I am interested in the possibility of sending unmanned reconnaisance spacecraft to study exoplanets, especially Earth-like ones with the ultimate goal of detecting alien life.

Certainly earth-based telescopes will become incredibly powerful in the next century, allowing us to study exoplanets in much greater detail. But I would think an unmanned spacecraft sent for a flyby of an expolanet would be a revolutionary step in this field.

  1. What kinds of technologies would we need for a flyby of an expolanet.

  2. What are the different approaches or methods for accomplishing a flyby. Please mention time to get to destination (ex: can reach alpha centauri in 60 years)

Please mention which technologies/approaches are the most feasible.


6 Answers 6


There are incredible, mindboggling distances involved in merely reaching an exoplanet. Alpha Centauri Bb, the closest known exoplanet, is 4.365 light years away. That's 41,295,000,000,000 km (25,660,000,000,000 mi), or 276,000 AU (the distance from the Earth to the Sun).

Voyager 1 has left most of our solar system behind and is traveling a blistering 17.3 km/s. If it were headed toward Alpha Centauri Bb, it would take nearly 75,700 years to get there.

At interstellar distance scales, the technical challenge is all about attaining speed. Project Icarus is proposing to use a nuclear fusion to attempt interstellar speeds. Beamed propulsion is a proposal to use light sails powered by gigantic lasers.

Another way to get a boost is the Krafft Arnold Ehricke trajectory, recently mentioned in this question, from which I quote:

. . . the best trajectory . . . to get out of Solar System with the highest velocity $V_\infty$ will have to use:

  • A gravity assist from Saturn
  • A gravity assist from Jupiter
  • A perihelion propulsive maneuver (aka firing engines) near the Sun (as close as allowed by the spacecraft's thermal control system).

Even the most optimistic predictions say perhaps within a hundred years we might attain speeds sufficient to get our probes there in hundreds of years. Thus another technical challenge will be designing a probe that will last that long, will still have a functioning power system upon arrival, will still have functioning instruments, radios, etc. This is, of course, complicated by the fact that there is not truly anything like empty space anywhere. The particles in interstellar space are extremely sparse, but when one is traveling at an appreciable fraction of the speed of light, the slightest mote has great damage potential.

Then there's the question of how to decelerate so that when we get to the destination, we don't flash by so quickly that no practical amount of data can be absorbed by our instruments. Most designs call for accelerating to speed, then decelerating. Magnetic sails have been put forth as a possible deceleration technique, but that presupposes sufficient magnetic flux in the interstellar medium to render that feasible.

Getting the data from the exoplanet flyby back to Earth is itself a technical challenge. Assuming your probe manages to capture a scientifically significant cache of data about the target system and planet(s), the vast distance the telemetry has to transmit means a significantly attenuated signal by the time it gets to any receiver in the solar system. One approach might be to send a series of relay probes after the primary vehicle to receive, boost, and then relay the signal.

  • $\begingroup$ Actually I wrote that before the above post was significantly edited. I then tried to remove that comment but the website went down. I have removed it now. $\endgroup$
    – math_lover
    Jan 7, 2015 at 5:33
  • $\begingroup$ @JoshuaBenabou yes, I too noticed the site go offline for a moment. It's usually much more robust. :) $\endgroup$ Jan 7, 2015 at 5:35

But I would think an unmanned spacecraft sent for a flyby of an expolanet would be a revolutionary step in this field.

Better said: Impossible, at least using anything close to current technology.

Getting to Alpha Centauri in 60 years would require an average velocity of nearly 22,000 kilometers per second, or 0.5 astronomical units per hour. That high of a velocity would give the vehicle just hours to conduct its mission. That pretty much rules out a flyby mission. The vehicle needs to come to rest at the target star, which ups the ante even further on the "we don't know how to do that" scale (aka the technology readiness level). The technologies that might be able to do that are optimistically rated at TRL 3. That's very low, and even that number requires looking at things through very rose colored glasses.

Even if we had the propulsion technology to do that, what good will it do if the vehicle is dead on arrival or if it can't communicate with Earth? The vehicle needs a power system that will last for 60 years (we don't know how to do that), a computer system that can survive for 60 years (we have some idea how to do that), and a communications system that can broadcast at a reasonable rate across 4.365 light years (we don't know how to do that).

To illustrate the last problem, the Voyager satellites have had to regularly throttle back its transmission rate as they has receded further and further from the solar system. Some time this year, Voyager 1 will have to reduce its high rate transmission to 1.4 kilobits per second. That will mark the end of those high rate transmissions; the playback system on the spacecraft can't work at that low a rate. That 1.4 kilobits per second at 132 AU becomes 28 bits per day at 4.365 light years. It would take almost a year to transmit a single 256x256 gray scale image compressed by a factor of 10:1 at that rate. We need a communication system three orders of magnitude better than that to be useful. One order of magnitude improvement is doable. Three orders of magnitude? We don't know how to do that.

  • $\begingroup$ I don't understand about the transmission rates - why don't they transmit in bursts? Does it have to do with signal power or S/N ratio? $\endgroup$
    – kim holder
    Jan 7, 2015 at 15:45
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    $\begingroup$ @briligg - They do transmit in bursts, at a bit over 1.4 kb/s right now. (Real-time data are transmitted at a much reduced rate.) The spacecraft record data for about six months, then send that recorded data to Earth as fast as possible. That "as fast as possible" is limited by the ability of the 70 meter dish (the biggest NASA has) to obtain and maintain bit sync with the signal. $\endgroup$ Jan 7, 2015 at 15:56
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    $\begingroup$ @briligg - The signal strength at the transmitter and the distance signal has to travel dictates the signal strength at the receiver. Noise sources include the transmitter itself, the atmosphere, and the receiver. These give the signal to noise ratio. The receiver has to detect the carrier, then the modulated signal, then identify bits in that modulated signal, and finally find CCSDS frame boundaries. Signal strength at the receiver needs to be high enough so that the receiver can receive something. It needs to be higher yet to find the modulated signal. ... $\endgroup$ Jan 7, 2015 at 16:55
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    $\begingroup$ Once the receiver has locked onto that, it can look for those bit boundaries. This is where noise is very important. Way too much noise and the signal doesn't look anything like bits. If the receiver locks onto the bit signal (bit synchronization), the next step is to find frame boundaries. Space data are transmitted in fixed length frames, with a known pseudonoise pattern marking the frame boundary. Too high a bit error rate means that that frame boundary can't be found. ... $\endgroup$ Jan 7, 2015 at 17:00
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    $\begingroup$ One way of overcoming a weak (but not too weak) signal at the receiver is to just have the transmitter take more time between bit transitions. This extra time lets the bit signal rise out of the noise at the receiver. $\endgroup$ Jan 7, 2015 at 17:01

The most feasible that I've seen, which isn't all that high on the feasibility scale, is a very large, very low mass wire mesh containing sensors that is accelerated and powered by a microwave beam from Earth. See Starwisp.

  • $\begingroup$ Forward's original paper: path-2.narod.ru/design/base_e/starwisp.pdf $\endgroup$ Jan 10, 2015 at 18:09
  • $\begingroup$ Still haven't found the Starwisp paper by Landis this side of a paywall, but this one has some analysis of Starwisp. 2 papers, 2nd one looks like draft of the one co-authored by J. Benford. $\endgroup$ Jan 10, 2015 at 18:13


The responses from Jerard Pucket and David Hammond are very good. As Jerard Pucket indicated, most people do not realize the great exponential increase in distances between the practical planetary/planetoid edge of our solar system, (such as Pluto and Sedna) and even the closet stars. The layman analogy that lays it out mathematically scaled down but proportional to the distances involved is the Football Stadium analogy for our Solar System... Imagine the Sun as a Clementine at the 50 Yard line, center field. Earth is 10 feet away. At this scale, Pluto is at the stadium edge, the farthest seats, where the lowest cost ticketed fans are. To go to the nearest star at this scale ? It's an Airline flight or a long car ride... Alpha Centauri is approx. a 1,000 miles at that scale. Gliese 581c, 21 Light years away... If the stadium is in Cleveland, then Gliese 581 is near Scotland, across the Atlantic. Pluto is about 7 to 9 years away from Earth with current tech (to the fans in the far stadium seats)... Get the picture... going the Gliese 581 in Scotland when its 7 to 9 years to reach the stadium edge. Marc Millis of Tau Zero Foundation has a good article on estimated resources and time lines for sending an interstellar probe... see: http://arxiv.org/ftp/arxiv/papers/1101/1101.1066.pdf - also see also the "incessant obsolescence postulate" (again from Marc Millis, Tau Zero Foundation) http://www.tauzero.aero/discoveries-log/getting-there/motives/
David Hammond gave a good basic review on radio transmissions back the Sol (Earth) from Alpha Centauri. Here is some more "sobering" info derived from an article written by David Woolley with references from Project Cyclops, (ISBN 0-9650707-0-0), Reprinted 1996, by the SETI League and SETI Institute; and Radio Astronomy, John D. Kraus, 2nd edition, Cygnus-Quasar Books, 1986.

How BIG should this interstellar probe be?
How much power should it generate for communications ? For propulsion ? (to reach Alpha Centauri in a life-time)

The Transmitter/Receiver (and it's size): Even if we took the biggest radio dish telescope today, the 1,000 foot Arecibo Radio Telescope and "magically" transported in orbit of Alpha Centauri Bb, we could NOT detect ANY Television, Radio, Radar or any other main stream communication from Earth ! Using the S- Band at 2.380 GHz with a band width of 0.1Hz (possible today, but would limit the data stream transfer rate) add a ludicrous transmission power of one giga-watt (well beyond anything we presently do, but possible if truly required). All this would have a detection range of 5 Light years with the Arecibo dish. A very precise pointing laser communications system is another option, with reduced power, but still power outputs in the multi-mega watt range and it would represent developing technology, but likely possible in your lifetime. For the Probe to reach Alpha Centauri Bb in 90 years, the probe must achieve a max speed of 10 % of light, decelerate and then orbit. The bottom line would be a probe utilizing a VERY robust nuclear fusion generator with lots of fuel (which we do NOT yet have nuclear fusion, it is still beyond us). The probe would be several times bigger than the USS Aircraft Carrier Nimitz (mainly for fusion fuel to reach 10 % of light speed twice), with just the communication system having enough power generation to run a large city. This probe's propulsion system would generate power which would DRAWF the entire World's energy production capacity today... The cost: The economics of building it would bankrupt the world many times over at this point in history, and oh yes, the technology is NOT yet available, so the whole point is mute... However, read the information from Marc Millis as today's stepping stone technologies could lead to building such a probe... in several hundred years. Additionally, the probe's size would come down an order of magnitude with advancing far future technologies, but the probe would still be larger than anything we have placed into orbit at this point.


Things aren't quite as negative as the other posters are making it out to be.

While we simply can't do it yet we aren't as far away from such a probe. Look at Robert L. Forward's Starwisp concept--both the boosting and the power for the flyby are done with beamed power, greatly simplifying matters. The construction of the probe is beyond current technology but it doesn't require fundamental breakthroughs like any fuel-carrying rocket capable of that kind of speed would.

Starwisp also has the advantage that a big part of the system (the huge solar power -> microwave satellite) stays here and can be used for launch after launch after launch. This makes additional launches much cheaper, if you're going to send out such probes you can send them to every star that's close enough. On the other hand, they can't communicate without being energized by the beam--if you want to know if the probe has survived you have to direct a power beam to it--and the design inherently can't have any sort of shield against the interstellar gas and dust.

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    $\begingroup$ I like the Starwisp concept -- but a microwave lens 560 km in diameter? A transmitter rated at 56 gigawatts? Not to mention the problem of manufacturing, thermal absorption . . . $\endgroup$ Jan 8, 2015 at 20:16
  • $\begingroup$ @JerardPuckett Compared to other approaches to getting up to that speed those are trivialities. $\endgroup$ Jan 9, 2015 at 2:27
  • $\begingroup$ Improvements might be to send a convoy of probes which refocus the beam to the next probe in front. The beam should not only provide propulsion, but also be the power supply and be used for deceleration. The probe should be self-reconstructable in order to deal with damages from collisions and radiation, to switch from shipping state to arrival state, and to be of the most modern design when it arrives. I imagine that it would be mainly biological. $\endgroup$
    – LocalFluff
    Jan 27, 2015 at 8:03

EMDrive + Solar sail = 80-100 years. A little high, but seems in the realm of possibility.

One "cutting edge"** technology being talked about is the EMDrive. Its basically a light powered ion engine that could perpetually accelerate, allowing for crazy high speeds. (.1 times the speed of light)

Mr. Joosten and Dr. White stated that “a one-way, non-decelerating trip to Alpha Centauri under a constant one milli-g acceleration” from an EM drive would result in an arrival speed of 9.4 percent the speed of light and result in a total transit time from Earth to Alpha Centauri of just 92 years.

However, if the intentions of such a mission were to perform in-situ observations and experiments in the Alpha Centauri system, then deceleration would be needed.

This added component would result in a 130-year transit time from Earth to Alpha Centauri – which is still a significant improvement over the multi-thousand year timetable such a mission would take using current chemical propulsion technology.

Evaluating NASA’s Futuristic EM Drive

Sneak in a solar sail (to both power it, and use sunlight to give it that extra boost), and you can decrease the time by up to a factor of two.

JAXA engineers used Doppler radar measurements of the Ikaros craft to determine that sunlight is pressing on the probe's solar sail with a force of about 1.12 millinewtons (0.0002 pounds of force).

Solar Sail Passes Big Test In Deep Space

**NOTE: The credibility of the EMDrive is up for debate.

  • $\begingroup$ Note: The EM-Drive is totally busted. Power cables in Earth's magnetic field--when they build the apparatus and send power it works the same whether the power goes to the drive or to a simple load. $\endgroup$ Jul 13, 2023 at 17:58

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