Deceleration of an interstellar probe

Several areas of investigation for deceleration of an interstellar probe have been identified and these include: Reverse engine thrust, solar sail, Magsail, Medusa sail, microwave sail, orbital slingshot, aerobraking. This is an analysis by the Icarus project.

Reverse engine thrust simply means using the same engine as used during the boost phase, but to decelerate the probe. However, there are several issues with doing this. First, this necessitates that the large mass engine (or a separate engine) must be carried by the probe into the target system, which minimises the staging potential. Second, the amount of propellant goes as the square of the mass ratio assuming an equal acceleration-deceleration profile, so the propellant mass would be increased to an unreasonable amount. Instead, one can go at a much slower cruise velocity and minimise the propellant mass required overall, but for a maximum mission requirement of 100 years this doesn’t allow for much margin.

Here is a quick back of envelope calculation using the existing Daedalus design and assuming its stage exhaust velocities. With a propellant mass of 46,000 tons (first stage) and 4,000 tons (second stage), and with a stage structure mass of 1,690 tons (first stage) and 980 tons (second stage), including the 450 tons science payload. If the vehicle undergoes its normal first stage acceleration for 2.05 years it will reach a cruise velocity of around 21,900 km/s or 0.073c where it will reach a distance of around 4,733 AU or 0.075 light years. But now rather than ignite the second stage, instead allow the vehicle to cruise at this speed for 41 years, until it reaches a distance of 3 light years, which is a total of around 3.1 light years when you add on the boost distance. Now use the second stage engine for reverse acceleration (deceleration) which takes just under 2 years (but we neglect this) and the vehicle will achieve a velocity increment of around 14,970 km/s or 0.05c, but in the opposite direction. Subtracting this from the cruise velocity already obtained, the vehicle now has a new cruise velocity of 6,930 km/s or 0.023c. Coasting at this speed it will reach the nearest stars 4.3 light years away in around 55 years from the end of the deceleration phase.

A solar sail will receive an intense flux of photons from the target star and thereby through momentum transfer decelerate the probe. However, in order for the sail to be effective it would have to be very large and be deployed near the target star. Also, there are concerns about a solar sail successfully unfurling and deploying after several decades of storage. In any case, initial calculations performed by the Icarus designer Pat Galea indicate that a sail for deceleration may not sufficiently slow down the probe. In work presented at the July 2010 New York Solar Sail symposium Galea showed that for a 50 ton probe travelling at a speed of 0.1c to perform the entire deceleration to a parabolic orbit capture using the sail alone, the calculated idea sail area would be seven hundred billion meters representing a circular sail of diameter nearly 1,000 km. Although one wonders what size of sail would be required for a much slower cruise velocity. Using the same data provided by Galea for 50 ton probe the sail diameter requirements become ever smaller for decreasing velocities: 472 km (0.05c), 377 km (0.04c), 283 km (0.03c), 189 km (0.02c), 94 km (0.01c) and 47 km (0.005c). For a sail of order 10 km in diameter, this would necessitate a cruise speed of around 0.001c. These numbers at least suggest further examination of the potential for solar sail deceleration is required.

A MagSail uses a magnetic field to deflect any charged particles radiated from the target star in a sort of plasma wind and use this deflection to impart deceleration to the main probe. The idea for using the MagSail in an interstellar context was first discussed in 1988 by the physicists Dana Andrews and Robert Zubrin in their paper “Magnetic Sails and Interstellar Travel”. The Icarus designer Adam Crowl has conducted some initial calculations which indicate that the deployment of this technology would be more useful when decelerating down from a higher velocity. This work is ongoing and shows good potential.

The Medusa sail uses the technology of nuclear explosion emitted from so called ‘units’, similar to Project Orion conducted in the 1950s and 1960s. The idea was first discussed in a set of papers published in 1999 and 2000 by the physicist Johndale Solem. Instead of using a large rear pusher plate to subtend a small angle as done for Project Orion (the shock absorbers of which also limit the performance), the Medusa sail utilizes a large sail (spinnaker) ahead of the probe, probably constructed out of a very high strength nanotechnology polymer such as aligned polyethylene. The detonations occur in front of the vehicle but behind the hemispherical sail so accelerating it and pulling the probe along. The cable line between the sail and the spacecraft is long, so that longer shock absorbers can be used which also (arguably) allows for an improved performance. A specific impulse of order 100,000 seconds is claimed to be possible for this propulsion system, where the velocity obtained from a single detonation increases for a larger caopy area and the closer the detonation point. The velocity will also be proportional tothe energy from the detonation and so the number of units used. The technology of a Medusa sail could similarly be used for deceleration of the Icarus probe, either by using direct nuclear explosions or even many inertial confinement pellets. Initial calculations for this concept are underway by the design team.

A Microwave sail is an alternative to a solar sail or direct laser propelled system. Microwaves are sent out using a maser. This concept was explored in a landmark 1985 paper titled “Starwisp: An Ultra-light Interstellar Probe”, by the physicist Robert Forward. Like the solar or laser sail, the microwave sail offers a rocket-less solution for an unmanned probe, where the wavelength of any microwave beam would be greater than the holes within the wire sail mesh that would constitute a Starwisp type probe. In theory, the Icarus vehicle could deploy a large number of Starwisp probes in the target system and decelerate them using an on-board microwave beam.

One option however is that instead of adopting any one of the above is to adopt an element of each method in order to gradually impart deceleration to the main probe. Utilizing a hybrid deceleration system along with the adoption of a low cruise velocity ~0.08-0.09c (see blog article “Daedalus & Icarus: Flyby versus Deceleration”) would seem a sensible option to ensure that the probe can be slowed down to a reasonable velocity in the target solar system and thereby ensure that the encounter time in the system is at least many months. One scenario would see the probe gradually slowed down by the use of a MagSail as it approached the target system and then a Medusa sail could be deployed to decelerate further still. On approaching the solar system, a laser/sail deceleration of a sub-probe could be deployed, perhaps accompanied by later engine burn to bring it down into orbital velocity. A complex set of manoeuvres for a presumably autonomous probe several light years away. But perhaps what is really needed for the ‘deceleration problem’ is some out of the box thinking, hence the concept of induced deceleration.

To induce something is to bring about or stimulate the occurrence of it. In the context of Project Icarus, Induced deceleration refers to the artificial creation of a desirable physics phenomena that doesn’t otherwise exist naturally in a sufficient amount or magnitude. The Starwisp is one example of this in fact, because the microwaves don’t exist naturally in sufficient abundance, so instead the vehicle carries along its own maser beam. Another example of induced deceleration is to create an intense particle flux field which impacts the probe shield and imparts momentum. This would be achieved by firing a projectile into an asteroid or comet ahead of the main probe, although assuming an isotropic debris field the probe would only receive a fraction of the explosion, hence the impact would need to be closely timed for maximum effect. Another example would be to detonate a large energy device inside a dense molecular cloud, causing the generation of a shock wave which the main probe then fly’s into and undergoes some drag. A sub-probe could also be launched ahead of the main probe when ready and deploy a laser (or maser) to irradiate a large sail which has been deployed on the main probe, in the absence of sufficient natural solar irradiation. The principle of induced deceleration is to identify the deceleration option and physics effect required, then assess if its available naturally in a sufficient amount, if not create it artificially using the on-board technology.

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