Hmm, what should NASA do next after the spectacular Curiosity mission? A solar observer? Seen that. A lunar orbiter? Like the ones China and India already did? A Mars orbiter? Like the several that are already there? Even I hadn’t heard of these probes – IRIS, LADEE, and MAVEN – and I actually like this stuff. It’s kind of nice that these missions are becoming routine, but they aren’t contributing to one of NASA’s main purposes, which is technical spectacle.
But I recently came across an old proposed mission that really would get the reporters out – the Interstellar Probe.
This was a plan to send a probe into true interstellar space, beyond the shell of plasma emitted by the sun called the heliosphere. That’s amazingly far out, about 20 billion miles, 7 times the distance to Pluto. Rockets would take forever to get there. Even Voyager 1 is only 12 billion miles out, and it was launched 35 years ago. So instead they planned to use a solar sail 400 m in diameter:
After launch from the earth, it would be deployed at an angle to the sun. Photons bouncing off it would slow it down so that it fell inward. The sail is deployed by spinning, and steered by shifting the mass of the main probe in the center:
It would fall to inside the orbit of Mercury, about 20 million miles out, where the intensity of sunlight is 16 times what it is at earth. Then it would turn face-on to the sun and get blasted outward.
It’ll take almost a year to fall into the sun, but then only four months to blast past Jupiter. At that point it’ll be moving at 70 km/s, the fastest probe ever. That’s four times faster than the current record-holder, the New Horizons probe to Pluto. It would jettison the sail near the orbit of Jupiter to make sure it didn’t interfere with the instruments. It would reach the orbit of Pluto (32 AU) in a bit over two years, and the heliopause (200 AU) in 15.
They wanted a mission lifetime of 30 years, so the probe would be 400 AU out at that point. The probe would be powered by radio-isotope generators, which have a limited life. The Voyager ones will run out by 2025. It would have a 2.7m main antenna, which would still be able to deliver 350 bits/sec even at 200 AU. The whole probe would weigh about 250 kg, of which 100 kg would be the sail.
The key technology limit is the areal density of the sail. They need to get it down to 1 gm/m2, even though sail materials as of 2000 were at about 10. In 2010 the Japanese launched a solar sail probe, IKAROS, to Venus. It had a 20 m sail made of 7.5 um polyimide, which would come in at about 7 gm/m2. They’re planning a 50m sail + ion propulsion mission to Jupiter in the late 2010s.
The other limit is the max temperature that the sail can withstand. This mission would get it up to 550 C at perihelion. An even deeper dive would be even hotter, but would get even more speed. In “The Starflight Handbook”, Eugene Mallove and Gregory Matloff calculate that a dive to within 0.01 AU with a sail that only weighed 0.02 gm/m2 would heat up to 1500 C. It would blast away from the sun at 440 Gs, and hit a terminal speed of 3400 km/s, about 1.2% of c. Then it’s only 350 years to Alpha Centauri! Space is just really, really big.
Unfortunately for all these plans, the Interstellar Probe mission appears to have been shelved by about 2002. NASA does have a sail project in the works: the Sunjammer, with a 40 m sail weighing only 30 kg. If all goes well, it should launch in 2014. If they scaled that up by a factor of 10, then threw it at the sun while aiming at the stars, they would get some press!