Napkin analysis: Interstellar comet hitch-hiking
Today is perihelion of Comet 2I/Borisov, the second interstellar object ever observed inside our solar system. It was first spotted on Aug. 30, 2019 (4 months before nearest apsis to Earth) by Gennady Borisov, a comet hunter in Crimea.
Back in 2014, before any interstellar objects had been discovered, a JPL report on the Comet Hitchiker described a craft that could spear asteroids to achieve a fast trajectory to a wide range of destinations within the Solar System and also produce energy. The craft would include a tether — between 62 and 620 miles long made of Zylon, Kevlar, or carbon nanotube — and a diamond harpoon. NASA’s Comet Hitchhiker will help create a guide to the galaxy
I wondered if a craft like this could tether an interstellar comet to give us a lift out of the solar system towards our nearest neighbor star, Alpha Centauri. Here’s my back of the napkin analysis:
The speed of 2I/Borisov: 20 mi/s = 32km/s . For reference:
- NASA’s “fastest” spacecraft, Parker Solar Probe will reach speeds of 192km/s, but that’s moving toward the sun (using a lot of gravity)
- Previously, it was the Juno probe at 75km/s (using Jupiter’s gravity)
- Voyager 1 travels at 17km/s
Light travels 10,000 times the speed of comet, and Alpha Centauri is 4.37 light-years from the Sun. So hitch-hiking even to Alpha Centauri on a comet going the speed of Borisov would take >40,000 years; and that’s if a comet happened to be going in that direction, or could somehow be redirected in that direction. That is some ten thousands of years too many.
Another idea: We could hitchhike out of the solar system, then launch craft when out of the sun’s orbit; this would save a lot of fuel. Something like this:
- Hit comet with some course-changing force, a missile or other projectile. The amount of force required would be determined by the trajectory correction and the mass of the comet... I don’t know what it’s mass would be, but this comet’s about a mile wide. https://www.quora.com/How-much-does-a-comet-weigh Note: Any energy spent changing the comets direction is energy that could have been used to just propel ourselves forward out of the solar system — something to factor into efficiency calculations.
- Harpoon comet — if we could know its material content in advance, we could reel ourselves in to mine the comet for energy resources on the ride out of solar system. There’s also a lot of water and maybe even sugar.
- Detach from comet and increase speed — baseline speed after detachment would equal speed of comet, 30km/s. Propulsion could be a big sudden burst of energy or slowly dispersed energy (gaining speed over time). Note: if we could go 1/5 the speed of light, we would arrive at Alpha Centauri in just four years. Currently the Parker probe, our fastest craft, is on the order of (but less than) 1/1000 the speed of light and only goes that fast because it’s hurtling toward the sun (using the sun’s gravity, not fuel).
- Chill for a few years riding out Newton’s 1st Law.
- Use Alpha Centauri’s gravity for low-fuel / fuel-less orbit insertion.
- Land on Proxima Centauri — requires some reserve fuel.
An obstacle would be the 14x Earth-sized hypervelocity dust environment, a cloud of tiny, fast-moving particles surrounding the comet, some of which will hit the spacecraft. This is not a new problem. The Parker Solar Probe carries Kevlar blankets to protect itself. Cassini has traveled through Saturn’s D-ring region, where the spacecraft has traveled amidst dust particles travelling at speeds as high as 31.4 km/s (Risk Assessment of Hypervelocity Impact of Saturn Dust on Cassini Sun Sensors).
Dozens of interstellar objects are thought to pass through the solar system every year unseen. Perihelion of 2I/Borisov is today 12/8, but the comet will come closest to Earth 12/28.
Could We Intercept Interstellar Comet C/2019 Q4 Borisov?
Comet Interceptor a “fast-class” concept consisting of three spacecraft that will wait in space until a pristine comet appears