed underground, the surface of Mars would act as a shield against cosmic rays. However, construction of these habitats would require a great deal of effort and heavy machinery.


A journey to Mars using conventional rockets would require several months each way, which may be tolerable. However, traveling to the nearest stars will require the highest possible speeds to minimize the length of the journey. Achieving those speeds with conventional rockets is theoretically possible, but would require enormous amounts of fuel (most of the ship would be fuel!).

Travel to the stars will require a propulsion system that does not require such a huge payload of fuel. Three options are engines based on anti-matter (very rare) and fusion (possible, but difficult) or a light sail pushed by a giant laser from Earth or light from the Sun.