How do you catch an interstellar asteroid? Go fast ... really, really fast
In September this year, the first interstellar object ever witnessed visiting the Solar System passed close to the Sun. While interstellar asteroid 'Oumuamua (1I/2017 U1) may have been and gone, that hasn't stopped scientists from figuring out ways to catch up to it. A team from the Initiative for Interstellar Studies (i4is) has put forward a proposal for Project Lyra, a plan to chase and intercept the asteroid in the most ambitious space mission ever attempted.
'Oumuamua caused quite a sensation when it was first detected on October 19, 2017 by the University of Hawaii's Pan-STARRS 1 telescope on Haleakala. First plotted as part of a NASA survey to identify near-Earth objects, later analysis of its orbit and light curve indicated that it was an asteroid that had been flying through interstellar space for hundreds of million of years before briefly passing through our system.
Because 'Oumuamua has an origin different from anything in history that scientists have been able to study close up, sending a probe to the interstellar asteroid is a very high priority. Unfortunately, as the i4is team admit, such a mission would be more ambitious than any other deep space flight to date. It would require reaching speeds that have never been obtained by a man-made spacecraft as well as sending a probe into an orbit far out of the ecliptic. This would require energy that would tax even the largest rockets on the drawing boards today.
The problem is that 'Oumuamua is heading away from us on an open-ended hyperbolic trajectory with at a velocity of 26 km/s (59,000 mph, 95,000 km/h). To give some perspective, Voyager 1 is the fastest object ever launched and it's only managing a mere 16.6 km/s (37,000 mph, 60,000 km/h). But sending a probe to 'Oumuamua isn't just a question of matching speeds. It has to be set on an intercept course at a velocity fast enough to chase and overtake the target, and this speed gets greater the farther away 'Oumuamua gets.
Announced on October 30 by the non-profit i4is, Project Lyra (named after the constellation from which 'Oumuamua came) sets out the challenges and alternatives of a mission to the asteroid using NASA's SLS or SpaceX's BFR boosters. This includes a flyby of the Sun and Jupiter or refueling the booster stage in orbit and sending the probe direct to 'Oumuamua.
Since 'Oumuamua is already speeding its way out of the Solar System, the ideal time to launch and intercept mission would be now, but because neither the SLS or the BFR is in service, to try such a mission using current boosters, even if a spacecraft was available, wouldn't be practical. A planetary flyby can reduce the size of the rocket needed, but it vastly increases the time required to intercept 'Oumuamua. The team also says that a mission within five years to 10 years is highly unlikely.
This is a problem because an earlier mission requires reaching a high speed of at least 26.5 km/s (59,300 mph, 95,400 km/h) to reach 'Oumuamua in 20 years, but this minimum gets higher the further back the launch date is. In 10 years, for example the velocity becomes 37.4 km/s (87,000 mph, 135,000 km/h). In 30 years, this becomes 76 km/s (170,000 mph, 274,000 km/h). These velocities for missions in later years exceed the capacity for any chemical, electric, or solar sail technology. Another factor is distance.
The later the launch, the farther the probe will be from Earth when it catches 'Oumuamua.
If it launches today, the probe will intercept inside a year and only 5.3 AU (493 million mi, 793 million km) away from the Sun. If the probe launches in 2027 or later, the intercept point will be 100 to 200 AU (15 billion km to 30 billion km). That's farther away than the Voyager probes are at the moment. This means including nuclear power sources, heavy thermal radiation, and special communications systems.
Another tradeoff is speed over time at the destination. A very fast probe will get to 'Oumuamua quicker, but it will also just flash past the objective with little chance of observation. Ideally, the probe would be equipped with some form of electric propulsion to slow it down to match trajectories or orbit 'Oumuamua. On the other hand, a fast approach velocity would allow the probe to release an impactor before slowing down. If timed right, the impactor could hit 'Oumuamua before the probe's arrival, allowing it to make a spectroscopic analysis of the impact flash and debris.
According to the researchers, the most promising way to reach 'Oumuamua would be using a laser sail propelled by a multi-megawatt laser in Earth or heliocentric orbit. This would not only allow the probe to reach 'Oumuamua in a short time, but could also help with deceleration of a single large probe or a swarm of small ones, much in the way a ship slows down by backing its sails.
The only problem is that such laser sail technology is still years in the future. The team says that this approach may be too late for 'Oumuamua, but could be built and at the ready when another interstellar visitor enters our system.