Time Capsules in Space


TIME CAPSULES IN SPACE

Several time capsules have already been launched into space:

PIONEER PLAQUES

The Pioneer plaque is a gold-anodized aluminum plate attached to the Pioneer 10 and 11 spacecraft, launched in 1972 and 1973. It shows a nude man and woman, a hydrogen atom, and a map of the Solar System and the Galaxy. The plaque was designed by Carl Sagan and Frank Drake for the two spacecraft. Now in interstellar space.

VOYAGER DISCS

The Golden Record was launched into space on board the Voyager 1 and Voyager 2 spacecraft. The information contained is a mini time capsule of humanity as of 1977. The disc can be played like a phonograph record. Instructions are shown on the top of the disc showing play details and waveforms used to encode the data. The bottom left image is a map of pulsars seen from Earth to help locate our planet. Spacecraft are now in interstellar space.

LUCY PLAQUE

The latest time capsule, in the form of a plaque, is carried aboard the NASA spacecraft LUCY, bound for the Trojan asteroids, near Jupiter’s orbit. Because the rocks are like space fossils, NASA named the spacecraft after Lucy, the fossilized human ancestor discovered in 1974.

DSM-BARC

This time capsule is a proposal from the University of Michigan. A cubesat, it is part of the Demonstration of Systems for Michigan Bicentennial Archive (DSM-BARC) which will be launched into geosynchronous orbit in 2018. It will weigh a maximum of 2 kg with 2 mm thick walls to shield it from radiation as it passes the Van Allen Belts in orbit. High-resolution photos of campus and student life, as well as 1000 interviews collected from U-M students, faculty, staff and alumni, will be etched onto silicon wafers.

“We decided to etch the interviews on a silicon wafer so the data could be stored in space for 100 years without any damage…any electronically stored data would not survive the space radiation environment for 100 years,” says MDP team member and CLASP alumni Hashmita Koka.

In addition, CubeSat will carry synthetic DNA samples as an experiment to determine its viability as a long-term storage method for information.

There is no record of this satellite being launched (2024)


DESIGN CONSIDERATIONS


ORBITAL REGIMES


LOW EARTH ORBIT

The graph below shows the approximate numbers of orbital objects in LEO as of a few years ago. The launch of large numbers of satellites for megaconstellations such as Starlink have now produced a large increase around 500 km altitude. Previous most popular orbits are 700 – 800 km and ~1400 km. Average orbital velocity is 7-8 km/sec and average collision velocity is ~10 km/sec. Approximate finite lifetimes due to atmospheric drag are shown at right. Collision probability over 100+ years is high and increasing. The lower altitudes do provide some protection from the higher Van Allen radiation belts. The two principal perturbing forces in LEO are atmospheric drag and gravitational asymmetry due to an oblate Earth. 

 Satellite     Satellite
 Altitude      Lifetime

  200 km          1 day

  300 km          1 month

  400 km          1 year

  500 km         10 years

  700 km        100 years

  900 km       1000 years


MIDDLE EARTH ORBIT

The big problem with MEO (Middle Earth Orbit) is the presence of high radiation levels due to the Van Allen radiation belts. The inner belt has high energy protons and the outer belt has mostly high energy electrons. These energies will easily penetrate the surface of satellites and quickly destroy the internal electronics unless special precautions are taken. This orbital regime is used mostly by PNT satellites (Precision Navigation and Timing) such as GPS, Glonass, Galileo and Beidou which use electronics made with Gallium Arsenide and Gallium Nitride which have higher resistance to radiation damage than silicon electronics. The population of these satellites is only a few hundred.


GEOSYNCHRONOUS EARTH ORBIT

The image above shows some of the commercial communication satellites in the popular geosynchronous orbit – there are many other types of satellites as well as these! The orbit is popular because a satellite in this orbit appears stationary in the sky and does not have to be tracked. Small satellites are not welcome because any object less than 30cm in size cannot be reliably tracked by many satellite tracking networks at this altitude. GEO is just above the outer van Allen radiation belt.


HIGH EARTH ORBIT

High Earth Orbit is mostly used by satellites in highly elliptical orbits (also called HEO) and high inclinations orbits (such as the Molniya and Tundra orbits shown). These are used mostly by Russia and other high latitude countries where GEO satellites are seen at very low elevations and thus do not provide good coverage. Perturbing forces in this regime are gravitational asymmetry (due to oblate Earth), lunar and solar gravity and solar wind pressure. Small satellites are very difficult to track in this orbital regime. This regime also has higher radiation due to galactic cosmic radiation (continuous) and high energy solar particle events (transient).


THE SPACE ENVIRONMENT

We used to think that space was a vacuum and benign as far as satellites were concerned. It is now realised that space is traversed by low and high energy particles and electromagnetic radiation from radio to gamma rays. These can all cause degradation to spacecraft in different ways. These include erosion of surfaces, change in surface properties, penetration into satellite, partial or total destruction due to hypervelocity collisions and induced electronic damage (transient of permanent) due to electric fields and discharges and cosmic rays. Of particular concern are degradation of solar cells which results in a progressive loss of power to the spacecraft. For spacecraft with exposed optical surfaces, erosion of the surface results in a loss of efficiency and function.


SATELLITE DESIGN

For time capsule data storage only a small satellite is required. The obvious choice is a “cubesat” which is a design based on a unit cube of dimensions 10 x 10 x10 cm (length by width by depth). This is called a unit cube or 1U. Actual satellites are made by joining several of these cubes together, eg a 3U or 6U cubesat. The design is standardised so that they multiple cubesats can be stacked together for ‘parallel’ launch – to reduce costs. Cubesats can contain solar panels for power, electronics for control, processing and communications, antennae for communications and stabilisation systems for orientation. A very basic 1U cubesat kit can be purchased for between $1K and $10K according to the features and whether it is educational or to be launched into space.


LAUNCHING A CUBESAT INTO ORBIT

The cost to launch a cubesat into orbit can range from $10K to $500K US dollars SpaceX charges $275,000 for 50 kg and NanoRacks costs $90,000 per 1U CubeSat A nanorack is a device that can hold multiple cubesats that are launched at the same time.

Sometimes ‘free’ rides can be obtained by a company that is testing a new rocker etc. NASA also has a cubesat launch initiative for educational institutions: Link: Cubesat Launch

However, all these launches are to low Earth orbit (LEO). Although people have talked about launching a cubesat into lunar orbit these are likely to be much expensive even if a permit is granted. The problem with launching a 1U cubesat into orbits above 10,000 km is that they cannot be tracked because of their size unless they carry a (radio) beacon.


CUBESAT LAUNCH DETAILS


LAUNCH POSSIBILITES FOR HEO

A second rocket burn is then made which places the satellite into the HEO with the desired perigee (note that a burn at perigee raises the apogee of the orbit while leaving the perigee unchanged and vice versa. When the Russian satellite reaches apogee the cubesat could fire a motor to raise its perigee and place it into a more circular orbit in the HEO regime. This requires cubesat motor! Australians have used Russia to launch their satellites but I image the cost would still be very high?

Note that the Australian government requires any launch by an Australian citizen to have a launch permit issued by them to satisfy UN international space law.


TIME SCALE CONSIDERATIONS

At this point it is probably appropriate to consider the time scale over which you want your time capsule to last and retain the data in a still readable form.

Are you looking for hundreds of years or millennia ?

This probably depends on the period of time you feel it would take for civilisation to recover from a dinosaur type asteroidal impact or a full scale nuclear war. And recover not just to a bow and arrow stage, but to a technological level that would allow the civilisation to reach, retrieve and read the orbital time capsule.

We should remember that the longest spacecraft in space is only around 50 years old and we really have no practical experience of the absolute lifetime of solar cells and other spacecraft components – only projections from ground tests and experience and a best guess consideration of the space environmental effects over longer time frames.


DATA STORAGE MEDIUM AND FORMAT

Previous time capsules sent into space have etched material onto metals.

A gold layer is resistant to degradation. A very hard material such as titanium might also be considered although a lot harder to work with. Note that most material used in spacecraft construction is Aluminium or its alloys for lightness.

Suggestions have been made to etch data onto a silicon substrate using the low cost fabrication methods to manufacture integrated circuits. This might be appropriate for discs or wafers that are enclosed inside a satellite.

Data format would invariably be in binary for numeric and symbolic data. The choice of symbol encoding would require a lot of thought as would voice encoding. There are no ‘natural’ choices that I can see in these areas. Image encoding is done by prime numbers.


WHAT SHOULD GO INTO THE CUBESAT

On the basis of past experience it would seem unlikely that even the best electronics and solar power systems are going to last past one hundred years. This basically precludes having a beacon on board.

It is also best to avoid batteries because exploding batteries and unspent fuel is the greatest cause of internal satellite fragmentation / destruction.

A satellite does need to be stabilised to be able to be tracked and retrieved later. Passive stabilisation may be achieved either by magnets (to align the spacecraft with Earth’s magnetic field) or by a boom with a weight on the end (this works by ‘gravity gradient’ stabilisation). Such a boom could be a flexible metal tape measure that is folded up and automatically deploys when the satellite is released into orbit by the carrier. The latter is probably better than using the Earth’s magnetic field which changes. However GGS becomes less effective as you move to higher orbits.

Thus for a long lasting time capsule it appears that only the data in the form of discs (for easy replay) just under 10cm in diameter is the only item likely to last and still be functional.


LONG TERM ORBITAL CHANGES

Over the course of 100 to 1000 years or more there will be orbital changes in HEO. This will be due to asymmetries in the Earth’s gravitation field (due to a non-spherical Earth). In HEO lunar gravity and solar gravity also perturb the satellite orbit. Pressure due to the solar wind is also a factor. All of these together make it virtually impossible to predict a precise orbit after only 100 years.

What this means is that some form of ‘beacon’ will be necessary to find the satellite after long periods of time. Radio beacons will not survive. It is possible that a very simple beacon comprising one or more high output LEDs (Light Emitting Diode) powered by a solar cell(s) might last a few tens of years ++?, and this may be worth including.

Micrometeoroid (debris from asteroids and comets) impacts will abrade satellite surfaces which is particularly important for optical surface.


TIME CAPSULE RECOVERY

Finding the time capsule cubesat after a long time may prove the most difficult part of the project. If a flashing LED can be made to work over the appropriate time period that would provide an optical beacon, but it would probably not be visible from the ground with any but a very large telescope. And the larger the telescope the narrower the field of view so a long duration scanning search would be required. A flashing (and coloured) beacon would be the only type distinguishable from stars etc. The only long lasting ‘beacon’ is probably a passive beacon or retroreflector such as the astronauts left on the moon. These are fashioned from mirrors or usually right angled or ‘corner’ prisms. These have the property that they reflect light back the way it came, so a laser scanning space would eventually get its own light back to the originating point. A laser and telescope together form a lidar or light radar, and could be used for detection.


GROUND LIDAR FOR CUBESAT LOCATION

The above diagram shows a generic scanning LIDAR that might be used to find a cubesat carrying retroreflectors in orbit. Output parameters would need to include look angles (eg Azimuth and Elevation) together with range to allow an orbit to be computed.




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