Satellites – FAQ

There are many questions about satellites. From how big and how fast, to how much do they cost, and what type of orbits do they operate in.  Below are answers to the most frequently asked questions:

How Many and How Big and How Much?

  • How many satellites are there?
    • Currently, in 2020, there 2,666 satellites in orbit around earth. The vast majority of these, 1,918, are in Low Earth Orbit (LEO).
  • When was the first satellite launched?
    • The first-ever satellite was Sputnik, launched by Russia on 4th October 1957 at 7:28pm. It orbited for just three weeks before it ran out of power and then fell back to earth two months later.
  • How big is a satellite?
    • They vary substantially in weight and size:
      • Weight: a few kilos to several tonnes, with a typical commercial satellite being a few tones.
      • Size: From tens of centimeters to something the size of a bus.
    • Is the size a problem for launch?
      • Since satellites need to fit in the fairing on top of a launcher all appendices (e.g. antennas and solar arrays) are folded up and only deployed when the satellite is in orbit.
    • What is the biggest satellite?
      • The largest-ever commercial satellite is the TerreStar-1. This was launched on 13:52 EDT on 2009 an Ariane 5 ECA from the Guiana Space Center in French Guiana and weighed 6,910 kg.
      • The largest ever recorded satellite is the KH-111 Kennen at 17,000 Kg, nearly three times heavier than the largest commercial satellite and 6 tonnes heavier than the Hubble telescope
    • What is the smallest satellite?
      • The smallest satellite is the Kalamsat-V2. It is just 1.26 kg and fits in the palm of your hand.
    • Who has the most satellites?
      • The United States has the most with 1,308. This is almost half of all satellites in orbit and nearly four times as many satellites as the next nearest competitor – China who has 356.
      • Russia, who has been in the space industry for much longer than China has just 167. Combined the EU (including the UK) would be in third place with 291 satellites.
    • How much does a satellite cost?
      • This price is driven by complexity and size. The smaller commercial ones typically cost a few million Euros, while larger satellites may cost billions of Euros.
      • However, this price is changing, and the costs are coming down. In the emerging ‘new space’ industry significant effort is made to reduce the cost of satellites, and several successful programs are now launching constellations of satellites that only cost a few hundred thousand Euros each.

Satellites – Fuel and Speed

  • How do satellites get into space?
    • Satellites get into space by riding on top of a launcher (rocket). The basic principle has not changed significantly since the launch of the first satellite, but recent development has greatly reduced cost and enabled more commercial activity.
  • How fast do satellites move?
    • The speed of depends on the orbit – Geostationary satellites orbit at about 3.1 km/s (over 10,000 kmph) while satellites in Low Earth orbit has a speed of about 7 km/s (over 25,000 kmph)
  • What powers a satellite?
    • The onboard electrical systems are powered via solar panels.
    • Some satellites have thrusters to move around in orbit, this relies on fuel – with some of the latest ones also using electric propulsion. However, even electric propulsion needs a type of fuel to push out of the thrusters.
  • Can a satellite run out of fuel?
    • While the internal system in the satellite run are powered by solar arrays, the satellite orbit is maintained using thrusters with some sort of fuel.
      Even the new types of electric propulsion becoming available need to have some sort of fuel to expel.
  • How long does a satellite last?
    • This depends on several factors. Modern satellites can, in theory, stay indefinitely, though some may only last a month. Even ones in orbits that last forever, will have a design life and eventually stop functioning.
  • Why do satellites sometimes fall to earth?
    • Below a certain altitude (~600km) there is still a very thin atmosphere. This atmosphere exerts a drag on the satellite. If it does not have thrusters and fuel to compensate for this drag, the orbit will decay fall to earth.
    • Note that it is very rare any pieces make it to the surface, most will be incinerated in the denser lower atmosphere.

Satellite Orbits

  • Why do satellites stay in orbit?
    • Orbital mechanics ensure they stay in orbit – essentially the satellite is always falling towards earth, but as long as it has sufficient velocity it will always miss.
    • Satellites in low earth orbit will see some drag from a very thin atmosphere at these altitudes. To make sure the satellite stays in the intended orbit, we need to boost the satellite with a thruster every now and then. If we fail to do so the orbit will decay with the satellite finally incinerating in the lower atmosphere.
  • Do all satellites orbit in the same direction?
    • No, not necessarily, but there is a preference to launch satellites towards the east. In doing so there is the advantage of the earth’s rotation when trying to give the satellite sufficient speed to orbit.
  • What types of orbits are there?
    • The primary types of orbit are:
      • Low Earth Orbit (LEO), Medium Earth Orbit (MEO) , Geostationary Earth Orbit (GEO) and Elliptical Orbit
        • These orbits refer to their distance in relation to the earth. Other types of orbit include:
      • Sun Synchronous, Equatorial, Polar, and Molniya
        • These orbits refer to how the satellite orbits in relation to the earth and the sun. E.g. satellites could have a LEO and by sun-synchronous,
    • What does geostationary orbit mean?
      • A geostationary satellite will, when viewed from the surface of the earth, appear to be stationary in the sky. This can be achieved by having the satellite orbiting along the equator, with exactly one revolution per day. This orbit is favored by communication satellites and results in an orbital altitude of ~36.000km.
    • What does sun-synchronous orbit mean?
      • A sun-synchronous orbit (SSO) is one where the earth’s surface is always illuminated by the sun from the same angle relative the satellite. This is desirable because it makes comparing data easier. A SSO is characterised by a parameter called LTAN – Local Time Ascending Node. A LTAN of for example 1300 implies the time is always 1 pm. When and where the satellite crosses the equator. Note that it does not necessarily mean the satellite will always pass overhead at a particular time.
    • What is a low earth orbit?
      • Low Earth Orbit, or LEO, is when a satellite has an altitude of is less than 2,000km (1,200miles) or least list 11.25 periods per day. Over 70% of all satellites are in Low Earth Orbit, LEO

Can you see them?

  • What satellites are visible in space?
    • Typically satellites in low earth orbit, and of some size, can be seen if the conditions are right. What you want is a clear dark night with few man-made light sources in the vicinity, and satellites in an orbit where they are lit by the sun.
  • What satellites can I see in the sky tonight?
    • There are several sites predicting what satellites are visible for your location. One such tracker is SkyandTelescope.Org
  • Can you see satellites during the day?
    • Typically, you will not be able to see satellites during the day due to the light in the atmosphere.

Satellites Beyond Earth

  • Do all satellites orbit the earth?
    • No, there are satellites around several planets and moons in the solar system.
  • Do satellites orbit the moon?
    • Yes, there are several satellites orbiting and observing the moon. There are plans for placing manned space stations in orbit around the moon – the lunar gateway. This would provide a staging post for lunar activities as well as future journeys to Mars.
  • Do satellites orbit the sun?
    • Yes, there are several satellites orbiting the sun. One recent such is Solar Orbiter which will contribute to our understanding of the complex processes in the sun.
  • Do satellites orbit mars?
    • Yes, Mars has a large number of satellites. Only the earth has more satellites than mars.
  • Do satellites have lights?
    • Satellites do not typically have lights. If you want to see them, you have to rely on them being lit by the sun.
  • Do satellites flash?
    • Satellites themself do not flash, as they don’ have lights. But there are instances where sunlight reflected off large surfaces such as solar arrays and antennas, generate a visible flash or flare when viewed from earth.

Can they see you?

  • Can you read a car number plate from a satellite?
    • No, with commercial satellites this is not possible
  • Can you see people from satellites?
    • Yes, you can see groups of people and, in theory, you could see an individual, if they were in a plain background (especially if there was a good shadow) – however, you can’t recognize individuals. You couldn’t see a single person in a crowd, nor could you pick out an individual from busy street.
  • Can a satellite see me?
    • Satellite can’t recognize and identify individual people.
  • Can you see somebody’s face via satellites?
    • No, you cannot see somebodies face or their feature